Diamond-Modified AFM Probes: From Diamond Nanowires to Atomic Force Microscopy-Integrated Boron-Doped Diamond Electrodes

Abstract

In atomic force microscopy (AFM), sharp and wear-resistant tips are a critical issue. Regarding scanning electrochemical microscopy (SECM), electrodes are required to be mechanically and chemically stable. Diamond is the perfect candidate for both AFM probes as well as for electrode materials if doped, due to diamond's unrivaled mechanical, chemical, and electrochemical properties. In this study, standard AFM tips were overgrown with typically 300 nm thick nanocrystalline diamond (NCD) layers and modified to obtain ultra sharp diamond nanowire-based AFM probes and probes that were used for combined AFM-SECM measurements based on integrated boron-doped conductive diamond electrodes. Analysis of the resonance properties of the diamond overgrown AFM cantilevers showed increasing resonance frequencies with increasing diamond coating thicknesses (i.e., from 160 to 260 kHz). The measured data were compared to performed simulations and show excellent correlation. A strong enhancement of the quality factor upon overgrowth was also observed (120 to 710). AFM tips with integrated diamond nanowires are shown to have apex radii as small as 5 nm and where fabricated by selectively etching diamond in a plasma etching process using self-organized metal nanomasks. These scanning tips showed superior imaging performance as compared to standard Si-tips or commercially available diamond-coated tips. The high imaging resolution and low tip wear are demonstrated using tapping and contact mode AFM measurements by imaging ultra hard substrates and DNA. Furthermore, AFM probes were coated with conductive boron-doped and insulating diamond layers to achieve bifunctional AFM-SECM probes. For this, focused ion beam (FIB) technology was used to expose the boron-doped diamond as a recessed electrode near the apex of the scanning tip. Such a modified probe was used to perform proof-of-concept AFM-SECM measurements. The results show that high-quality diamond probes can be fabricated, which are suitable for probing, manipulating, sculpting, and sensing at single digit nanoscale.

Fabrication and characterization of boron-doped nanocrystalline diamond-coated MEMS probes

“…Apart from those techniques, scanning electrochemical microscopy (SECM) provides localized information on interfacial and biological processes on challenging subjects such as cellular signalling at molecular level [9].…”

“…These properties make diamond particularly attractive, also for optoelectronic devices at micro-and nanoscales that are suited for critical applications in extreme conditions [9]. Rapid technical advances now allow resolutions of 0.02 nm in depth, 50 nN in load and 50 nm in spherical diamond tip radius [24].…”

“…Recently, Drew et al [26] fabricated non-conductive hydrogen-terminated NCD-coated silicon AFM tips for photochemical force spectroscopy showing enhanced fluorescence when subjected to carboxylic acidselective labelling conditions. Smirnov et al [9] developed a focused ion beam (FIB)-fabricated CVD diamond nanoprobes with a resonance frequency of 15 kHz and a nominal force constant of 0.5 N m -1 . The dimensions of the coated cantilever were 450, 50 and 2 lm in length, width and height, respectively.…”

High-throughput scanning electrochemical microscopy brushing of strongly tilted and curved surfaces

“…the oxygen reduction) for determining the working distance was also reported [20]. An alternative route is the combination of SECM with other scanning probe techniques that can control the probe-sample distance, such as scanning force microscopy (SFM) [24][25][26][27][28][29][30], electrochemical scanning tunnelling microscopy (ECSTM) [31,32], and scanning ion-conductance microscopy (SICM) [33,34]. Other SECM hybrid techniques for distance control rely on the reduced damping of a vertically or laterally vibrated probe in close proximity to the substrate.…”

Electrochemical current-sensing atomic force microscopy in conductive solutions

“…These probes were insulated with various coatings, such as Si 3 N 4 [18,22], SiO 2 [23], Si 3 N 4 /SiO 2 sandwich layers [19], poly(p-xylylene) (parylene) [20] or plasma-deposited fluorocarbon films [21]. Recently, the group of Kranz reported on diamond-coated tips with integrated boron-doped diamond (BDD) nanoelectrodes [24].…”