Matthias Muoth

Atomically precise bottom-up fabrication of graphene nanoribbons

Graphene nanoribbons—narrow and straight-edged stripes of graphene, or single-layer graphite—are predicted to exhibit electronic properties that make them attractive for the fabrication of nanoscale electronic devices. In particular, although the two-dimensional parent material graphene exhibits semimetallic behaviour, quantum confinement and edge effects should render all graphene nanoribbons with widths smaller than 10 nm semiconducting. But exploring the potential of graphene nanoribbons is hampered by their limited availability: although they have been made using chemical, sonochemical and lithographic methods as well as through the unzipping of carbon nanotubes, the reliable production of graphene nanoribbons smaller than 10 nm with chemical precision remains a significant challenge. Here we report a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation. The topology, width and edge periphery of the graphene nanoribbon products are defined by the structure of the precursor monomers, which can be designed to give access to a wide range of different graphene nanoribbons. We expect that our bottom-up approach to the atomically precise fabrication of graphene nanoribbons will finally enable detailed experimental investigations of the properties of this exciting class of materials. It should even provide a route to graphene nanoribbon structures with engineered chemical and electronic properties, including the theoretically predicted intraribbon quantum dots, superlattice structures and magnetic devices based on specific graphene nanoribbon edge states.

Hysteresis-free operation of suspended carbon nanotube transistors

Single-walled carbon nanotubes offer high sensitivity and very low power consumption when used as field-effect transistors in nanosensors. Suspending nanotubes between pairs of contacts, rather than attaching them to a surface, has many advantages in chemical, optical or displacement sensing applications, as well as for resonant electromechanical systems. Suspended nanotubes can be integrated into devices after nanotube growth, but contamination caused by the accompanying additional process steps can change device properties. Ultraclean suspended nanotubes can also be grown between existing device contacts, but high growth temperatures limit the choice of metals that can be used as contacts. Moreover, when operated in ambient conditions, devices fabricated by either the post- or pre-growth approach typically exhibit gate hysteresis, which makes device behaviour less reproducible. Here, we report the operation of nanotube transistors in a humid atmosphere without hysteresis. Suspended, individual and ultraclean nanotubes are grown directly between unmetallized device contacts, onto which palladium is then evaporated through self-aligned on-chip shadow masks. This yields pairs of needle-shaped source/drain contacts that have been theoretically shown to allow high nanotube-gate coupling and low gate voltages. This process paves the way for creating ultrasensitive nanosensors based on pristine suspended nanotubes.

Narrowing SWNT diameter distribution using size-separated ferritin-based Fe catalysts.

Sensors and devices made from single-walled carbon nanotubes (SWNTs) are most often electrically probed through metal leads contacting the semiconducting SWNTs (s-SWNTs). Contact barriers in general and Schottky barriers (SBs) in particular are usually obtained at a metal-semiconductor interface. The unique one-dimensional structure (1D) of SWNTs allows tailoring of the SB heights through the contact metal type and the size of the s-SWNT bandgap. A large workfunction reduces the SB height (e.g. using Pd as the metal contact material). The bandgap of an SWNT is inversely proportional to its diameter. Ohmic contacts--the preferable choice--are achieved for s-SWNTs with diameters greater than 2 nm on Pd metal leads. SWNT device reproducibility, on the other hand, requires a narrow distribution of the SWNT diameters. Here, we present a method to fabricate SWNTs with a large and adjustable mean diameter (1.9-2.4 nm) and very narrow diameter distribution (+/- 0.27 nm at mean diameter 1.9 nm). The results are achieved through a size separation of the ferritin catalyst particles by sedimentation velocity centrifugation prior to their use in the chemical vapor deposition (CVD) formation of SWNTs.

Ultra-low power operation of self-heated, suspended carbon nanotube gas sensors

We present a suspended carbon nanotube gas sensor that senses NO2 at ambient temperature and recovers from gas exposure at an extremely low power of 2.9 μW by exploiting the self-heating effect for accelerated gas desorption. The recovery time of 10 min is two orders of magnitude faster than non-heated recovery at ambient temperature. This overcomes an important bottleneck for the practical application of carbon nanotube gas sensors. Furthermore, the method is easy to implement in sensor systems and requires no additional components, paving the way for ultra-low power, compact, and highly sensitive gas sensors.

Aqueous dispersion and dielectrophoretic assembly of individual surface-synthesized single-walled carbon nanotubes

The successful dispersion and large-scale parallel assembly of individual surface-synthesized large-diameter (1-3 nm) single-walled carbon nanotubes (SWNTs), grown by chemical vapor deposition (CVD), is demonstrated. SWNTs are removed from the growth substrate by a short, low-energy ultrasonic pulse to produce ultrapure long-term stable surfactant-stabilized solutions. Subsequent dielectrophoretic deposition bridges individual, straight, and long SWNTs between two electrodes. Electrical characterization on 223 low-resistance devices (R(average) approximately 200 kOmega) evidences the high quality of the SWNT raw material, prepared solution, and contact interface. The research reported herein provides an important framework for the large-scale industrial integration of carbon nanotube-based devices, sensors, and applications.

Suspended CNT-FET piezoresistive strain gauges: Chirality assignment and quantitative analysis

Carbon nanotubes can withstand large elastic deformation and show pronounced piezoresistive effects which make them promising candidates for low-power strain sensors in the large strain regime. Integration of individual suspended nanotubes into complex microelectromechanical devices, including gate structures, is still a challenge. Here, ultraclean carbon nanotubes spanning across actuated MEMS electrodes are operated as suspended Carbon Nanotube Field-Effect Transistors (CNT-FETs) under repeated variable strain up to 4.5%, thus acting as small-sized piezoresistive strain gauges whose gauge factor is electrostatically tuned by the gate. We present the first quantified electromechanical analysis of suspended CNT-FETs to uniaxial strain applied by on-chip micro actuators.

Transfer of carbon nanotubes onto microactuators for hysteresis-free transistors at low thermal budget

A dry transfer process for single-walled carbon nanotubes allows for suspended, ultraclean, and hysteresis-free nanotube field-effect transistors on micro-actuated electrodes without exposing the device die to elevated nanotube growth temperatures. Nanotubes are grown on a separate die, between the arms of a fork structure, and are subsequently transferred onto receiving electrodes of a device die. The device die is maintained at room-temperature; and thus, it can in principle contain temperature-sensitive readout circuitry. The liquid-free, room-temperature transfer is performed under light microscopy observation, while placement is detected by monitoring the current through the device electrodes. In contrast to manipulation under electron beam observation, electron-beam-induced carbonaceous contamination is prevented. Moreover, pre-selection of nanotubes by Raman spectroscopy is rendered possible, avoiding any contamination of the nanomaterial during selection and integration into NEMS.

Advances in NO2 sensing with individual single-walled carbon nanotube transistors

Summary The charge carrier transport in carbon nanotubes is highly sensitive to certain molecules attached to their surface. This property has generated interest for their application in sensing gases, chemicals and biomolecules. With over a decade of research, a clearer picture of the interactions between the carbon nanotube and its surroundings has been achieved. In this review, we intend to summarize the current knowledge on this topic, focusing not only on the effect of adsorbates but also the effect of dielectric charge traps on the electrical transport in single-walled carbon nanotube transistors that are to be used in sensing applications. Recently, contact-passivated, open-channel individual single-walled carbon nanotube field-effect transistors have been shown to be operational at room temperature with ultra-low power consumption. Sensor recovery within minutes through UV illumination or self-heating has been shown. Improvements in fabrication processes aimed at reducing the impact of charge traps have reduced the hysteresis, drift and low-frequency noise in carbon nanotube transistors. While open challenges such as large-scale fabrication, selectivity tuning and noise reduction still remain, these results demonstrate considerable progress in transforming the promise of carbon nanotube properties into functional ultra-low power, highly sensitive gas sensors.

Clean carbon nanotubes coupled to superconducting impedance-matching circuits

Coupling carbon nanotube devices to microwave circuits offers a significant increase in bandwidth (BW) and signal-to-noise ratio. These facilitate fast non-invasive readouts important for quantum information processing, shot noise and correlation measurements. However, creation of a device that unites a low-disorder nanotube with a low-loss microwave resonator has so far remained a challenge, due to fabrication incompatibility of one with the other. Employing a mechanical transfer method, we successfully couple a nanotube to a gigahertz superconducting matching circuit and thereby retain pristine transport characteristics such as the control over formation of, and coupling strengths between, the quantum dots. Resonance response to changes in conductance and susceptance further enables quantitative parameter extraction. The achieved near matching is a step forward promising high-BW noise correlation measurements on high impedance devices such as quantum dot circuits.

Integration of clamped-clamped suspended single-walled carbon nanotubes into SOI MEMS

Processes aiming at integrating suspended single-walled carbon nanotubes (SWNTs) into Micro-Electromechanical Systems (MEMS) have to deal explicitly with the fragility of SWNTs. In general, SWNT growth is delayed to the last step of a process so as to avoid any contamination and physical damage, which restricts the flexibility of an integration process. Here, we present a novel integration process involving a sacrificial bridge structure that provides SWNTs with physical support throughout the process. Thus, post-growth wet processes are enabled, and mechanical rupturing is prevented. With this novel approach, clamped-clamped suspended SWNTs with resistances down to 115 kΩ (with 2 µm suspended segment) are integrated into SOI micro-structures.