Terahertz scanning tunneling microscopy on metals, semiconductors, and carbon nanostructures
Creators: Marin Calzada, Jesus Alejandro
Publisher: name: University of Alberta Library
Year: 2024

<p>Scanning probe microscopes routinely provide atomic resolution of numerous materials, but lack the tools to investigate their ultrafast dynamics. Laser pulses can be generated in the femtosecond or even attosecond regime, but their spatial application is restricted by the diffraction limit. Coupling ultrafast laser pulses to scanning tunneling microscopes (STM) has recently opened a window into an unexplored world where subnanometer spatial resolution and subpicosecond temporal resolution can be achieved simultaneously. As these techniques evolve in the scientific community, different characterization methods have been developed to gain a deeper understanding of the different aspects behind their operating principles. Great efforts are dedicated on this front, since any improvement in their performance will help to push the technological boundaries even further.<br>
In this thesis, single-cycle terahertz pulses (1 THz bandwidth) were coupled to the tip of a scanning tunneling microscope (THz-STM), which enhances and localizes the fields of the incident pulse at the tip apex. The surface of Au(111) was first examined to establish a benchmark of the THz-STM system. A comparison of these measurements with previous results on Cu(111) shows agreement and similarities between these two metals. Nanostructures are ideal candidates for THz-STM because their dynamic response can be studied individually, thanks to the nanometer resolution of the system. Therefore, the properties of single-walled carbon nanotubes (6,5) were explored with THz-STM, but were found to be unstable in our experimental setting. Graphene islands, on the contrary, exhibited high stability and the first THz-STM images of graphene islands reported here show their capability to identify structural defects that have otherwise very similar profiles in a conventional topographic image. An attempt to perform a pump-probe experiment on these nanostructures revealed an undesired electron emission from the sample substrate. Consequently, the electron photoemission occurring at the STM junction under illumination by 70 fs ultrafast near-infrared laser pulses centered at 800 nm was investigated. Photoemission experiments with W and Au tips on an Au(111) substrate revealed that multiphoton photoemission (MPP) was the main emission mechanism in our experimental setting.<br>
The use of a wide bandgap semiconductor as a substrate is proposed to eliminate its photoemission and facilitate optical pump-terahertz probe experiments on nanostructures. Three different semiconductive samples were studied: p-doped GaN, n-type Si-doped GaAs(110), and p-type Zn-doped GaAs(110). The results demonstrated that MPP was suppressed on a wide bandgap semiconductor, such as GaN, confirming it is a good candidate as a substrate for ultrafast pump-probe experiments. An initial attempt to perform an optical pump-THz probe experiment on a single-walled carbon nanotube on GaN is presented. However, the pump-probe signal closely resembled the THz near field waveform at the tip apex observed with photoemission sampling. The thermal expansion of the semiconductors was additionally investigated in the STM since thermal effects from a pump beam can also interfere with the experiments. The measurements showed that the tip expansion is usually small compared to that of the sample, and that the least thermal expansion occurred on the GaN sample which reinforces the proposal to use it as a substrate.<br>
Finally, the design and construction of a home-built ambient STM is included. Basic THz-STM measurements on a single-walled carbon nanotube and the acquisition of a THz-induced photoemission waveform proved that the system is capable of performing THz-STM and ultrafast optical pump-THz probe experiments. </p>