Molecular electronics. Lecture 2

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Hillary Rose
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1 Molecular electronics Lecture 2

2 Molecular electronics approach

3 Electrodes and contacts Basic requirement for molecular electronics: connection of the molecule of interest to the outside world, i.e. electrode type of links Covalent bond Au-thiol bond is the most used one Au-Se and Ag-S under investigation van der Waals deposition of Langmuir-Blodget films + only planar surface with proper wetting properties required - poor stability

4 Bond length effect If the distant is short enough that delocalized state of the molecule overlaps with the metallic electronic states, than the common delocalized state is formed and electron can be transmitted through the system. If the overlap is not achieved the wave function can be treated independently and the whole situation can be modeled as tunneling of an electron from electrode to a molecule.

5 Molecular wires Molecular wires are, generally, rod-like structures with delocalized p-system, the longer the structure the lesser the difference between the frontier orbitals and the Fermi level of the electrode polyene alternating system of single and double bonds; polythiophene polyphenylenevinylene polyphenyleneethynylene thyophenylsubstituted benzene

6 Molecular insulator (spacer) insulating molecular structures could be used as spacers i.e. have to be significantly insulating to preserve energy difference, but still allow tunneling. torsion angle is important alkanes good insulating properties, lack rigidity; adamantyl cage good rigidity and insulating properties, synthetically demanding tetramethylsubstituted bephenyl - single bonding connecting two rings with perpendicular p-systems metal-organic insulator (?) relative position is important meta connected aromatic are insulators opposite to ortho- and para- connections

7 Molecular rectifier (Diode) The idea (Aviram and Ratner, 1974): donor-acceptor system separated by a spacer so their p-systems don t overlap. the system will have preferential charge transfer direction V

8 Switches and Memory Bistable molecular systems: molecules that can exist in 2 (meta)stable states with different properties switches can be triggered by light, ph etc.

9 Switches and Memory light triggered switch

10 Switches and Memory Voltage triggered switch: catenane molecule can be switched between two state (rotation of a ring) by applying positive (+2V) or negative pulses (-2V), reading voltage is 0.1V Collier et al, Science 289, 1172 (2000)

11 Theory considerations: Resonance transport Landauer-Buttiker theory: electron is transmitted through a state with a certain probability (transparency) Tr(t,t ) electron states in the molecule E F electrodes

supermolecule involving last few metal atoms should be in the calculations HOMO: depleted

12 Theory considerations: Contacts and MO overlap contribution of different MO to the current may very strongly depending on their spatial arrangement issue of contact is important: a supermolecule involving last few metal atoms should be in the calculations HOMO: depleted at the contacts LUMO: depleted in the middle lower lying MO (~1eV) is important for current

13 Role of contacts

14 Theory considerations: Coulomb blockade Charging effects on the nanoscale are important Geometrical effect: depends on the particle size and geometry of the contacts 1nm cluster: Ec ~ 0.5 ev

15 Design Rules for Molecular Circuits Applicability of superposition principle is restricted as molecular parts can not be treated independently. Effect of molecular structure on density of states and geometry of MO should be considered Coulomb blockade effects: conductance will depend on charge on subunits and the capacitance to the gate Interference effects

16 Molecular devices The challenges: how to attach molecules to the electrodes how to arrange them in the same direction

17 Langmuir-Blodgett Approach Molecular rectifier: Metzger, R. M.; Xu, T.; Peterson, I. R.,. Journal of Physical Chemistry B 2001, 105, (30), arachidic acid rectifier acid

18 Molecular rectifier Very fragile device! Need for cold evaporation technique

19 Scanning probe methods SAM domains molecular rods protruding advanced possibility: from dodecylmercaptan layer molecules can be selectively desorbed by applying a voltage pulse or by mechanical forces (AFM)

20 Scanning probe approach advanced possibilities: molecules can be selectively desorbed by applying a voltage pulse or by mechanical forces (AFM)

21 Molecular rectifier: pore approach Molecular layer is sandwiched between gold layers in a SiN nanopore

22 Mechanically controlled break junction

23 Mechanically controlled break junction

24 Fixed three terminal technique

25 Electromigration break junctions electromigration technique: small current applied to a notched e-beam fabricated wire, electromigration causes thinning of the notched part. various gate voltages

Introduction to Molecular Electronics. Lecture 1: Basic concepts

Introduction to Molecular Electronics Lecture 1: Basic concepts Conductive organic molecules Plastic can indeed, under certain circumstances, be made to behave very like a metal - a discovery for which