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Posted: Jul 14, 2008
Electrometers for nanotechnology
(Nanowerk Spotlight) Electrometers are instruments that measure electric charge or electrical potential difference by means of electrostatic force. While early electrometers such as developed by Lord Kelvin in the 19th century were crude instruments, modern electrometers based on solid state technology are high-precision electronic devices that, in extreme cases, are so sensitive they can count individual electrons as they pass through a circuit.
As the dimensions of electronic devices shrink further, the probes required to measure the voltage inside a miniature conductor have to be miniaturized, too. An alligator clip cannot be scaled down indefinitely to perform such tasks. Furthermore, as devices reach the nanoscale, the perturbation of the measurement on the device itself cannot be neglected and must be assessed.
A few techniques, many of which are based on scanning a small object such as an atomic force microscope (AFM) tip, have been developed in the past to address this challenge. Each technique has its pros and cons.
The design principle of a novel technique to probe local voltage distributions inside a nanostructure – in essence, a nanoelectrometer – combines the spatial resolution and force sensitivity of an AFM and the single-electron charging of a gold nanoparticle. Developed by researchers at Cornell and Penn State Universities, its fabrication, calibration and operation are demonstrated in a looped carbon nanotube (CNT).
Techniques like this one are essential for answering questions such as how an electric current flows through a nanoscale conductor or where the resistance comes from, and they are necessary for understanding the properties and improving the performance of today's ever shrinking electronic devices.
"Our scheme combines locally placed gold nanoparticles with AFM-based single-electron force sensing microscopy to function as weakly perturbative, highly sensitive nanoelectrometers" Dr. Jun Zhu explains to Nanowerk. "The gold nanoelectrometer senses the local electrostatic potential via its single-electron charge states. Its large input impedance, produced by a molecular junction and measured by dissipation force microscopy, ensures weak coupling to the device. We demonstrate the operation of our gold nanoelectrometers by mapping the local potential profile of a looped carbon nanotube. This new type of nanoparticle-based electrometer can be generalized to map potential distributions in other nanostructures such as nanowires and two-dimensional conductors. The impedance measurement can be used to probe the resistance of molecular junctions."
"Instead of using hookup wires, the gold electrometers in our design are tethered to the device under test – in this case a CNT – through molecules that conduct," Zhu explains the device. "Using this geometry and a high quality factor AFM cantilever, we are able to measure the resistance of the molecular junction by examining the dissipation of the cantilever, essentially using it as a sensitive calorimeter. The energy dissipation we measured is extremely small: ∼10-17W, demonstrating the superb sensitivity of the measurements."
Coulomb oscillation behavior of gold nanoparticles. (a) Topographic AFM image of a section of the CNT device, showing selective attachment of gold nanoparticles. The CNT diameter is 3.2 nm. (b) A series of scans above the CNT device with the oscillation amplitude of the cantilever plotted in color scale. Coulomb oscillations of the gold nanoparticles appear as concentric, dark rings centered on individual particles. Charge motion causes additional damping to the oscillation of the cantilever in the tunneling regime of the gold particles, producing a general decrease of 10-20% in oscillation amplitude with extremely dark spots reaching 50%. White dashed line indicates the location of the CNT. All images are taken with Vtip = 3 V, Vg = -2 V, and tip height z = 60 nm. (c) A close-up image of the gold nanoparticle indicated by the arrow in b. Several concentric rings are clearly visible. Image taken with Vtip = 3 V, Vg = -1.5 V, and tip height z = 60 nm. (Image: Dr. Zhu, Pennsylvania State University)
This novel approach circumvents the difficulty of having to make a nano gap to bridge a molecular conductor and is an important new addition to the existing tool bag of molecular electronics.
To fabricate their device, the researchers integrated a gold nanoparticle with a mean diameter of 12 nm into a CNT field effect transistor via chemical
linkers (comparison experiments show that without the linker molecule, gold nanoparticles do not adhere to CNTs).
Zhu notes that, due to their small size and weak coupling to the nanotube, the nanoparticles exhibit single-electron charging behavior at 77 K. "The charge state of a gold nanoparticle is set by nearby electrostatic potentials, which is the principle behind its application as a local electrometer. The linker molecules form tunnel barriers between the CNT and the gold nanoparticles; this sets the input impedance of the nanoparticle electrometer."
The commercial availability of AFM cantilevers of a range of resonant frequencies, gold nanoparticles of different sizes, and linker molecules of varying impedances expands the measurement range and applicability of this technique.
"By using suitable linker chemistries, it should be straightforward to implement the approach we described here to other material systems such as silicon nanowires and graphene" says Brink, who together with Zhu, contributed equally to this paper. "Additionally, the impedance measurement provides a novel method to determine the resistance of single or few molecular junctions. The fabrication process is significantly simplified since only one electrical contact to the nanoparticle is required. The molecules themselves need not form an ordered monolayer or bridge electrodes separated by ∼1 nm gap."
Zhu adds that local diagnostic tools play increasingly important roles in understanding the behavior of small yet complex artificial nanostructures. "Our current tool set is quite small but there still is a lot of work to be done to develop all the new tools required to suit the growing needs of nanoscience and nanotechnology."