Scanning Tunnelling Microscopy of Si(111)-7x7: Local Atomic Manipulation and Light Emission

  • Rebecca Purkiss

Student thesis: Doctoral ThesisPhD


In this thesis, the injection of tunnel current from the STM tip is utilised to locally manipulate individual toluene molecules and also result in the emission of light from the tunnel junction.

Current injection pulses result in the local manipulation of toluene molecules on the Si(111)-7x7 surface. Investigating the rate of manipulation shows an increase with the tunnel current, indicative of a 1-electron process for electron injection at +1.6 V. For hole injection at -1.3 V, this trend exists for low current (2 – 10 pA), but plateaus for high current injections (10 - 900 pA), indicating a 0-electron process with manipulation invariant to the injected current.

The probability of manipulation is measured to decrease for high current hole injection, at small tip-sample separation distances. We propose that this occurs as a result of the tip quenching the excited-state lifetime of the molecule by over two orders of magnitude, due to the creation of a tip-dependent interface state on the molecule, creating a new decay channel for the excited state.

The ability to influence the manipulation process has led to investigating the branching ratio of this system, to determine whether the outcome of the manipulation event can be selected. The branching ratio has been measured and found to be invariant as a function of: voltage, the corresponding tip height, electric field and position. This suggests a 1.4 V threshold to local manipulation and that the manipulation events each occur with the same excited state form. Charge must undergo ultra-fast relaxation to 1.4 V before initiating manipulation when transitioning from this state.

Light emission has been investigated on the same Si(111)-7x7 surface, with the voltage dependence of emission revealing a 1.9 ± 0.3 V threshold, matching that of non-local manipulation. We propose that the mechanism of non-local manipulation and light emission must have a common origin. Once the injected charge has relaxed to the bottom of the surface state (~2 V), transitions from this state can occur via two inelastic decay channels. One results in the manipulation of molecules and the other, light emission. This mechanism is used to re-interpret results previously published by other research groups.

The possibility of observing these effects on a new system of graphite dosed with oxygen atoms is documented, with preliminary work to reliably dose and image the surface.
Date of Award3 Apr 2018
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorSergey Gordeev (Supervisor) & Peter Sloan (Supervisor)

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