Fabrication of cone-shaped boron doped diamond and gold nanoelectrodes for AFM-SECM

Diamond Colloidal Probe Force Spectroscopy

Diamond is a highly attractive coating material as it is characterized by a wide optical transparency window, a high thermal conductivity, and an extraordinary robustness due to its mechanical properties and its chemical inertness. In particular, the latter has aroused a great deal of interest for scanning probe microscopy applications in recent years. In this study, we present a novel method for the fabrication of atomic force microscopy (AFM) probes for force spectroscopy using robust diamond-coated spheres, i.e., colloidal particles. The so-called colloidal probe technique is commonly used to study interactions of single colloidal particles, e.g., on biological samples like living cells, or to measure mechanical properties like the Young’s modulus. Under physiological measurement conditions, contamination of the particle often strongly limits the measurement time and often impedes reusability of the probe. Diamond as a chemically inert material allows treatment with harsh chemicals without degradation to refurbish the probe. Apart from that, the large surface area of spherical probes makes sensitive studies on surface interactions possible. This provides detailed insight into the interface of diamond with other materials and/or solvents. To fabricate such probes, silica microspheres were coated with a nanocrystalline diamond film and attached to tipless cantilevers. Measurements on soft polydimethylsiloxane (PDMS) show that the manufactured diamond spheres, even though possessing a rough surface, can be used to determine the Young’s modulus from a Derjaguin-Muller-Toporov (DMT) fit. By means of force spectroscopy, they can readily probe force interactions of diamond with different substrate materials under varying conditions. The influence of the surface termination of the diamond was investigated concerning the interaction with flat diamond substrates in air. Additionally, measurements in solution, using varying salt concentrations, were carried out, which provide information on double-layer and van-der-Waals forces at the interface. The developed technique offers detailed insight into surface chemistry and physics of diamond with other materials concerning long and short-range force interactions and may provide a valuable probe for investigations under harsh conditions but also on biological samples, e.g., living cells, due to the robustness, chemical inertness, and biocompatibility of diamond.

Conductive diamond: synthesis, properties, and electrochemical applications

This review summarizes systematically the growth, properties, and electrochemical applications of conductive diamond.

An improvement in scanning electrochemical microscopy based on a plasmon-accelerated electrochemical reaction

A plasmon-accelerated electrochemical reaction coupled with SECM provides an opportunity to improve the performance of SECM.

Fabrication of Scanning Electrochemical Microscopy-Atomic Force Microscopy Probes to Image Surface Topography and Reactivity at the Nanoscale

Concurrent mapping of chemical reactivity and morphology of heterogeneous electrocatalysts at the nanoscale allows identification of active areas (protrusions, flat film surface, or cracks) responsible for productive chemistry in these materials. Scanning electrochemical microscopy (SECM) can map surface characteristics, record catalyst activity, and identify chemical products at solid-liquid electrochemical interfaces. It lacks, however, the ability to distinguish topographic features where surface reactivity occurs. Here, we report the design and fabrication of scanning probe tips that combine SECM with atomic force microscopy (AFM) to perform measurements at the nanoscale. Our probes are fabricated by integrating nanoelectrodes with quartz tuning forks (QTFs). Using a calibration standard fabricated in our lab to test our probes, we obtain simultaneous topographic and electrochemical reactivity maps with a lateral resolution of 150 nm.

Direct-Write Ion Beam Lithography

Patterning with a focused ion beam (FIB) is an extremely versatile fabrication process that can be used to create microscale and nanoscale designs on the surface of practically any solid sample material. Based on the type of ion-sample interaction utilized, FIB-based manufacturing can be both subtractive and additive, even in the same processing step. Indeed, the capability of easily creating three-dimensional patterns and shaping objects by milling and deposition is probably the most recognized feature of ion beam lithography (IBL) and micromachining. However, there exist several other techniques, such as ion implantation- and ion damage-based patterning and surface functionalization types of processes that have emerged as valuable additions to the nanofabrication toolkit and that are less widely known. While fabrication throughput, in general, is arguably low due to the serial nature of the direct-writing process, speed is not necessarily a problem in these IBL applications that work with small ion doses. Here we provide a comprehensive review of ion beam lithography in general and a practical guide to the individual IBL techniques developed to date. Special attention is given to applications in nanofabrication.

Electrochemical current-sensing atomic force microscopy in conductive solutions

Insulated atomic force microscopy probes carrying gold conductive tips were fabricated and employed as bifunctional force and current sensors in electrolyte solutions under electrochemical potential control. The application of the probes for current-sensing imaging, force and current-distance spectroscopy as well as scanning electrochemical microscopy experiments was demonstrated.

Diamond-modified AFM probes: from diamond nanowires to atomic force microscopy-integrated boron-doped diamond electrodes

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.