This project will measure the properties and phases of matter in the form of a single layer of atoms or simple molecules deposited on one, two, or bundled single-walled carbon nanotubes. With already designed, constructed and tested very sensitive one- and two-suspended nanotube mechanical oscillators vibrating in the upper megahertz range to act as extremely sensitive mass balances (at the level of a few atoms), this project will measure mass adsorption isotherms using atoms from the He isotopes to Xe, and the interesting molecules of hydrogen, nitrogen and oxygen. Due to the pristine surface and very low mass of the suspended nanotubes, this project will map accurately phase boundaries, measure critical and tri-critical parameters of two-dimensional (2d) matter, study 2d to one-dimensional (1d) crossover behavior as a function of temperature and adsorbed mass, obtain excellent values of heats of adsorption for both classical and quantum mechanical monolayers, and study the effect of surface adsorbates on the electronic properties of the substrate, which may lead to the fabrication of new ultra sensitive matter specific detectors. For those systems of interest, this project will measure the crystalline structure and the phonon spectrum of the adsorbates through an international collaboration using the facilities in Grenoble, France. Graduate and undergraduate students participating are being trained in contemporary electronic, thermodynamic, scanning and scattering measurement techniques, all applicable to nanotechnology.
Films of atoms or molecules just one layer thick (said to be "two-dimensional") condense and solidify at lower temperatures than regular "three-dimensional" matter. Lines of atoms (the extreme "one-dimensional" case) are expected not to condense or solidify even all the way down to absolute zero of temperature. To explore this fascinating limit, this project will measure the properties of matter in the form of a single layer of atoms or simple molecules confined to the surface of one or two individual single-walled carbon nanotubes. With nanotube mechanical oscillators which we have already designed, constructed and tested acting as extremely sensitive mass balances (which can detect just a few atoms on the surface), this project will measure mass deposition at many different temperatures, with atoms ranging from the isotopes of helium (low mass) to xenon (large mass), and the molecular forms of hydrogen, nitrogen and oxygen which are of great practical interest. This project also will study their effect on the electrons moving in the nanotubes, which may lead to new, more sensitive and gas-specific detectors. For certain systems this project will also measure the arrangement and the vibrational spectrum of the adsorbates through an international collaboration using facilities in Grenoble, France.This project involves graduate and undergraduate students who are being trained in contemporary electronic, thermodynamic, scanning and scattering measurement techniques applicable to nanotechnology, over a wide range of temperatures.
We were able to perfect a nano-size, extremely sensitive mass balance, for use in studying single layers of atoms (Z. Wang et al, Science, Vol. 327, p. 552, year 2010). We have used a technique developed elsewhere (Cornell U.) for measuring the frequency of a vibrating, single wall carbon nanotube (a very long pure carbon tube about a million times smaller diameter than a human hair, suspended across a trench of width a hundred times smaller than the diameter of the same human hair). It acts like a guitar string vibrating at radio frequencies (about 100 MHz), rather than the much lower sound frequencies (15 Hz to 20 kHz). The sensitivity of our mass balances is the mass of a few atoms. In parallel, we developed techniques for sensitive measurements of the electrical conductance of the suspended nanotube,and how it is affected by its coating. The overall outcome has been very detailed measurements of the plating (adsorption) of the suspended nanotube by various non-reactive gases, all done over a wide range of temperatures. We studied helium, argon, krypton and xenon, between -268 C (five degrees above abosolute zero) and -128 C. We were able to observe some of the two-dimensional gas, liquid, and/or solid phases known to occur for the same gases adsorbed on graphite and other substrates. We also observed other phases not clearly visible before (like a two-dimensional liquid Kr layer). In addition to the adsorption studies, we have been able to measure the small changes in the conductance of the nanotube by the plating made from few to many adsorbed atoms, a line of work with promise for mass and species sensitive devices (see H-C. Lee et al, Journal of Low Temperature Physics, Vol. 169, p. 338, year 2012). Three doctoral students and seven undergraduate students have participated in this project, where they mastered vacuum, electronic, cryogenic, atomic force and scanning electron microscopy techniques, as well as various aspects of our in-house, private vendor and national facilities capabilities for the fabrication of nanodevices. The images attached show a) A set of Argon isotherms at various temperatures; phi = 0.17 is one atomic layer coating, and b) A scanning electron microscope image of a suspended nanotube. This device was entirely built in our lab. For further discussion see the two references above. D. H. Cobden's Nano Devices lab web site, https://sharepoint.washington.edu/phys/research/nanodevicelab/Pages/default.aspx, has frequent updates on progress along this line of research. The last period of this report corresponds to a no-cost extension that allowed us to perfect the technique for reliable fabrication of devices. Most of the outcomes described above were done during the previous three years.
