We engineer electrical, vibrational, magnetic and optical properties of nanoscale molecular devices for various applications such as molecular electronic building blocks, sensors, molecular spintronics, thermoelectric, piezoelectric and optoelectronic devices.
Overview of Research Topics
- Nanoscale materials modelling and discovery i.e. Quantum, phonon and spin transport; Environmental effects
- Molecular electronics i.e. Quantum and phonon interference; Energy harvesting e.g. Thermoelectricity and Piezoelectricity; Biological sensing; Spintronic; Optoelectronics; Molecular electronic building blocks i.e. switches, transistors and rectifiers
- One and two dimensional materials i.e. Van-der Waals heterostructures; Graphene electrodes; Molecular graphene nanoribbons
- Multiscale modelling i.e. Density functional theory; Tight-binding modelling; Molecular dynamics; Quantum transport
- GOLLUM: We co-develop the next generation quantum transport simulation tool: GOLLUM
Nanoscale materials modelling and discovery
Quantum, phonon and spin transport
Any nanoscale device consists of two or more electrodes (leads) connected to a scattering region. The electrodes are perfect waveguides where electrons and phonons can transmit without any scattering. The main scattering occurs either at the junction to the leads or inside the scattering region.
The goal is to understand the electrical. vibrational and magnetic properties of nano and molecular junctions where a nanoscale scatter or a molecule is located between electrodes. There are different approaches to study these though, our main focus is on "scattering theory" and "master equation" approaches.
The electron, phonon and spin transport in a nanoscale junction could also be investigated in the absence or presence of surroundings, such as an electric field (gate and bias voltages or local charge), a magnetic field, a laser beam or a molecular environment (water, solvent, gases, biological spices, donors and acceptors, etc).
We co-develop the next generation quantum transport simulation tool: GOLLUM which is a program that computes the charge, spin and thermal transport properties of multi-terminal nano-scale junctions. For more information, visit GOLLUM page.
Quantum and phonon interference
Electrons and phonons (i.e. vibrations due to heat) both behave quantum-mechanically like waves and so they can exhibit interference. When a single molecule is attached to metallic electrodes, de Broglie waves of electrons entering the molecule from one electrode and leaving from the other form a complex interference pattern inside the molecule. These patterns called "quantum interference" could be utilized to optimize the single-molecule device performance.
It turns out that constructive or destructive interference of electrons and phonons within individual organic molecules can be engineered precisely by carefully selecting the connection of the molecule to external electrodes and the addition of various atomic groups to the molecule.
For many years, the attraction of the single-molecule electronics has stemmed from their potential for sub-10nm electronic switches and rectifiers and from their provision of sensitive platforms for single-molecule sensing. In the recent years, their potential for removing heat from nanoelectronic devices (thermal management) and thermoelectrically converting waste heat into electricity has also been recognized.
The efficiency of a thermoelectric device for power generation is characterized by the dimensionless figure of merit ZT = GS^2T/κ, where G is the electrical conductance, S is the thermopower (Seebeck coefficient), T is temperature, and κ is the thermal conductance. Therefore, low-κ materials are needed for efficient conversion of heat into electricity, whereas materials with high κ are needed for thermal management.
Inorganic materials for thermoelectricity have been extensively studied and have delivered ZT values as high as 2.2 at temperatures over 900 K. However, this level of efficiency does not meet the requirements of current energy demands, and furthermore, the materials are difficult to process and have limited global supply.
Organic thermoelectric materials may be an attractive alternative, but at present the best organic thermoelectric material with a ZT of 0.6 in room temperature is still not competitive with inorganics. In an effort to overcome these limitations, single organic molecules and self-assembled monolayers have attracted recent scientific interest, both for their potential as room temperature thermoelectric materials and for thermal management.
One implementation of this approach is based on measurement of the variation in the ionic current through a solid-state or biological nanopore, due to the translocation of a DNA strand through the pore. However, the current leakage through such pores, low signal-to-noise ratios, and poor control of the speed of the strand through the pore create significant obstacles. To overcome the key technical problems, we study an alternative strategy that involves measuring changes in the electrical conductance of the membrane e.g. containing the pore, rather than variations in an ionic current passing through the pore.
Coherent manipulation of electron spins is essential for quantum and neuromorphic computing and data storage and transfer. We showed recently that the spin coherence time in the range of microseconds at room temperature is possible in bottom up molecular nanoribbons with well-defined zigzag edges decorated with organic radical molecules that bear electron spins. Our focus is on studying spin manipulation in magnetic molecules and molecular nanoribbons and the interplay between their transport and magnetic properties.
One of the main focuses of nanoscale electronics is on switching electrical conductance by an external stimulus such as an external electric field, redox chemistry, or light. In the latter, the electrical current is switched on or off in photochromic molecules in the presence of light or a change in its intensity.
In Piezoelectric molecules, electrical current is generated due to the deformation of molecule. This was observed recently in 4,16-dibromo[2.2]paracyclophane and heptahelicene derived molecules. To enhance the Piezoelectric response, molecules with low conductance and a high dipole moment are required.
One and two dimensional materials
Two dimensional materials such as graphene, molybdenum disulphide (MoS2), phosphorene, silicene and other transition metal dichalcogenides have attracted considerable scientific interest in the past couple of years due to their fascinating transport properties in both in-plane as well as stacked configurations (cross plane van der Waals structures). In nanoscale transport group, we study both in-plane and cross plane transport to explore novel physical phenomena induced by quantum coherent electronic states in these low dimensional systems and application based on novel functionalities.
Bottom-up graphene nanoribbons
Fabrication of novel nanometers long electronic devices with single-atom level precision has been recently possible in chemically-synthesised bottom up molecular graphene nanoribbons (MGNRs). It has been predicted that MGNRs should display half-metallicity, magnetic and topological effects solely when their edges are shaped with molecular precision. Although MGNRs now provide the necessary structural control, but their potential for quantum electronics remains unexplored. In nanoscale transport group, we exploit MGNRs as building blocks for nanoelectronic applications.
For the purpose of attaching molecules to such metallic electrodes, a variety of anchor groups have been explored. These fundamental studies have demonstrated clear correlations between molecular structure and function but are not scalable, not CMOS compatible, and in many cases do not allow gating by a nearby electrostatic gate. Silicon-based platforms have been proposed but so far such technologies remain in their infancy. To overcome some of these limitations, strategies for contacting single molecules based on carbon nanotubes and graphene have been developed. In particular, electroburnt graphene junctions have been shown to deliver stable electrode gaps below 5 nm, which allow electrostatic gating through buried or side gates. Recently graphene-based molecular junctions were used to realize a stable and reversible photoswitch and to study quantum interference, electron transport, vibrational properties, and molecular magnetism in single molecules. However, compared with metal/molecule/metal junctions, graphene-based molecular electronics is still in an early phase of development. In nanoscale transport group, we study the potential of graphene electrodes to form reproducible, stable and reliable junctions.