The main scope of my research in the past twelve years has been studying electronic transport in conjugated molecular wires and DNA molecules using the mechanically controllable break junction (MCBJ) technique. I measure fundamental properties of single molecule devices, seeking to understand the interplay of physics, chemistry and engineering at the nanometer scale. The underlying focus of my research is to fabricate single molecule or a few molecule circuits, a molecule attached to two electrodes, with varied functionality, where the circuit structure is defined with atomic precision. I measure how electronic transport in these devices relate to the molecular structure, what is the influence of the metal contacts and linking bonds, and what is the influence of external stimuli such as light and gate to the electronic transport. These experiments provide a deeper understanding of the fundamental physics of electron transport, while laying the groundwork for technological advances at the nanometer scale. I joined the field of the single molecule electronics at its very beginning and was one of the first to use MCBJ junction technique for studying the charge transport in conjugated functional molecular wires.

 

Mechanically Controllable Break Junction Technique (MCBJ)

The MCBJ technique is an elegant way to control the spacing between two metallic electrodes with subatomic (10 picometer) resolution. The layout of the MCBJ technique is schematically presented in Fig. 2. The base is a phosphor bronze substrate, which has a certain degree of flexibility. On top of it a thin insulating layer (polyimide) is fabricated. After this a thin gold layer (80 nm) is deposited. The gold film is patterned with electron beam lithography to form a very narrow construction with a neck of about 100 nm. At the end the polyimide film is under-etched to produce a free hanging construction. Then the substrate is bent, by means of a three-point bending mechanism which can be controlled with micrometer precision. Due to the large mechanical "attenuation factor" the electrode spacing, therefore, can be controlled with a precision comparable to, or even better than that of STM.

MCBJ technique: a) Artistic impression of a single molecule three – terminal device. b) Lay-out of the mechanically controllable break junction (MCBJ) set up.   c) Micrograph of a three –terminal MCBJ device. d) Micrograph of a two –terminal MCBJ device.

Upon bending of the substrate, a nanosized gap is formed between the electrodes, which can be mechanically adjusted and which is impressively stable of the order of several hours, even at room temperature. The layout of the technique is presented in Figure b.
The next step is the assembly of the molecules on the electrodes. This can be done by dissolving the molecules in an appropriate solvent. The thiol end groups of the molecules will now attach to the electrodes, and will form a self-assembled monolayer. The junctions are opened in vacuum and all experiments are preformed in vacuum at ambient or cryogenic temperatures (6K).
In contrast to other experimental methods for studying the charge transport on a single molecular scale, MCBJ technique allows us to study, both, the low-bias conductance as a function of the electrode displacement (similar to STM break junction technique) and perform current–voltage (I-V) measurements at different electrode spacings in order to gain spectroscopic information in the wide voltage-bias regime. In addition we can fabricate three – terminal devices with an additional gate electrode  (c) which allows electrostatic tuning of the energy levels of the molecular junction (similar to planar three-terminal device).
This unique combination is essential for studying the charge transport in structurally more complicated, but more interesting - functional molecules, such as porphyrines.

 

Diamond based molecular electronic devices
The goal of this research is to investigate and implement new top-down diamond based nanostrutures with bottom-up molecular system for future non-conventional solution in opto-electronics. Due to its intrinsic high carrier mobility, largest thermal conductivity of any material, and wide band gap diamond can potentially deliver outstanding performance when used as the basis for high-frequency, high–power and high-temperature electronic devices. Here, we will exploit some of these outstanding properties to develop alternative routes in molecular electronics. The advantage of using transparent diamond nanoelectrodes combining with photosensitive molecular systems is to allow both electrical and optical manipulation. In addition, the electron affinity of diamond can be tuned by controlling its surface termination and organic molecules can be attached via chemically stable C-C bonding, which are advantageous for efficient charge transfer and robust applications.  Among the appealing aspects one is the incorporation of single nitrogen vacancy (NV) color center between conducting or semiconducting diamond nanoelectrodes in order to tune changes in the charge transport upon the attachment of a molecule.