Surface electron perturbations and the collective behaviour of atoms adsorbed on a cylinder

Abstract

A single-walled carbon nanotube presents a seamless cylindrical graphene surface and is thus an ideal adsorption substrate for investigating the physics of atoms and molecules in two dimensions and approaching the one-dimensional limit [1][2][3][4][5][6][7] . When a suspended nanotube is made into a transistor, frequency shifts of its mechanical resonances allow precise measurement of the adsorbed mass down to the single-atom level [8][9][10] . Here we show that its electrical characteristics are also modified by the adsorbed atoms and molecules, partly as a result of a small charge transfer between them and the carbon surface. We quantify this charge transfer, finding it similar for many di erent species, and use the associated sensitivity of the conductance to carry out the studies of phase transitions, critical scaling, dynamical fluctuations and dissipative metastable states in a system of interacting atoms confined to a cylindrical geometry.Graphite is the substrate of choice for investigating the collective two-dimensional (2D) behaviour of adsorbed atoms and molecules, including transitions between 2D solid (S), liquid (L) and vapour (V) phases 11,12 . In addition, the physical interactions of adsorbates with carbon are important in filters, electrodes, sensors and gas storage, but very little is known experimentally about their effects on the electrons in the surface. Using the unique combination of assets of suspended nanotube transistors, held in equilibrium vapour 6 , we are able to detect the charge transfer from neutral atoms or molecules. Surprisingly, it is of a similar magnitude for all the simple gases tested ( 4 He, Ar, Kr, Xe, N 2 , CO and O 2 ), and although small (much less than predicted [13][14][15] ), at gate voltages near threshold it can produce a large change in conductance. Thus, simply by monitoring the conductance we are able to explore the phase transitions of atoms on a cylinder, seeing 2D critical and triple points and critical behaviour matching the 2D Ising universality class with a finite-size cutoff. We also observe intriguing features in the phase transition dynamics, and discover nonlinear effects of adsorbates interacting with electrical current.Each device, containing a nanotube of diameter ∼2 nm and suspended length ∼1 µm, is mounted in a vapour cell at temperature T and pressure P, the latter being deduced from the pressure P g on an external gauge, as indicated in Fig. 1a (see also Supplementary Information 1). The nanotubes have small bandgaps, producing a minimum in the conductance G near zero gate voltage 16 V g , as can be seen in the characteristics of a device (YB11) shown in Fig. 1b. At lower temperatures, contact barriers cause the nanotube to act as a single-electron transistor exhibiting reproducible Coulomb blockade (CB) oscillations, visible in the characteristic at 4.3 K (blue).Using the arrangement indicated in Fig. 1a, while measuring the conductance we can also detect mechanical resonances and deduce the coverage, φ, which is the number of adsorbates pe...

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