Molecular electronics faces many problems in practical device implementation, due to difficulties with fabrication and gate-ability. In these devices, molecules act as the main conducting channel. One could imagine alternate device structures where molecules act as quantum dots rather than quantum wires, with assembled molecular adsorbates acting as scattering centers in commercial silicon-based FETs.
We describe a theory of electrostatic detection in ChemFETs (Chemically modified Field Effect Transistors) and quantum detection in SurfFETs (Surface-modulated Field Effect Transistors). Using DFT for attachment geometry and depletion electrostatics for band-bending, we calculate the threshold voltage shifts caused by accumulation of charge carriers in a silicon-on-insulator (SOI) pseudo-MOSFET device with a backgate and a molecular overlayer. Our results are in good agreement with experimental data from Rice University on threshold modulation and UPS/IPES of molecular dipoles on these structures.
We also analyze surface-modulated transistors in which covalent bonding of a quantum dot with silicon can transfer the dot dynamics to the channel. We illustrate this with a few examples, where the channel conduction is modulated by the trap dynamics, ranging from stochastic signals created by gate resonance (random telegraph noise) to deterministic signals created by interaction with monochromatic light impulse (Rabi oscillations). Modeling quantum detection is more complicated and needs attention to quantum many-body effects such as Coulomb Blockade in the molecular dots. We also need a formal description of how the nanodots ‘talk’ to their parent silicon macrochannels. Here the temporal response of the scatterer, obtained from the equations of motion of its electronic creation and destruction operators, is incorporated into a fully time-dependent nonequilibrium Green’s functions (TD-NEGF) formalism for the channel current. Numerical results are obtained through the time-domain decomposition technique. We contrast two different mechanisms of molecular-scattering: short-range quantum interference vs long-range Coulomb blockade.
Finally, we discuss possible applications of the considered systems as well as general considerations on the distinct physics underlying nano- and microsystems.
Burton Morgan Building, Purdue University, West Lafayette, IN