Using a graphene nanoribbon suspended over a trench, a team of researchers at the University of Science and Technology of China has discovered a link between the nanomechanical motion and conductance through a single-electron transistor (SET). This nano-electromechanical system (NEMS) exploits a gold gating contact in the bottom of the trench that is used to actuate, or trigger, the nanoribbon and to explore its energy landscape.
NEMS devices made from two-dimensional (2D) materials are rapidly becoming a key area of focus for the research community. Simply put, they are devices that combine electrical and mechanical phenomena at the nanoscale. The potential capabilities are almost endless, and graphene is a prime candidate because it has the second highest Young’s modulus ever observed. Moreover, graphene has an intrinsic strength five times that of steel, all the while being a fraction of the density.
The small width of the graphene ribbon means there is a reduced number of states available for electron conduction, and so it acts as a quantum dot when a voltage is applied between its ends. Typically, quantum dots exhibit sharp peaks and troughs in current–voltage measurements, which correspond to the in and out flow of electrons. An electron enters the dot, exhibiting a peak, and has to exit before another can take its place – in other words, it is a one-in and one-out arrangement. This is important because it means the ribbon can be used to detect the effects a single electron has on the system.
A voltage applied to the gate contact in the bottom of the trench results in a force acting on the ribbon due to the electrostatic interaction. The magnitude of the voltage, and therefore the force, controls the tension, mechanical oscillation frequency and electrochemical potential of the ribbon.
It was when Guo-Ping Guo and his team measured the current through the ribbon that they made some truly remarkable discoveries. As they tuned the frequency of an alternating gate voltage applied to the ends of the ribbon, they tracked the resonant mechanical oscillation frequency and compared this to the current flow through the ribbon. Incredibly, they found that the mechanical motion was coupled to the flow of a single electron in and out of the ribbon (Nanoscale 2017, Advance Article).
By driving the ribbon at higher powers, the system enters a non-linear regime that is useful for mass sensing at higher sensitivities, and their calculations reveal some truly astonishing numbers. The mass and force sensitivities are of the orders 10–21 g and 10–19 N Hz–0.5, respectively. Putting this into perspective, haemoglobin and other typical proteins have masses on this scale.
The unique applications of these devices could provide a reliable method for cooling the mechanical modes of resonators to the ground state as well as ultra-high force/mass sensors. They also provide a route to exploring nano-scale phenomena that is beyond the resolution of current technology and could shed light on problems in a range of fields. The full report can be found in Nanoscale .
About the author
Matthew Holwill is a PhD student at the National Graphene Institute at the University of Manchester, UK. Matthew is a member of nanotechweb.org’s network of student contributors.
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Source: Nanotech Web