Electrochemistry at the Edge of a van der Waals Heterostructure.

Artificial van der Waals heterostructures, obtained by stacking two-dimensional (2D) materials, represent a novel platform for investigating physicochemical phenomena and applications. Here, the electrochemistry at the one-dimensional (1D) edge of a graphene sheet, sandwiched between two hexagonal boron nitride (hBN) flakes, is reported. When such an hBN/graphene/hBN heterostructure is immersed in a solution, the basal plane of graphene is encapsulated by hBN, and the graphene edge is exclusively available in the solution. This forms an electrochemical nanoelectrode, enabling the investigation of electron transfer using several redox probes, e.g., ferrocene(di)methanol, hexaammineruthenium, methylene blue, dopamine and ferrocyanide. The low capacitance of the van der Waals edge electrode facilitates cyclic voltammetry at very high scan rates (up to 1000 V s-1), allowing voltammetric detection of redox species down to micromolar concentrations with sub-second time resolution. The nanoband nature of the edge electrode allows operation in water without added electrolyte. Finally, two adjacent edge electrodes are realized in a redox-cycling format. All the above-mentioned phenomena can be investigated at the edge, demonstrating that nanoscale electrochemistry is a new application avenue for van der Waals heterostructures. Such an edge electrode will be useful for studying electron transfer mechanisms and the detection of analyte species in ultralow sample volumes.

Chiral Transport of Hot Carriers in Graphene in the Quantum Hall Regime.

Photocurrent (PC) measurements can reveal the relaxation dynamics of photoexcited hot carriers beyond the linear response of conventional transport experiments, a regime important for carrier multiplication. Here, we study the relaxation of carriers in graphene in the quantum Hall regime by accurately measuring the PC signal and modeling the data using optical Bloch equations. Our results lead to a unified understanding of the relaxation processes in graphene over different magnetic field strength regimes, which is governed by the interplay of Coulomb interactions and interactions with acoustic and optical phonons. Our data provide clear indications of a sizable carrier multiplication. Moreover, the oscillation pattern and the saturation behavior of PC are manifestations of not only the chiral transport properties of carriers in the quantum Hall regime but also the chirality change at the Dirac point, a characteristic feature of a relativistic quantum Hall effect.

Graphene-Si CMOS oscillators.

Graphene field-effect transistors (GFETs) offer a possibility of exploiting unique physical properties of graphene in realizing novel electronic circuits. However, graphene circuits often lack the voltage swing and switchability of Si complementary metal-oxide-semiconductor (CMOS) circuits, which are the main building block of modern electronics. Here we introduce graphene in Si CMOS circuits to exploit favorable electronic properties of both technologies and realize a new class of simple oscillators using only a GFET, Si CMOS D latch, and timing RC circuit. The operation of the two types of realized oscillators is based on the ambipolarity of graphene, i.e., the symmetry of the transfer curve of GFETs around the Dirac point. The ambipolarity of graphene also allowed to turn the oscillators into pulse-width modulators (with a duty cycle ratio ∼1 : 4) and voltage-controlled oscillators (with a frequency ratio ∼1 : 8) without any circuit modifications. The oscillation frequency was in the range from 4 kHz to 4 MHz and limited only by the external circuit connections, rather than components themselves. The demonstrated graphene-Si CMOS hybrid circuits pave the way to the more widespread adoption of graphene in electronics.

A Graphene-Edge Ferroelectric Molecular Switch.

We show that polar molecules (water, ammonia, and nitrogen dioxide) adsorbed solely at the exposed edges of an encapsulated graphene sheet exhibit ferroelectricity, collectively orienting and switching reproducibly between two available states in response to an external electric field. This ferroelectric molecular switching introduces drastic modifications to the graphene bulk conductivity and produces a large and ambipolar charge bistability in micrometer-size graphene devices. This system comprises an experimental realization of envisioned memory capacitive ("memcapacitive") devices whose capacitance is a function of their charging history, here conceived via confined and correlated polar molecules at the one-dimensional edge of a two-dimensional crystal.

Fully inkjet-printed two-dimensional material field-effect heterojunctions for wearable and textile electronics.

Fully printed wearable electronics based on two-dimensional (2D) material heterojunction structures also known as heterostructures, such as field-effect transistors, require robust and reproducible printed multi-layer stacks consisting of active channel, dielectric and conductive contact layers. Solution processing of graphite and other layered materials provides low-cost inks enabling printed electronic devices, for example by inkjet printing. However, the limited quality of the 2D-material inks, the complexity of the layered arrangement, and the lack of a dielectric 2D-material ink able to operate at room temperature, under strain and after several washing cycles has impeded the fabrication of electronic devices on textile with fully printed 2D heterostructures. Here we demonstrate fully inkjet-printed 2D-material active heterostructures with graphene and hexagonal-boron nitride (h-BN) inks, and use them to fabricate all inkjet-printed flexible and washable field-effect transistors on textile, reaching a field-effect mobility of ~91 cm2 V-1 s-1, at low voltage (<5 V). This enables fully inkjet-printed electronic circuits, such as reprogrammable volatile memory cells, complementary inverters and OR logic gates.

High-Gain Graphene Transistors with a Thin AlOx Top-Gate Oxide.

The high-frequency performance of transistors is usually assessed by speed and gain figures of merit, such as the maximum oscillation frequency f max, cutoff frequency f T, ratio f max/f T, forward transmission coefficient S 21, and open-circuit voltage gain A v. All these figures of merit must be as large as possible for transistors to be useful in practical electronics applications. Here we demonstrate high-performance graphene field-effect transistors (GFETs) with a thin AlOx gate dielectric which outperform previous state-of-the-art GFETs: we obtained f max/f T > 3, A v > 30 dB, and S 21 = 12.5 dB (at 10 MHz and depending on the transistor geometry) from S-parameter measurements. A dc characterization of GFETs in ambient conditions reveals good current saturation and relatively large transconductance ~600 S/m. The realized GFETs offer the prospect of using graphene in a much wider range of electronic applications which require substantial gain.

On-Chip Magnetic Platform for Single-Particle Manipulation with Integrated Electrical Feedback.

Methods for the manipulation of single magnetic particles have become very interesting, in particular for in vitro biological studies. Most of these studies require an external microscope to provide the operator with feedback for controlling the particle motion, thus preventing the use of magnetic particles in high-throughput experiments. In this paper, a simple and compact system with integrated electrical feedback is presented, implementing in the very same device both the manipulation and detection of the transit of single particles. The proposed platform is based on zig-zag shaped magnetic nanostructures, where transverse magnetic domain walls are pinned at the corners and attract magnetic particles in suspension. By applying suitable external magnetic fields, the domain walls move to the nearest corner, thus causing the step by step displacement of the particles along the nanostructure. The very same structure is also employed for detecting the bead transit. Indeed, the presence of the magnetic particle in suspension over the domain wall affects the depinning field required for its displacement. This characteristic field can be monitored through anisotropic magnetoresistance measurements, thus implementing an integrated electrical feedback of the bead transit. In particular, the individual manipulation and detection of single 1-µm sized beads is demonstrated.

Suspended monolayer graphene under true uniaxial deformation.

2D crystals, such as graphene, exhibit the higher strength and stiffness of any other known man-made or natural material. So far, this assertion has been primarily based on modelling predictions and on bending experiments in combination with pertinent modelling. True uniaxial loading of suspended graphene is not easy to accomplish; however such an experiment is of paramount importance in order to assess the intrinsic properties of graphene without the influence of an underlying substrate. In this work we report on uniaxial tension of graphene up to moderate strains of ∼0.8%. This has been made possible by sandwiching the graphene flake between two polymethylmethacrylate (PMMA) layers and by suspending its central part by the removal of a section of PMMA with e-beam lithography. True uniaxial deformation is confirmed by the measured large phonon shifts with strain by Raman spectroscopy and the indication of lateral buckling (similar to what is observed for thin macroscopic membranes under tension). Finally, we also report on how the stress is transferred to the suspended specimen through the adhesive grips and determine the value of interfacial shear stress that is required for efficient axial loading in such a system.

Elastic coupling between layers in two-dimensional materials.

Two-dimensional materials, such as graphene and MoS2, are films of a few atomic layers in thickness with strong in-plane bonds and weak interactions between the layers. The in-plane elasticity has been widely studied in bending experiments where a suspended film is deformed substantially; however, little is known about the films' elastic modulus perpendicular to the planes, as the measurement of the out-of-plane elasticity of supported 2D films requires indentation depths smaller than the films' interlayer distance. Here, we report on sub-ångström-resolution indentation measurements of the perpendicular-to-the-plane elasticity of 2D materials. Our indentation data, combined with semi-analytical models and density functional theory, are then used to study the perpendicular elasticity of few-layer-thick graphene and graphene oxide films. We find that the perpendicular Young's modulus of graphene oxide films reaches a maximum when one complete water layer is intercalated between the graphitic planes. This non-destructive methodology can map interlayer coupling and intercalation in 2D films.

Scaling of graphene integrated circuits.

The influence of transistor size reduction (scaling) on the speed of realistic multi-stage integrated circuits (ICs) represents the main performance metric of a given transistor technology. Despite extensive interest in graphene electronics, scaling efforts have so far focused on individual transistors rather than multi-stage ICs. Here we study the scaling of graphene ICs based on transistors from 3.3 to 0.5 µm gate lengths and with different channel widths, access lengths, and lead thicknesses. The shortest gate delay of 31 ps per stage was obtained in sub-micron graphene ROs oscillating at 4.3 GHz, which is the highest oscillation frequency obtained in any strictly low-dimensional material to date. We also derived the fundamental Johnson limit, showing that scaled graphene ICs could be used at high frequencies in applications with small voltage swing.

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems.

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

Parallelization of thermochemical nanolithography.

One of the most pressing technological challenges in the development of next generation nanoscale devices is the rapid, parallel, precise and robust fabrication of nanostructures. Here, we demonstrate the possibility to parallelize thermochemical nanolithography (TCNL) by employing five nano-tips for the fabrication of conjugated polymer nanostructures and graphene-based nanoribbons.

Gigahertz integrated graphene ring oscillators.

Ring oscillators (ROs) are the most important class of circuits used to evaluate the performance limits of any digital technology. However, ROs based on low-dimensional nanomaterials (e.g., 1-D nanotubes, nanowires, 2-D MoS2) have so far exhibited limited performance due to low current drive or large parasitics. Here we demonstrate integrated ROs fabricated from wafer-scale graphene grown by chemical vapor deposition. The highest oscillation frequency was 1.28 GHz, while the largest output voltage swing was 0.57 V. Both values remain limited by parasitic capacitances in the circuit rather than intrinsic properties of the graphene transistor components, suggesting further improvements are possible. The fabricated ROs are the fastest realized in any low-dimensional nanomaterial to date and also the least sensitive to fluctuations in the supply voltage. They represent the first integrated graphene oscillators of any kind and can also be used in a wide range of applications in analog electronics. As a demonstration, we also realized the first stand-alone graphene mixers that do not require external oscillators for frequency conversion. The first gigahertz multitransistor graphene integrated circuits demonstrated here pave the way for application of graphene in high-speed digital and analog circuits in which high operating speed could be traded off against power consumption.

Cascading wafer-scale integrated graphene complementary inverters under ambient conditions.

The fundamental building blocks of digital electronics are logic gates which must be capable of cascading such that more complex logic functions can be realized. Here we demonstrate integrated graphene complementary inverters which operate with the same input and output voltage logic levels, thus allowing cascading. We obtain signal matching under ambient conditions with inverters fabricated from wafer-scale graphene grown by chemical vapor deposition (CVD). Monolayer graphene was incorporated in self-aligned field-effect transistors in which the top gate overlaps with the source and drain contacts. This results in full-channel gating and leads to the highest low-frequency voltage gain reported so far in top-gated CVD graphene devices operating in air ambient, A(v) ∼ -5. Such gain enabled logic inverters with the same voltage swing of 0.56 V at their input and output. Graphene inverters could find their way in realistic applications where high-speed operation is desired but power dissipation is not a concern, similar to emitter-coupled logic.

Homogeneity of Ge-rich nanostructures as characterized by chemical etching and transmission electron microscopy.

The extension of SiGe technology towards new electronic and optoelectronic applications on the Si platform requires that Ge-rich nanostructures be obtained in a well-controlled manner. Ge deposition on Si substrates usually creates SiGe nanostructures with relatively low and inhomogeneous Ge content. We have realized SiGe nanostructures with a very high (up to 90%) Ge content. Using substrate patterning, a regular array of nanostructures is obtained. We report that electron microscopy reveals an abrupt change in Ge content of about 20% between the filled pit and the island, which has not been observed in other Ge island systems. Dislocations are mainly found within the filled pit and only rarely in the island. Selective chemical etching and electron energy-loss spectroscopy reveal that the island itself is homogeneous. These Ge-rich islands are possible candidates for electronic applications requiring locally induced stress, and optoelectronic applications which exploit the Ge-like band structure of Ge-rich SiGe.

Nanolithographic templates using diblock copolymer films on chemically heterogeneous substrates.

The orientation of the lamellae formed by the phase separation of symmetric diblock copolymer thin films is strongly affected by the wetting properties of the polymer blocks with respect to the substrate. On bare silicon wafers the lamellae of polystyrene-b-polymethylmethacrylate thin films tend to order parallel to the wafer surface, with the polymethylmethacrylate block preferentially wetting silicon. We have developed a methodology for inducing the arrangement of lamellae perpendicular to the substrate by using chemically modified substrates. This is done by chemisorbing a self-assembled monolayer of thiol-terminated alkane chains on thin gold films deposited on silicon wafers. We also show that it is possible to spatially control the perpendicular orientation of the lamellae at sub-micron length scales by using simple chemical patterns and etch them, in order to produce nanolithographic templates. This method may be of great technological interest for the preparation of well-defined templates using block copolymer thin films.

Size Evolution of Ordered SiGe Islands Grown by Surface Thermal Diffusion on Pit-Patterned Si(100) Surface.

The ordered growth of self-assembled SiGe islands by surface thermal diffusion in ultra high vacuum from a lithographically etched Ge stripe on pit-patterned Si(100) surface has been experimentally investigated. The total surface coverage of Ge strongly depends on the distance from the source stripe, as quantitatively verified by Scanning Auger Microscopy. The size distribution of the islands as a function of the Ge coverage has been studied by coupling atomic force microscopy scans with Auger spectro-microscopy data. Our observations are consistent with a physical scenario where island positioning is essentially driven by energetic factors, which predominate with respect to the local kinetics of diffusion, and the growth evolution mainly depends on the local density of Ge atoms.

Vertical arrays of nanofluidic channels fabricated without nanolithography.

Vertical arrays of sealed nanofluidic channels, in which both cross-sectional dimensions are controllable down to 10 nm, were fabricated by selective side etching of a SiGe heterostructure comprised of layers of alternating Ge fractions. Capillary filling of these nanochannel arrays with fluorescent dye solutions was investigated using a confocal microscope. The feasibility of using nanochannels for size-based separation of biomolecules was demonstrated by imaging aggregates of tagged amyloid-beta peptide. The ability to integrate a large number of nanochannels shows promise for high throughput applications involving lab-on-a-chip systems.