Scanning tunnelling microscopy is a powerful technique for imaging surfaces with extremely high resolution down to the atomic level. The scanning tunnelling microscope (STM) has proven to be suitable not only for very high resolution topographic and spectroscopic imaging, but also for precisely controlled manipulation of atoms and molecules.
With the purpose of developing electronic devices dimensionally reduced to the atomic scale, experimental research has been carried out to investigate the properties of atoms and molecules adsorbed at surfaces. My research project, coordinated by Prof. Philip Moriarty (Nanoscience Group, University of Nottingham), involves STM and, ultimately, atomic force microscope (AFM) measurements at room and low temperature to investigate the behaviour of C60 molecules on the Ag-terminated Si (111) surface. In a preliminary experiment spectroscopic measurements and manipulation attempts have been successfully performed in ultrahigh vacuum (UHV) with an Omicron VT STM/AFM system.
The STM measurements have been carried out at room temperature in UHV with a base pressure better than 2 x 10 -10 mbar. Topographic images have been acquired in constant current mode. Electrochemically etched tungsten tips were used. Even that the new tips were cleaned in first instance by scanning gold (111) surface, still further treatment was needed. During scanning, conditioning procedures like voltage pulsing and light indentations were applied. In the VT STM-AFM system the tip is biased. Scanning data were acquired and processed by the Matrix software.
High quality atomic resolution has been achieved on both the Si (111)-7x7 and Ag-terminated Si (111) √3x√3 R30o reconstructions. C60 molecules have been evaporated onto Ag-Si (111). The molecules preferentially adsorb to defective sites and at step edges. In STM mode the molecules have been imaged as bright spherical protrusions.
Silicon wafers used for the experiments have the following characteristics: p-type semiconductor, Boron doped, 0.01-0.02 Ω cm, 1x10 mm2 laser cut. Prior to their introduction to UHV, the silicon samples are sequentially ultrasonicated for a few minutes in three different solvents: acetone, methanol, and isopropanol. The flash anneal procedure, which leads to a 7x7 reconstruction, involves heating the sample for 10-20 seconds at 1200 oC, then maintaining for few minutes the silicon at 600- 700 oC with slow cool down.
Si (111) 7x7 - empty states image, room temperature, dark spots represent missing atoms or adsorbates while the white bright spots are adatoms – all considered defects of the reconstruction. Scan parameters: 40 nm, 1V, 20 pA, 0.5%.
Silver deposition is carried out by thermal evaporation in UHV from a crucible using an UHV Omicron Evaporator EFM 3T. Silver is evaporated onto a Si (111) 7x7 reconstructed sample. In order to achieve the (√3x√3) reconstruction, the silicon surface must be maintained at 400- 500 oC during evaporation.
Ag-Si (111) √3x√3 R30o , honeycomb-chain-trimer (HCT) model, room temperature, 10 nm, 0.4 V, 1 nA, 1%.
The Omicron EFM 3T UHV Evaporator has been used to deposit C60 on silver terminated silicon surfaces.
Low coverage C60 molecules on Ag-Si(111) imaged as bright protrusions by STM tip, room temperature. Random distribution, preferential for defects and steps sites Scanning parameters: 35 nm, -1.6 V, 5pA, 1.2 %.
The VT STM/AFM system consists of two main vacuum chambers. Being fitted with a turbo molecular pump and an ion pump, the system is capable of attaining a pressure of 10-10 mbar and lower. The main chamber enables the transfer and manipulation of the sample, with the help of a sample manipulator, mounted in the left-hand part of the setup. A sister chamber supporting the scanning probe microscope is attached to the main chamber.
Omicron UHV VT STM/AFM setup. The system consists of the transfer and STM/AFM chambers, fitted with four pumps through the transfer chamber for achieving UHV. In this image Cristina Chiutu (me), 2nd year PhD, Nanoscience Group Nottingham.
Key future goals of my work include the investigation of C60 molecule interaction with Ag-Si (111) by means of atomic force microscope (AFM). In this work it is proposed to realize AFM manipulations of C60 molecules and to measure force spectroscopic data on single C60 molecule, in order to determine the nature, the force and the energy of the molecule bonding configuration with Ag-Si (111) surface.