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An Atomic-Resolution Nanomechanical Mass Sensor: Supplementary materials

K. Jensen, Kwanpyo Kim and A. Zettl
Department of Physics, University of California at Berkeley
Center of Integrated Nanomechanical Systems, University of California at Berkeley
Materials Sciences Division, Lawrence Berkeley National Laboratory
Berkeley, CA 94720, U.S.A.

The following media files are intended for public access, and may be reproduced with credit: "Courtesy Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley." All rights reserved ©2008.

Introduction

What is the smallest mass that a simple mechanical balance can measure? A barely visible speck of dust? A single bacterium? Or perhaps a nanocrystal composed of a few thousand atoms? In fact, we have developed a new form of mechanical balance, based on a vibrating carbon nanotube, that can determine the mass of a single atom!

Our nanotube-based mass sensor belongs to a family of balances known as inertial balances. These balances consist of a spring with an attached mass, which is free to oscillate. By monitoring shifts in oscillation frequency, it is possible to detect changes in mass. In our device, the nanotube serves as the spring, and by monitoring the frequency of its vibrations, we can detect changes in mass caused by single atoms adsorbing to the nanotube's surface. To be more precise, our device's current sensitivity is 1.3 × 10-25 kg/Hz1/2 or equivalently 0.40 Gold atoms/Hz1/2. Thus, by scaling the concept of the inertial balance down to the nanoscale, we have increased its sensitivity by orders-of-magnitude.

Our nanotube-based mass sensor has numerous advantages over traditional high-resolution mass spectrometers. First, our device does not require the potentially destructive ionization of the test sample. (Large molecules such as proteins are often destroyed by ionization.) Second, our device becomes more sensitive at higher mass ranges in contrast with traditional mass spectrometers. Finally, our nanotube-based mass sensor is compact and could eventually be incorporated on a chip.

Artwork

If you use any of the following images, please include the credit "Courtesy Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley."

Gold atoms raining on a nanotube resonator.

Raining atoms (TIFF 19.3 MB)
Raining atoms (small) (JPEG 523 KB)

Raining atoms, blue nanotube (TIFF 19.2 MB)
Raining atoms, blue nanotube (small) (JPEG 574 KB)

Raining atoms, blue nanotube, black background (TIFF 23.9 MB)
Raining atoms, blue nanotube, black background (small) (JPEG 655 KB)

Representation of a gold atom being weighed in a historical balance.

Historical balance (TIFF 24.0 MB)
Historical balance (small) (JPEG 464 KB)

Images

If you use any of the following images, please include the credit "Courtesy Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley."

Transmission electron microscope image of a typical nanotube-based mass sensor. Carbon nanotubes are the ideal material for constucting nanomechanical mass sensors because their small size and low density gives them an incredibly small mass (about 10-21 kg). We used double-walled, rather than single-walled, carbon nanotubes because of their uniform electrical properties (mostly metallic).

Double-walled carbon nanotube resonator (TIFF 7.26 MB)
Double-walled carbon nanotube resonator (small) (JPEG 322 KB)

How does one detect the vibrations of a carbon nanotube? As nanotubes are smaller than the wavelength of light, standard optical techniques for detecting vibrations fail. Although you can see the vibrations using a transmission electron microscope, this method is too slow to be useful and not practical for commercial applications. The solution is to detect the vibrations electrically. We use a modified version of our "Nanotube Radio" experimental setup to detect the nanotube's vibrations. In effect, we listen for the vibrations of the nanotube. A partial schematic of this setup is shown to the left (click below for full schematic).

Nanotube mass sensor schematics (TIFF 6.45 MB)
Nanotube mass sensor schematics (small) (JPEG 190 KB)

As gold atoms land on the nanotube resonator, they increase the effective mass of the resonator, which lowers the resonance frequency of the resonator. This plot shows the resonance frequency of the nanotube over the course of an experiment. In the shaded regions, no atoms are loaded onto the nanotube and the resonance frequency remains relatively constant. In the unshaded regions, we load gold atoms onto the resonator and the resonance frequency shifts downward. In the first open section, we load just 51 gold atoms onto the resonator. Even when no atoms are loaded onto the nanotube, there is still noise in the system, which limits our resolution. As the third shaded region indicates, this noise is less than the equivalent of a single gold atom.

Gold mass loading data (TIFF 16.4 MB)
Gold mass loading data (small) (JPEG 310 KB)

Acknowledgements

This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, which provided for nanotube synthesis, detailed TEM characterization, and UHV testing of the nanomechanical mass sensor, and by the National Science Foundation within the Center of Integrated Nanomechanical Systems, under Grant No. EEC-0425914, which provided for design and assembly of the nanomechanical mass sensor.


Last modified: Thu Jul 17 17:24:55 Pacific Daylight Time 2008