Topic 3: Uni-molecular devices (wires/diodes/switches)

Question 1

• … law says that the number of … in a computer chip has doubled every two years.
• It has now reached length scales below … nm, final target being … nm

Answer 1

• Moore’s law says that the number of transistors in a computer chip has doubled every two years.
• It has now reached length scales below 10 nm, final target being 1 nm

Question 2

• What is the problem approaching as we reach smaller transistor length scales?

Answer 2

• Controlling dimensions becomes difficult due to quantum size/confinement effects
• Using traditional lithographical techniques for system <10 nm is not possible, greatly increasing the cost of production

Question 3

• What is a possible solution to the issue of size in transistor production?

Answer 3

• Use of small molecules (1-3 nm) that are optically and electronically well-defined, showing no variance from quantum size/confinement effects
• The energy levels of which can be tailored to fit the specific application.

Question 4

• What are quantum size/confinement effects?

Answer 4

• The quantum confinement effect is observed when the size of the particle is too small to be comparable to the wavelength of the electron.
• Confinement of motion of randomly moving electron restricts its motion in specific energy levels (discreteness) and quantum reflects the atomic realm of particles. So as the size of a particle decreases until we a reach a nano scale, the decrease in confining dimension makes the energy levels discrete
• This increases/ or widens up the band gap and ultimately the band gap energy also increases, causing an overall variance in the system

Question 5

• What are some of the main challenges in unimolecular electronics?

Answer 5

• Connecting individual molecules between electrodes
• Understanding nature of contact between electrode and molecule (mixing between frontier orbitals)

Question 6

• What are two methods of making nanogaps

Answer 6

• Electro-migration: nanowire between two electrodes
• Scanning Probe Microscope break junction

Question 7

• What is electro-migration?

Answer 7

• The motion of atoms subject to a large current

Question 8

• Outline the proposed mechanisms for electro-migration

Answer 8

• First mechanism based on transfer of conduction electron momentum to lattice imperfections, pushing them apart, forming a gap and causing a drop-in conductance
• Second is nano-gaps formed by direct force of electric field on charged defects, derived from trapped charges at grain boundaries in crystallites of wire.

Question 9

• What is Joule heating and how can it be avoided?

Answer 9

• At very high currents, the heat produced via joule heating can cause breaks in nanowires, but causes them to melt
• To supress, the critical thickness of the wire is < 20 nm which suppresses the current, lowering joule heating (I²R)

Question 10

• What is the downside of reducing thickness of a nanowire?

Answer 10

• When made too thin, size effects reduce the MP of the nanowire, making it more susceptible to dual heating (L6)

Question 11

• What is process of using a SPM break junction to create a nano-gap?

Answer 11

• Sharp AFM/STM probe repeatedly pushed into and retracted from a substrate in the presence of a dilute solution of molecules.
• Conductance histogram of many measurements has peaks at integer multiples of fundamental value of solution smallest step chance of which corresponds to 1 molecule between surface and tip.

Question 12

• What material is generally used for electrodes and why?

Answer 12

• Gold or platinum as they are inert

Question 13

• What is a molecular diode?

Answer 13

• Molecular device where current only flows one way

Question 14

• Its is important for both electrodes are made of the same material to avoid a built-in potential, what is a built-in potential?

Answer 14

• Unequal work functions (due to largely different µ) cause charge to flow from low ϕ electrode (2) à high ϕ electrode (1) until E_f is aligned
• This is because electrode 1 has more low-lying empty energy states
• The result is an electric field across the molecule and a dipole across the electrodes.

Question 15

(?) Difference in electrode … gives rise to … behaviour which is not a true … …

Answer 15

(?) Difference in electrode ϕ gives rise to diodic behaviour which is not a true molecular diode

Question 16

• What is the problem with the built-in potential?

Answer 16

• Electrons must be pushed uphill (v.v for holes)

Question 17

• How can the built-in potential problem be solved?

Answer 17

• Flat band condition: apply a negative bias across built in potential to the point where electrons can be deposited in to HOMO and holes into LUMO

Question 18

• describe the various contributions to resistance across a molecular device connected to two electrodes in a circuit

Answer 18

• Contribution to the contact resistance by process of injection of electrons/holes at R₁/R₃.
• R₂ is the contribution to the resistance due to movement of electrons through molecule.

Question 19

• How is contact resistance minimised?

Answer 19

• Ensuring molecule is strongly bound to electrode, as a short bond is more transparent to the movement of charge.

Question 20

• What is the relationship between conductance and bond order in anchor/head groups in electrode-molecule contacts?

Answer 20

• A higher bond order (# of bonds between anchor-metal)
• Gives higher conductance

Question 21

• Give three examples of head groups in ascending binding strength

Answer 21

• Carboxylic acid, 1^o amine, thiol.

Question 22

• In addition to electrodes needing the same work function, the anchor moiety at either end of the molecule must be the same, why is this?

Answer 22

• Pushback/pillow effect contribution will be different, reducing the metals surface dipole to different extents either side of the molecule, causing overall electrode ϕ’s to differ from one another
• Charge density redistribution with bond formation between molecule and electrode
• Results in asymmetry I/V characteristic and a diodic charge, meaning a true molecular diode does not form
• For this reason, anchor groups must be the same, to ensure alignment between fermi level in metal and frontier orbitals of interest

Question 23

• Even when the same anchor group and electrode metals used, there can still be disparity in conductance, why is this?

Answer 23

• Conductance is also a function of the molecule-electrode contact geometry (i.e. the bonding site on the electrode)
• Binding to the hollow site results in a lower energy, stronger bond, as anchor group is bound to more atoms, giving higher conductivity
• The reverse is true for a top site bond

Question 24

• Geometric fluctuations in electrode-molecule contacts are due to binding site are common in practice, how can they be decreased?

Answer 24

• Use molecule that forms a bi/tri-dentate anchor group, where the most likely conformation across many sites is likely to be the same
• • Use a tertiary amine, with a soft lone pair bond which has inbuilt flexibility and insensitivity to binding angle, reducing variation of molecular binding.