Single Molecular Science Laboratory


Studying the electron transport properties at the electrode|molecule interface under the influence of external stimuli like electrochemical potential, magnetic field, light, temperature will have a variety of applications in several research fields like:
(i)  Molecular electronics and spintronics: Molecule based electronic technologies (like OLEDs) and other futuristic molecular electronics applications (like flexible and transparent electronic devices, artificial electronic skins etc), where electrode|molecule interface plays a crucial role in determining performance of the device.
(ii) Energy research: In solar cells, the charge or electron transfer at electrode|molecule interface plays a crucial role in determining efficiency. (iii) Electrocatalysis and photoelectrocatalysis: Activity/selectivity of a catalyst depends on the effectiveness of charge transport at electrode|catalyst interface under the influence of electrochemical potential and/or light.
(iv)  Sensors: Sensitivity and selectivity of a sensor may depend on the effective charge transport at electrode|molecule interface. Thus, the ability to measure and control charge transport across metal|molecule interface is of considerable fundamental interest. ​In our group, we focus on developing the instrumentation and experimental methodology to study the charge/heat transport properties at the electrode|molecule interface (at a single molecule or ensemble level) under the influence of external stimuli.  
We are aiming at constructing a detailed knowledge base of the physico-chemical properties of molecular junctions and the mechanistic insights behind the single (or ensemble) charge/heat transport. Such knowledge is the key to design the next generation of hybrid electronic high-performance devices for a wide variety of applications. Such knowledge is strongly desirable because of the imminent potential of single (or ensemble) molecular junctions as the prototype building block for functional materials and molecular electronic devices. The aim of our research work is to contribute to that knowledge.  

By studying the electronic properties at various level of complexity from single molecule to single cell, Structure property relationships can be established. Using these established structure property relations, we will finally explore the ways in which these relations can be translated to the real-world applications and will help to solve the unwound mysteries of the nature. We are trying to reveal how atomic-detailed nano-scaled structural modification in materials can affect the performance, energy consumption and longevity of molecular-scale devices. In terms of the practical point of view, our research holds a great the promise of developing flexible, lightweight, solution-processable, and low-cost molecular devices.
Measuring the charge transport/heat transport through a single molecule at room temperature under ambient conditions poses various experimental challenges. This needs a sophisticated instrumentation capable of trapping the molecule between two electrodes and simultaneously measuring the current/heat passing through this junction with very high accuracy and precision.  Since the molecule trapped between two electrodes at ambient conditions are subjected to thermal vibrations, as the result the lifetime of the metal|molecule| metal junction is of the order of 10-100s of milli-seconds. Any external mechanical instability will further decrease the stability of the metal|molecule|metal junction at the nanoscale makes it impossible to successfully measure the electronic/heat transport through the junctions. Thus, in order to probe the electronic/heat transport through the single molecular junction, a highly stable, robust and high frequency/sampling measurement system is required. We have designed and built a Mechanically controlled break junction (MCBJ) and scanning tunneling microscope break junction setups (STM-BJ) which provide this experimental capability.

1.Mechanically controlled break junction setup: In mechanically controlled break junction experiment, a nanosized gap is created between two gold electrodes by pushing the spring steel substrate from below on which gold wire notched in the middle is mounted (fixed) at two positions by epoxy resin. The nano resolution pushing of the spring steel is done with the help of suitable piezo stack which expands upon applying positive potential and contracts upon applying negative potential. The coarse movement is done with the help of Stepper motor on the axel of with piezo stack is attached for fine movement. Upon pushing the spring steel from below the attached gold wire (0.1 mm) breaks and an adjustable nanosized gap is created. When the nano gap is in comparison of the target molecule length, the anchoring groups of molecules (like thiol, pyridine etc.) attaches to gold electrodes. Upon more pushing the steel substrate the gap is enlarged as a result of which the molecule detaches from the electrode(s) and the junction is broken. Using current as feedback the instrument senses the broken junction and starts contracting the piezo. As a result, the gap reduces again, and the molecule is again trapped. Upon further decreasing the nanogap between two electrodes eventually, the 2 electrodes touch each other, and a short circuit current is measured. Then the opening cycle is repeated. These opening and closing cycles are repeated thousands of times to get statistically significant data.  The speed of opening and closing the gap and the frequency of data collection of current upon applying potential across the gold electrodes is controlled by careful movement of piezo stack and frequency adjustment in data collection system. Modular design of our setup allows us to couple our MCBJ with other instruments like Raman microscope to probe the spectroscopic signatures of molecular junctions and also we can incorporate external light source or magnetic field or temperature to stimulate/tune the charge transport properties.   

2.Scanning tunneling microscope break junction setup: In this setup a nanogap is created between a bottom flat electrode and top thin wire electrode (with single atom thick tip).  The movement of the top electrode is controlled by the piezo stack and stepper motor combination. Using this setup we can perform the single molecular conductance measurements under electrochemical conditions. Additionally, this setup also allows the heating of bottom electrode to create a temperature gradient across the molecular junction to measure the thermopower generated by single molecules attached to two electrodes at different temperatures. 

3.EGaIn based thermo-electric set-up: This setup is capable of simultaneous measurement of electric conductance and Thermoelectric power measurement of SAM based junctions. The top Electrode is based on non-Newtonian liquid metal, an Eutatic mixture(75.5% Ga and 24.5% In by weight) of Gallium and Indium(EGaIn), covered with a conductive, self-passivating Ga2O3 layer (nominal thickness of ∼1 nm). The EGaIn conical tip enables reproducible, noninvasive, and well defined top contacts on delicate molecular thin films such as self-assembled monolayers (SAMs) under ambient conditions. The top electrode is obtained as a hanging EGaIn conical tip from a micro-syringe containing bulk mixture. Template stripped Au/Ag(111) Surface  act as bottom electrode, on which self-assembled monolayer is deposited. Using this setup we explore the transport through ensemble molecular junctions and these experiments are complementary to our single molecular work. 

4. Atomic force microscopy: We also have a Bruker multi-mode STM/AFM setup. We use this setup to study morphology and electromechanical properties of molecular thin films. We are also developing electrochemical conducting probe AFM, to study the electromechanical properties of nanoscale systems with relevance to energy research.