Markus Brink

Tuning carbon nanotube band gaps with strain.

We show that the band structure of a carbon nanotube (NT) can be dramatically altered by mechanical strain. We employ an atomic force microscope tip to simultaneously vary the NT strain and to electrostatically gate the tube. We show that strain can open a band gap in a metallic NT and modify the band gap in a semiconducting NT. Theoretical work predicts that band gap changes can range between +/-100 meV per 1% stretch, depending on NT chirality, and our measurements are consistent with this predicted range.

Electrical Nanoprobing of Semiconducting Carbon Nanotubes Using an Atomic Force Microscope

We use an atomic force microscope (AFM) tip to locally probe the electronic properties of semiconducting carbon nanotube transistors. A gold-coated AFM tip serves as a voltage or current probe in three-probe measurement setup. Using the tip as a movable current probe, we investigate the scaling of the device properties with channel length. Using the tip as a voltage probe, we study the properties of the contacts. We find that Au makes an excellent contact in the p region, with no Schottky barrier. In the n region, large contact resistances were found which dominate the transport properties.

Assessment of silicon MOS and carbon nanotube FET performance limits using a general theory of ballistic transistors

A simple model for ballistic nanotransistors, which extends previous work by treating both the charge control and the quantum capacitance limits of MOSFET-like transistors, is presented. We apply this new model to MOSFET-like carbon nanotube FETs (CNTFETs) and to MOSFETs at the scaling limit. The device physics for operation at ballistic and quantum capacitance limits are explored. Based on the analysis of recently reported CNTFETs, we compare CNTFETs to MOSFETs. The potential performance advantages over Si that might be achieved at the scaling limit are established by using the new model.

Electrical cutting and nicking of carbon nanotubes using an atomic force microscope

In this letter, we demonstrate that voltage pulses from a metal-coated AFM tip can be used to permanently modify the electrical properties of NT devices. By adjusting the properties of the voltage pulses, we can either electrically break ~‘‘cut’’ ! NTs or create tunneling barriers ~‘‘nick’’ ! at any point along them. We demonstrate the utility of these techniques by creating single NT devices through the cutting of unwanted extra NTs, and making ultrasmall NT quantum dots can be created by nicking a NT at two places along its length. The NT devices used in this work were prepared following an approach similar to that of Kong et al. 21 First, catalyst islands containing Fe(NO 3 ) 3 i9H 2 O, MoO 2 (acac) 2 and alumina nanoparticles were defined on a degenerately doped silicon wafer with 200-nm-thick thermally grown oxide. Photolithography and etching were used to pattern a poly~methylmethacrylate! layer, which was subsequently used as a lift-off mask for the catalyst. NTs were then grown by chemical vapor deposition. 21 Metal electrodes consisting of Cr~ 5n m! and Au~50 nm! were patterned over the catalyst islands using photolithography and a lift-off process, with a spacing between source and drain electrodes between 1 and 3 mm. This process allowed the parallel production of hundreds of NT devices using only optical lithography. The samples were annealed at 600 °C for 45 min in an Ar environment to decrease the contact resistance between the NTs and the electrodes. Typical NT conductances obtained were 0.2‐ 2e 2 /h.

Hardware-efficient variational quantum eigensolver for small molecules and quantum magnets

Quantum computers can be used to address electronic-structure problems and problems in materials science and condensed matter physics that can be formulated as interacting fermionic problems, problems which stretch the limits of existing high-performance computers. Finding exact solutions to such problems numerically has a computational cost that scales exponentially with the size of the system, and Monte Carlo methods are unsuitable owing to the fermionic sign problem. These limitations of classical computational methods have made solving even few-atom electronic-structure problems interesting for implementation using medium-sized quantum computers. Yet experimental implementations have so far been restricted to molecules involving only hydrogen and helium. Here we demonstrate the experimental optimization of Hamiltonian problems with up to six qubits and more than one hundred Pauli terms, determining the ground-state energy for molecules of increasing size, up to BeH2. We achieve this result by using a variational quantum eigenvalue solver (eigensolver) with efficiently prepared trial states that are tailored specifically to the interactions that are available in our quantum processor, combined with a compact encoding of fermionic Hamiltonians and a robust stochastic optimization routine. We demonstrate the flexibility of our approach by applying it to a problem of quantum magnetism, an antiferromagnetic Heisenberg model in an external magnetic field. In all cases, we find agreement between our experiments and numerical simulations using a model of the device with noise. Our results help to elucidate the requirements for scaling the method to larger systems and for bridging the gap between key problems in high-performance computing and their implementation on quantum hardware.

Demonstration of Weight-Four Parity Measurements in the Surface Code Architecture

We present parity measurements on a five-qubit lattice with connectivity amenable to the surface code quantum error correction architecture. Using all-microwave controls of superconducting qubits coupled via resonators, we encode the parities of four data qubit states in either the X or the Z basis. Given the connectivity of the lattice, we perform a full characterization of the static Z interactions within the set of five qubits, as well as dynamical Z interactions brought along by single- and two-qubit microwave drives. The parity measurements are significantly improved by modifying the microwave two-qubit gates to dynamically remove nonideal Z errors.

Density scaling with gate-all-around silicon nanowire MOSFETs for the 10 nm node and beyond

We present results from gate-all-around (GAA) silicon nanowire (SiNW) MOSFETs fabricated using a process flow capable of achieving a nanowire pitch of 30 nm and a scaled gate pitch of 60 nm. We demonstrate for the first time that GAA SiNW devices can be integrated to density targets commensurate with CMOS scaling needs of the 10 nm node and beyond. In addition, this work achieves the highest performance for GAA SiNW NFETs at a gate pitch below 100 nm.

Single-electron force readout of nanoparticle electrometers attached to carbon nanotubes.

We introduce a new technique of probing the local potential inside a nanostructure employing Au nanoparticles as electrometers and using single-electron force microscopy to sense the charge states of the Au electrometers, which are sensitive to local potential variations. The Au nanoelectrometers are weakly coupled to a carbon nanotube through high-impedance molecular junctions. We demonstrate the operation of the Au nanoelectrometer, determine the impedance of the molecular junctions, and measure the local potential profile in a looped nanotube.

Frequency shift imaging of quantum dots with single-electron resolution

We employ atomic force microscope-based frequency shift microscopy to study the electronic properties of quantum dots formed in carbon nanotubes. The nontransport detection scheme of frequency shift allows us to probe nearly isolated quantum dots in a few electron regime. At 4K, we observe Coulomb oscillations of quantum dots with single-electron resolution and extract the charging energy of a quantum dot.

Wafer-scale integration of sacrificial nanofluidic chips for detecting and manipulating single DNA molecules

Wafer-scale fabrication of complex nanofluidic systems with integrated electronics is essential to realizing ubiquitous, compact, reliable, high-sensitivity and low-cost biomolecular sensors. Here we report a scalable fabrication strategy capable of producing nanofluidic chips with complex designs and down to single-digit nanometre dimensions over 200 mm wafer scale. Compatible with semiconductor industry standard complementary metal-oxide semiconductor logic circuit fabrication processes, this strategy extracts a patterned sacrificial silicon layer through hundreds of millions of nanoscale vent holes on each chip by gas-phase Xenon difluoride etching. Using single-molecule fluorescence imaging, we demonstrate these sacrificial nanofluidic chips can function to controllably and completely stretch lambda DNA in a two-dimensional nanofluidic network comprising channels and pillars. The flexible nanofluidic structure design, wafer-scale fabrication, single-digit nanometre channels, reliable fluidic sealing and low thermal budget make our strategy a potentially universal approach to integrating functional planar nanofluidic systems with logic circuits for lab-on-a-chip applications.