Topic 4: Conducting injected charge carriers + self-assembled monolayers

Question 1

• Saturated alkane chains possess a band gap, E_g of 8 eV, outline the mechanism by which charge is transferred.

Answer 1

• Off/non-resonant tunnelling: as our frontier (charge transferring) orbitals are not aligned with the electrode fermi level, charge is not injected and instead tunnels through gap

Question 2

• Describe the change conductance with chain length of di-thiol alkanes, using a sketch to support you answer

Answer 2

• Conductance decreases exponentially with molecular length
• G = G₀exp(-βL)
• Β – tunnelling decay coefficient; depends on nature of bonds between atoms in molecule
• Large β -conductance drops off quickly with increasing chain length
• Leads to poor molecular wires

Question 3

• How can the relationship between conductance (G and molecular length (L) be rewritten in terms on resistance?

Answer 3

• G= I/V=1/R
• –> R = R₀exp(βL); where R₀ is the contact resistance when length is 0

Question 4

• Outline what how simple tunnelling theory agrees with the proposed mechanism in alkanes. Relate your answer to temperature

Answer 4

• Conductance is temperature dependent; as barrier is very large, an increase in T doesn’t affect tunnelling probability
• Current exponentially decays with L (G ∝ I)
• Conductance for L >2-3 nm too small to measure

Question 5

• How does the conductance mechanism of conjugated chains compare to that of saturated?

Answer 5

• Charge transport still via off-resonant tunnelling in non conjugated (when molecule small - <4 units) as band gap smaller, but still large overall
• Rate of conductance decay with length is much slower
• This is due to conjugated chains being much more conductive than saturated

Question 6

• How is the decaying coefficient related to the barrier height?

Answer 6

• Β ∝ ϕ_B­
• QM says tunnelling coefficient of barrier is a function of its width and height

Question 7

• What is the transport mechanism more large conjugated chains (polyenes)?

Answer 7

• Beyond 5nm chain, charge transport mechanism changes to hopping transport via thermally activated conduction, where charge is now localised on the molecule
• As the HOMO-LUMO gap has become even smaller, hopping between FOs occurs in favour of tunnelling this now larger distance.

Question 8

What is the relationship which allows thermally activated conduction to occur?

Answer 8

• R ∝ L; however now as R decreases, T increases which is useful for heat dissipation

Question 9

• Sketch the effect of resistance with increasing chain length in conjugated molecular wires

Answer 9

• Note logarithmic scale

Question 10

• What is a Uni-molecular diode and what are the three mechanisms to form them in metal-organic-metal assemblies?

Answer 10

• Uni-molecular diode are devices that exhibit asymmetric conduction where current flows in 1 direction but not the other
• 1) Schottky: work function different between barriers breaks symmetry (seen before)
• 2) Uni-molecular diode via donor-bridge-acceptor (D-σ-A)
• 3) Bias-induced structural change

Question 11

• Sketch the TTF-TCNQ uni-molecular diode and describe the electronic properties of its components

Answer 11

• D – electron donor (small I_p)
• A - electron acceptor (large A_g)
• Both A/D are highly conjugated
• Resistive σ bridge decouples them

Question 12

• Describe the setup of a uni-molecular diode, using energy level diagrams to support you answer

Answer 12

• Current flows from D to A
• Same metal at each electrode, so E_f aligned (this is a large assumption c.f. geometry fluctuations)
• When biasing potential applied across system charge dense D-HOMO aligns with A-LUMO, so barrier can be tunnelled across
• Gradient of σ bridge much higher indicating large electric field falling across resistive part

Question 13

• Why is the spacing of D/A important in a uni-molcular diode?

Answer 13

• A/D too far apart: will not communicate/tunnel
• A/D too close: single mixing of ground state

Question 14

• Describe the features of this IV plot

Answer 14

• Diodic behaviour displayed where current can flow only one way
• Steps marked with arrows indicate regions where resonance achieved as a result of D-HOMO aligning with an A-LUMO, as are sets of orbitals.

Question 15

• Why can current flow only one way in uni-molecular diodes? Use energy level diagrams to support your answer

Answer 15

• In reverse bias, D-HOMO must overcome huge energy barrier to transfer charge to A-LUMO

Question 16

• What is a self-assembled molecular monolayer?

Answer 16

• Spontaneous assembly of molecules in to densely packed and well-ordered crystals on a supporting substrate

Question 17

Give three reasons why are self-assembled monolayers important in nanotechnology?

Answer 17

• More practical than single molecule devices as easier to make electrical connection
• Reduces impact of bond fluctuations
• Amplifies single molecule functionality into many as can be packed densely.

Question 18

• How can electrical measurements be taken with self-assembled monolayers? (MORE DETAIL NEEDED ON THIS - diagrams etc)

Answer 18

• Use a droplet of Hg/Ga on to a surface in a solution to measure electrical properties of individual assemblies of molecules
• However always get a diodic response with this method as electrodes are different due to use of liquid metal.