Whenever a fundamental new principle of science is discovered, the chances are that sooner or later a way will be found to use it for a new technology. The quantum mechanical principles of superposition and entanglement, identified nearly a century ago, are now understood to offer spectacular potential for technological applications. Superposition describes how an object can be in two states at once, as it were "here" and "there" at the same time. Two or more objects in superposition states can be entangled, so that measurements on each of them are correlated in a way that goes beyond anything we would expect from everyday intuition. Exploiting these effects in practical devices would provide new capabilities for fields such as molecular light harvesting and for molecular quantum technologies such as sensors, simulators, and quantum computers.
Successful laboratory experiments have shown that molecules of various kinds can exhibit these crucial quantum properties. Molecules are composed of electrons and atomic cores or "nuclei". Both electrons and nuclei can have a property called spin associated with them that makes them behave like tiny bar magnets. We have confirmed that electron and nuclear spins can be put into superpositions or entangled, and they can last for a long time in that condition. Most of the experiments so far have been in small test tubes. The crucial step now is to implement the same effects in nanometre scale electrical devices, such as single electron transistors consisting of single sheets of carbon rolled up as nanotubes or flat as sheets of graphene. By making hybrid technologies that combine molecules with nanoelectronics, we will lay the foundation for scaling up to more complex systems.
At this very small size, different atoms or molecules in different places affect the behaviour of the device. A breakthrough in the past few years enables us to see the positions of individual atoms in the materials which we want to use in our devices. The technique is aberration-corrected electron microscopy, and provided the electrons are not too energetic it is possible to look at the structures which we have made without damaging them. In this way we shall be able to relate the device performance to the atomic resolution microscopy of the component materials. Individual components of this vision include the following.
design the devices to build, based on a deep understanding of how to control their quantum states;
produce the materials, such as molecules with suitable spin states with carbon nanotubes and graphene for electrical substrates;
make nanoscale devices and examine them in a microscope to see where the individual atoms and molecules are;
perform the experiments to develop the quantum control and measurement for the effects which we aim to exploit;
undertake theoretical modelling to understand the electron behaviour and to design new materials systems for improved performance.
Large amplitude charge noise and random telegraph fluctuations in room-temperature graphene single-electron transistors.
We analyze the noise in liquid-gated, room temperature, graphene quantum dots. These devices display extremely large noise amplitudes. The observed noise is explained in terms of a charge noise model by considering fluctuations in the applied source-drain and gate potentials. We show that the liquid environment and substrate have little effect on the observed noise and as such attribute the noise to charge trapping/detrapping at the disordered graphene edges. The trapping/detrapping of individual charges can be tuned by gating the device, which can result in stable two-level fluctuations in the measured current. These results have important implications for the use of electronic graphene nanodevices in single-molecule biosensing.
A coherent nanomechanical oscillator driven by single-electron tunnelling
A single-electron transistor incorporated as part of a nanomechanical
resonator represents an extreme limit of electron-phonon coupling. While it
allows for fast and sensitive electromechanical measurements, it also
introduces backaction forces from electron tunnelling which randomly perturb
the mechanical state. Despite the stochastic nature of this backaction, under
conditions of strong coupling it is predicted to create self-sustaining
coherent mechanical oscillations. Here, we verify this prediction using
time-resolved measurements of a vibrating carbon nanotube transistor. This
electromechanical oscillator has intriguing similarities with a laser. The
single-electron transistor, pumped by an electrical bias, acts as a gain medium
while the resonator acts as a phonon cavity. Despite the unconventional
operating principle, which does not involve stimulated emission, we confirm
that the output is coherent, and demonstrate other laser behaviour including
injection locking and frequency narrowing through feedback.
Publisher Correction: Efficiently measuring a quantum device using machine learning (npj Quantum Information, (2019), 5, 1, (79), 10.1038/s41534-019-0193-4)
Understanding resonant charge transport through weakly coupled single-molecule junctions
Off-resonant charge transport through molecular junctions has been
extensively studied since the advent of single-molecule electronics and it is
now well understood within the framework of the non-interacting Landauer
approach. Conversely, gaining a qualitative and quantitative understanding of
the resonant transport regime has proven more elusive. Here, we study resonant
charge transport through graphene-based zinc-porphyrin junctions. We
experimentally demonstrate an inadequacy of the non-interacting Landauer theory
as well as the conventional single-mode Franck-Condon model. Instead, we model
the overall charge transport as a sequence of non-adiabatic electron transfers,
the rates of which depend on both outer and inner-sphere vibrational
interactions. We show that the transport properties of our molecular junctions
are determined by a combination of electron-electron and electron-vibrational
coupling, and are sensitive to the interactions with the wider local
environment. Furthermore, we assess the importance of nuclear tunnelling and
examine the suitability of semi-classical Marcus theory as a description of
charge transport in molecular devices.
Charge-state assignment of nanoscale single-electron transistors from their current-voltage characteristics.
The electronic and magnetic properties of single-molecule transistors depend critically on the molecular charge state. Charge transport in single-molecule transistors is characterized by Coulomb-blocked regions in which the charge state of the molecule is fixed and current is suppressed, separated by high-conductance, sequential-tunneling regions. It is often difficult to assign the charge state of the molecular species in each Coulomb-blocked region due to variability in the work-function of the electrodes. In this work, we provide a simple and fast method to assign the charge state of the molecular species in the Coulomb-blocked regions based on signatures of electron-phonon coupling together with the Pauli-exclusion principle, simply by observing the asymmetry in the current in high-conductance regions of the stability diagram. We demonstrate that charge-state assignments determined in this way are consistent with those obtained from measurements of Zeeman splittings. Our method is applicable at 77 K, in contrast to magnetic-field-dependent measurements, which generally require low temperatures (below 4 K). Due to the ubiquity of electron-phonon coupling in molecular junctions, we expect this method to be widely applicable to single-electron transistors based on single molecules and graphene quantum dots. The correct assignment of charge states allows researchers to better understand the fundamental charge-transport properties of single-molecule transistors.
Metal Atom Markers for Imaging Epitaxial Molecular Self-Assembly on Graphene by Scanning Transmission Electron Microscopy.
Direct imaging of single molecules has to date been primarily achieved using scanning probe microscopy, with limited success using transmission electron microscopy due to electron beam damage and low contrast from the light elements that make up the majority of molecules. Here, we show single complex molecule interactions can be imaged using annular dark field scanning TEM (ADF-STEM) by inserting heavy metal markers of Pt atoms and detecting their positions. Using the high angle ADF-STEM Z1.7 contrast, combined with graphene as an electron transparent support, we track the 2D monolayer self-assembly of solution-deposited individual linear porphyrin hexamer (Pt-L6) molecules and reveal preferential alignment along the graphene zigzag direction. The epitaxial interactions between graphene and Pt-L6 drive a reduction in the interporphyrin distance to allow perfect commensuration with the graphene. These results demonstrate how single metal atom markers in complex molecules can be used to study large scale packing and chain bending at the single molecule level.
Atomic Scale Imaging of Reversible Ring Cyclization in Graphene Nanoconstrictions.
We present an atomic level study of reversible cyclization processes in suspended nanoconstricted regions of graphene that form linear carbon chains (LCCs). Before the nanoconstricted region reaches a single linear carbon chain (SLCC), we observe that a double linear carbon chain (DLCC) structure often reverts back to a ribbon of sp2 hybridized oligoacene rings, in a process akin to the Bergman rearrangement. When the length of the DLCC system only consists of ∼5 atoms in each LCC, full recyclization occurs for all atoms present, but for longer DLCCs we find that only single sections of the chain are modified in their bonding hybridization and no full ring closure occurs along the entire DLCCs. This process is observed in real time using aberration-corrected transmission electron microscopy and simulated using density functional theory and tight binding molecular dynamics calculations. These results show that DLCCs are highly sensitive to the adsorption of local gas molecules or surface diffusion impurities and undergo structural modifications.
Physical Review B - Condensed Matter and Materials Physics
Shear alignment of fullerenes in nanotubular supramolecular complexes