In today's addition to our Application Note series we are looking at the future of electronics and the implications for research instrumentation. We are showing two examples of atomic force microscope (AFM) applications employed in this research. Current CMOS (complementary metal-oxide-semiconductor) technology used for making integrated circuits is constantly being scaled down. These devices will reach their ultimate physical limits in 10 to 15 years. As chip structures - which currently already have reached nanoscale dimensions - continue to shrink below the 20 nanometer mark, ever more complex challenges arise and scaling appears not to be economically feasible any more. And below 10 nm, the fundamental physical limits of CMOS technology will be reached. Researchers are therefore exploring novel concepts for future nanoelectronic devices.
Traditional techniques in cell biology involve chemical or pharmaceutical treatments of entire cells; however, in many cases it would be advantageous to target a single organelle or other structure within a cell without damaging overall cell structure. If scientists could inject a drug into a chosen organelle within the cell, or even destroy, extract or isolate the whole organelle without significantly harming the cell itself, new insight could be gained into the inner workings of the cell. In recent years, techniques have been developed which allow the manipulation of the individual nanoscale structures within biological cells. This manipulation, or 'nanosurgery', has the potential to provide new insight into the internal structure and dynamics of cells. Nanosurgical methods have been developed to target the cell's internal organelles, the cell membrane, and the structural protein filaments within the cell.
Today, in our Application Note series, we are covering Scanning Thermal Microscopy (SThM) - an atomic force microscopy (AFM) imaging mode that maps changes in thermal conductivity across a sample's surface. Similar to other modes that measure material properties, SThM data is acquired simultaneously with topographic data. The SThM mode is made possible by replacing the standard contact mode cantilever with a nanofabricated thermal probe with a resistive element near the apex of the probe tip. This resistor is incorporated into one leg of a Wheatstone bridge circuit, which allows the system to monitor resistance. This resistance correlates with temperature at the end of the probe, and the Wheatstone bridge may be configured to either monitor the temperature of a sample or to qualitatively map the thermal conductivity of the sample.
The folks at the European NanoHand project, whose nanogripper design we have covered in a previous Nanowerk Spotlight, seem to have loved playing with their plastic toy kits as kids. At least that's the impression you get when watching their latest video explaining their proof-of-principle study of scanning probe tips defined by planar nanolithography and integrated with AFM probes using nanomanipulation. They have prefabricated nanoscale needles, to be picked up by nanogrippers inside a scanning electron microscope. They then use these nanobits as ultralong tips in atomic force microscopes. The researchers call the needles 'nanobits' because they are a reminder of drill bits - you can have a library of different nanobits and then pick the one you want, and mount it where you want it.
Ferroelectric domain patterns attract increasing attention owing to their potential for integrated optical and novel electronic applications. Lately, Piezoresponse Force Microscopy (PFM) has become a standard technique for the investigation of such domain patterns due to the high lateral resolution of only about 10 nm even - so no specific sample preparation is needed. In addition, due to the frequency modulated PFM technique for recording ferroelectric domains, topography and domain patterns can be recorded simultaneously and independently. Piezoresponse force microscopy is based on the deformation of the sample due to the converse piezoelectric effect. Generally, all scanning force microscopes are suited for PFM operation as long as they allow application of voltages to the tip and separate readout of the cantilever movement.
A Japanese-UK research team has now demonstrated that cryoEM image analysis may be exploited to obtain structural information of sufficient resolution to reveal the absolute three-dimensional (3D) configuration of a designed DNA nanostructure. With their technique they have obtained structural information at sufficient resolution to visualize the DNA helix and reveal the absolute stereochemistry of a self-assembled DNA tetrahedron. Each edge is a 7 nm, 20 base pair duplex, and the edges are connected covalently through single unpaired adenosine nucleotides, making it a rigid, triangulated structure that could serve as a building block for larger 3D structures or as a molecular cage. This DNA tetrahedron is the smallest 3D nanostructure made by DNA self-assembly.
It its more than 25 years of existence, Scanning Tunneling Microscopy has predominantly brought us extremely detailed images of matter at the molecular and atomic level. The Scanning Tunneling Microscope (STM) is a non-optical microscope that scans an electrical probe over a surface to be imaged to detect a weak electric current flowing between the tip and the surface. It allows scientists to visualize regions of high electron density and hence infer the position of individual atoms and molecules on the surface of a lattice. Now, researchers in Japan have managed to partially sequence a single DNA molecule with a STM - a significant step towards the realization of electronic-based single-molecule DNA sequencing.
The Atomic Force Microscope (AFM) is a key tool for nanotechnology. This instrument has become the most widely used tool for imaging, measuring and manipulating matter at the nanoscale and in turn has inspired a variety of other scanning probe techniques. Originally the AFM was used to image the topography of surfaces, but by modifying the tip it is possible to measure other quantities (for example, electric and magnetic properties, chemical potentials, friction and so on), and also to perform various types of spectroscopy and analysis. Increasingly, the AFM is also becoming a tool for nanofabrication. Relatively new is the use of AFM in cell biology. Researchers in Switzerland have now demonstrated novel cell biology applications using hollow force-controlled AFM cantilevers - a new device they have called FluidFM.