W REPORTS Properties of Metallic Nanowires: From [local conductance minima associated with the presence of disorder (20)], is correlated Conductance Quantization to Localization with atomic-scale structural transforma- tions that occur during the layer-by-layer J. I. Pascual, J. Mendez, J. Gomez-Herrero, A. M. Barb, order-disorder elongation process. The N. Garcia, uzi andm man,* W. D. Luedtke, E. N. ~o~achek, dominance of disorder and the onset of H.-P. Cheng localization in long wires (longer than the localization length) are shown by a nonlin- ear dependen& of the resistance R on the Material structures of reduced dimensions exhibit electrical and mechanical properties length of the wire t as it is pulled contin- different from those in the bulk. Measurements of room-temperature electronic transport uously [that is, In R(t) - t2]. Moreover, in pulled metallic nanowires are presented, demonstrating that the conductance char- current versus voltage data, recorded at se- acteristics depend on the length, lateral dimensions, state and degree of disorder, and lected stages in the elongation process, in- elongation mechanism of the wire. Conductance during the elongation of short wires dicate gradual loss of metallic character as (length e - 50 angstroms) exhibits periodic quantization steps with characteristic dips, the long wire narrows. correlating with the order-disorder states of layers of atoms in the wire predicted by In our experiments, contact between the molecular dynamics simulations. The resistance R of wires as long as e - 400 angstroms tip and the substrate is produced either by exhibits localization characteristics with In R(e) - e2. the application of a voltage pulse or by indentation, starting from typical STM tun- nel conditions. Once the contact is pro- duced, as indicated by the electrical current Material systems of reduced size or dimen- wire and to a decrease in the number of flowing between the two electrodes, we sionality may, and often do, exhibit proper- transverse modes or channels in the elongate theocontact by retracting the tip ties different from those found in the bulk. stretched junction. Such quantized conduc- slowly (-1 A/s). To achieve optimal con- These include quantized conductance (1, tance steps have also been observed in 0th- trol over the STM operations, we have 2) in point contacts and narrow channels er measurements (16-18), as well as the developed a digital control unit (21) that whose characteristic (transverse) dimen- formation of long Pb wires at elevated tem- allows us to break the feedback loop at any sions approach the electronic wavelength, peratures (1 9). prescribed time and act on the piezo ele- localization phenomena in low-dimensional In this report we investigate the evolu- ment (z-piezo) that controls the relative systems (3), and mechanical properties tion of room-temperature electronic trans- displacement in the z direction between the characterized by a reduced propensity for port properties in Au nanowires, from quan- tip assembly and the substrate. We carried the creation and propagation of dislocations tized conductance [in 2e2/h or 2(2e2/h) out experiments at room temperature, using in small metallic samples (4-7). Such phe- steps, where h is Planck's ~onstant] with a homemade STM heads operating either un- nomena are of considerable scientific and spatial periodicity of -2 A during elonga- der ambient conditions or at ultrahigh vac- technological interest, particularly in the tion of short (-50 A) wir$s, to the onset pf uum (UHV). Gold evaporated onto mica area of miniaturized, highly compact, elec- localization in long (100 A < t < 400 A) and A u ( l l 0 ) single crystals were used as tronic devices. ones. Combining electronic conductance samples, and Pt-Ir and Au tips were used Most studies of electronic transport phe- measurements with molecular dynamics interchangeably [our results are insensitive nomena in microconstrictions are currently (MD) simulations of the wire elongation to the kind of tip used; most likely, the Pt-Ir performed on high-purity two-dimensional process reveals that for short wires, the pe- tip apex is covered with Au atoms once the (2D) electron-gas systems havipg a large riodic occurrence of quantized conduc- tip touches the sample (5-7)]. Because the Fermi wavelength (A, = 400 A) and are tance, accompanied by characteristic "dips" wires are more easily formed in air, the data conducted under cryogenic conditions (2). In contrast, our focus here is on room- temperature transport in 3D metallic nanowires with A, = 5 A. In such systems, where the width of the constriction is of the order of A, the atomic-scale structure, im- perfections, impurities, and diffusive bound- ary scattering are expected to play an im- -20- portant role. Nevertheless, room-tempera- 3 ture measurements (8) made with a scan- - - ning tunneling microscope (STM) have [ shown othat short and thin Au nanowires O 10- (-40 A long) exhibit conductance quanti- zation (1, 2, 9-15), attributed to changes in the contact area during elongation of the J. I. Pascual, J. Mendez, J. Gomez-Herrero, A. M. Baro, Departamento de Fisca de la Materia Condensada, Uni- bpiezo Displacement (A) versidad Autonoma de Madrid, E-28049 Madrid$ Fig. 1. (A) Current and conductance in a short wire (-50 A) during elongation; this process exhibits N. Garcia, Fsica de Sstemas Pequerios, Consejo Supe- rlor de Investigaclones Cientisieas, Universidad Au- room-temperature conductance quantization steps. Dashed lines denote 2e2/h intervals. Arrows indi- tonoma de Madrid, E-28049 Madrd, Span. cate dips, associated with disordered elongation stages, which become somewhat less pronounced U, Landman, W. D, Luedtke, E. N. Bogachek, H.-P. toward the breaking of the wire, (B) Left scale: Current in a 95 A wire as it is pulled 20 A more, to the Cheng, School of Physics, Georga lnsttute of Techno[- breaking point. A voltage of 500 mVwas applied in the measurement.The smoothed line IS introduced for ogy, Atlanta, GA 30332, USA clarity. Right scale: natural logarithm of the resistance of the wire during the elongation process. The solid *To whom correspondence should be addressed. line corresponds to In R = a + b(z - z,)~ with a = 2.8, b = 2.6 X and z, = 97 A. SCIENCE VOL. 267 24 MARCH 1995 1793