Simultaneous scanning tunneling/atomic force microscopy investigation of diamond surfaces in ultra high vacuum

Diamond has always been one of the most outstanding naturally existing materials due to its distinguishing thermal and mechanical properties. As it is known as the hardest material, diamond has been a field of interest in plenty of mechanical studies and has taken its place in industry for different uses, as well. In the last few decades, with the pursuit of adaptation of semiconductors to the technology, the search for more durable novel materials with manipulative electronic characteristics has ascended. Thus, diamond has been one of the most popular research areas, such as the synthesis processes and surface studies, including scanning probe microscopy. Atomic Force Microscopy (AFM) studies of both natural and CVD grown diamond surfaces revealed the topography of diamond surfaces with different surface orientations and reconstructions. Besides, Scanning Tunneling Microscopy (STM) studies of diamond have shown that the electronic structure of diamond is highly related to the type of dopants and surface termination. In this study, the diamond surface has been investigated using simultaneous Scanning Tunneling/Atomic Force Microscopy in ultra-high vacuum (UHV). The investigation of diamond using simultaneous STM/AFM in UHV has been focused on polycrystalline and single crystalline diamond surfaces. The cleaning techniques including the sputtering, annealing and acid etch have also been practiced, in order to obtain atomically clean and flat diamond surfaces. The simultaneous STM/AFM system used in this study operates as using the tunnel current as the feedback parameter. By oscillating the cantilever at a specific frequency well below its resonance frequency, oscillations with sub-Ångström amplitudes were obtained. The detection of such small cantilever oscillation amplitudes was enabled by using a fiber-optic interferometry setup. Scanning the surfaces using such small cantilever oscillation amplitudes enables the true simultaneous STM/AFM operation on the diamond surfaces, by the means of being at certain operating regimes while scanning the surface. With the simultaneous STM/AFM system used in this study, surface topography, force, interaction stiffness, tunnel current modulation (measure of barrier height), phase shift (energy dissipation) and tunneling current were obtained. In this study, the diamond surfaces were scanned using two different cantilevers, a Tungsten (k=53 N/m) and a highly boron doped diamond cantilever (k=70 N/m), which were calibrated using thermal noise method. Atomic resolution imaging of different regions on diamond surfaces was achieved. Moreover, force-distance spectroscopies were conducted on different regions on the diamond surfaces, leading to obtain the interaction stiffness between the diamond and the tip. The maximum negative interaction stiffness between the polycrystalline diamond surface and the W tip is measured around -2-2.5 N/m, where the interaction stiffness between the polycrystalline diamond surface and the highly boron doped diamond tip is measured as -1.2 N/m. For the single crystalline diamond, the maximum negative interaction stiffness between the sample surface and the W tip is measured as -2.5 N/m and -1.2 N/m, with an applied bias voltage of -1 V and 1 V respectively. The maximum negative interaction stiffness between the single crystalline surface and the highly boron doped diamond tip is measured as -1.5 N/m.