Measuring masses with a few atoms sensitivity

Oscar E. Vilches, University of Washington, DMR 0906690

This grant supports the research of faculty, graduate and undergraduate students studying the interaction between atoms and simple molecules deposited (adsorbed) on the graphite-like surface of single wall carbon nanotubes and associated changes in the nanotube electrical and mechanical properties. We are carrying out two main projects:

• A. The development and use of an extremely sensitive mass balance consisting of a single carbon nanotube as adsorbent surface and detector of individual atoms deposited on it, the focus of this slide, and

• B. Measurement of (three) adsorption isotherms on the same sample of carbon nanotube bundles at various temperatures from 4.2K to 125K, for most rare gases and simple molecules to quantify the onset of quantum effects

The mass sensitivity of the balances is about 10-24 kg.

We are studying the phases of Ar, Kr, Ne and 4He deposited on single nanotubes. At the same time, we measure the electrical conductivity of the nanotubes as a function of amount of gas adsorbed on their surface. Our research may lead to the development of very sensitive practical mass detectors, past the study of phases of matter in this particular type of nano-surface.

A

D

A. Cartoon of ultrasensitive mass balance: a single wall carbon nanotube is freely suspended between electrodes. It is one-thousand times longer than its diameter. We measure its vibration frequency, which is inversely proportional to the square root of the mass of the nanotube plus the mass of the atoms deposited on its surface.

A

C

Coverage is the fraction of Kr atoms to C atoms, Pressure is the pressure of Kr gas in equilibrium with the adsorbed gas on the surface. The vertical jump in Φ and R occur at a phase transition of the adsorbed Kr (vapor-liquid?)

B. Mass adsorption isotherm (Φ vs. P) and simultaneous resistance (conductance) measurement (R vs. P) for Kr adsorption on a single carbon nanotube at 77.4K. A potential difference (gate voltage) between the bottom of the trench and the nanotube controls the tension. Vertical arrow on inset shows Vgate at which main graph was measured.*

*Z. Wang, J. Wei, P. Morse, J.G. Dash, O. E. Vilches and D. H. Cobden, Science 327, 552 (2010)

The two projects mentioned involve several graduate and undergraduate students, all being trained in a variety of techniques useful in surface science and nano-technology: growth of carbon nanotubes, fabrication of devices, lithography, optical and electron microscopy, atomic force microscopy, area calibrations, sensitive electronic measurements, cryogenic techniques and the thermodynamics of adsorption.

Participants: Undergraduate: Erik Frederickson (junior), Evan Mattson (junior), An Hua (senior, BS 2010), Ben Thomas (junior), Justin Zeck (sophomore), Graduate: Zenghui Wang (PhD 2010), Jiang Wei (PhD 2010), Hao-chun Lee (2nd year), REU: Ricky Roy (UPS, Tacoma, WA), Faculty: David Cobden and Oscar Vilches (co-PIs).

A. Ben Thomas (l) and Evan Mattson (r) in front of Evan’s poster at the 2010 Undergraduate Research Symposium, UW. Evan, Ben and Justin have been doing the adsorption isotherms survey (so far 3He, 4He, H2, Ar, Kr, N2) B. From left to right, Ricky Roy, Zenghui Wang, Erik Frederickson and Hao-chun Lee pose in front of new cryocooler based low temperature adsorption isotherm system. They are doing the measurements described in the previous slide.

A

C

A

B

Measuring masses with a few atoms sensitivity

Oscar E. Vilches, University of Washington, DMR 0906690