Principle Scientist: Cheng Xiang
Chengxiang Xiang, Jung Yun Kim, and Reginald M. Penner*, "Reconnectable Sub-5nm Gaps in Ultra-Long Gold Nanowires", Nano Letters 9 (2009) 2133.pdf.
Nanogaps, consisting of nanometer-scale gaps in metal wires, have received a lot of attention mainly because they provide means for the study of electron transfer through molecules that can be located within this gap. But one can imagine a myriad of applications that exploit the ability to reproducibly form electrical contacts to a nanometer-scale void (that is, the nanogap) that can be filled with a functional material, such as a semiconductor. This was our motivation for developing an automated method for forming nanogaps in gold nanowires prepared using LPNE.
Figure 1. a) Optical micrograph of the device used to study the electromigration of long gold nanowires. b) SEM image of a single gold nanowire that was electrically contacted by two nickel pads. c) and d) Low and higher magnification SEM images of a typical gold nanowire used for these investigations.
Starting with a gold nanowire with typical dimensions of 40 nm x 100 nm (Fig 1), we first developed an algorithm for producing a nanogap. Nanogaps were produced using a programmed voltage algorithm that was controlled by a Labview program. The process flow for this program (Fig 2a) involved: 1) the application of an initial voltage bias (Eapp,i = 10-100 mV) and the measurement of an initial wire resistance, R0, 2) Eapp was then increased in a ramp at a rate of 5 mV/s, and the wire resistance was simultaneously measured, 3) when the resistance change ratio (R/R0) = R0 exceeded a predefined threshold (typically 1.5%), a new reference resistance value was measured at Eapp = 0.95Eapp,i and the cycle was repeated. This program terminated when the formation of a nanogap was signaled by the measurement of R > 10 kohms (Fig 2).
Figure 2. a) Flow chart for the Labview algorithm governing the feedback-controlled electromigration process. b) The applied potential (Eapp) and the nanowire resistance (R) as a function of time for the initial formation of a nanogap in a fresh nanowire. c) The current-voltage trace for the electromigration process for the fresh nanowire (dotted green line) and reconnected nanowire (solid green line). The arrow indicates the nanowire failure point. d) The applied potential (Eapp) and the nanowire resistance (R) as a function of time for the reconnected nanowire. e) The current-voltage trace for the electromigration process for the reconnected nanowire. The arrow indicates the failure point.
Here are some typical images of the nanogaps produced by this algorithm (Fig 3):
Figure 2. a), b), c) and d) High magnification SEM images of nanogaps formed using the algorithm of Fig 2a.
We were surprised to discover that these nanogaps can be closed simply by pulsing or scanning the potential above a threshold of 2-3 V (Fig 4). The success rate for effecting this re-closure of a nanogap was 42%. And nanogaps that reclose usually can be cycled between open and closed states many times simply by repeating the nanogap formation algorithm to close the nanogap, and then pulsing the potential above the closure threshold potential (Fig 4).
Figure 4. a) I as a function of Eapp plotted during the reconnection process. The dashed blue line is a linear extrapolation of the initial I-V curve measured for the initial fresh nanowire. b) Histogram of the reconnection threshold voltage for ten different nanowires. c) Histogram of the failure current density for fresh gold nanowires.
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