The Single-Atom Transistor: perspectives for quantum electronics on the atomic-scale

Europhysics News, Jul 2018

Ch. Obermair, F.-Q. Xie, Th. Schimmel

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The Single-Atom Transistor: perspectives for quantum electronics on the atomic-scale

Ch. Obermair 1 2 F.-Q. Xie 1 Th. Schimmel 0 1 0 Institute of Nanotechnology, Forschungszentrum Karlsruhe, Karlsruhe Institute of Technology (KIT) , 76021 Karlsruhe , Germany 1 Institute of Applied Physics and Center for Functional Nanostructures (CFN), University of Karlsruhe, Karlsruhe Institute of Technology (KIT) , 76131 Karlsruhe , Germany 2 DOI: 10.1051/epn/2010403 Controlling the electronic conductivity on the quantum level will impact the development of future nanoscale electronic circuits with ultralow power consumption. Here we report about the invention of the single-atom transistor, a device which allows one to open and close an electronic circuit by the controlled and reproducible repositioning of one single atom. It opens intriguing perspectives for the emerging fields of quantum electronics and logics on the atomic scale. - PERSPECTiVES FOR QuAN TuM ElE CTRONiCS ON THE ATOMiC-SCAlE ascinating physical properties and technological In our new approach, a three-terminal, gate-controlled perspectives have motivated investigation of ato- atomic quantum switch was fabricated by electrochemimic-scale metallic point contacts in recent years cal deposition of silver between two nanoscale gold [ 1-10 ]. e quantum nature of the electron is electrodes (see Fig. 1) [ 1,6 ]. A comparison of the expedirectly observable in a size range where the width of the rimental data with theoretical calculations indicates contacts is comparable to the Fermi wavelength of the perfect atomic order within the contact area without electrons, and conductance is quantized in multiples of volume or surface defects [ 10 ]. 2e2/h for ballistic transport through ideal junctions [ 2 ]. In metallic point contacts, which have been fabricated by Switching an atom mechanically controlled deformation of thin metallic We control individual atoms in the quantum point wires [ 2-4 ] and electrochemical fabrication techniques contact by a voltage applied to an independent gate [ 1,5-7 ], the conductance depends on the chemical valence electrode, which allows a reproducible switching of the [ 2,3 ]. Two-terminal conductance-switching devices based contact between a quantized conducting “on-state” and on quantum point contacts were developed both with an an insulating “off-state” without any mechanical moveSTM-like setup [ 8 ] and with electrochemical methods [ 9 ]. ment of an electrode (see Fig. 2). Article available or FiG. 1: Schematic of the single-atom transistor: the atomic switch is entirely controlled by an independ ent third gate electrode, allowing to open and close a metallic contact between the source and drain electrodes by the gate-volt age-controlled relocation of one single silver atom. FiG. 2: Schematic of the switching process: A metal atom (see arrow) is switched between a quantized conducting “on-state” (lower graph) and an insulating“off-state” (upper graph). To fabricate the initial atomic-scale contact we deposit As soon as the conductance exceeds a preset “target” silver within a narrow gap between two macroscopic value, the deposition is stopped and the voltage is revergold electrodes (gap width: typically 50 nm) by applying sed to dissolve the junction again. Aer the an electrochemical potential of 10-40 mV to a gate elec- conductance drops below a preset value, the depositrode [ 7 ]. e gold electrodes are covered with an tion/dissolution cycle is repeated automatically by the insulating polymer coating except for the immediate computer-controlled setup. During the first cycles, the contact area, and serve as electrochemical working elec- conductance at the end of the deposition step varies trodes. ey correspond to the “source” and “drain” strongly from cycle to cycle [ 11 ]. Aer repeated cycling, electrodes of the atomic-scale transistor. Two silver an abrupt change is observed from this irregular variawires serve as counter and quasi-reference electrodes. tion to a bistable switching between zero and a finite, e potentials of the working electrodes with respect quantized conductance value at an integer multiple to the quasi-reference and counter electrodes are set by of G0 (= 2e2/h). a computer-controlled bipotentiostat (see Fig.3). All experiments are performed at room temperature, the Controlling the junction electrolyte being kept in ambient air. For conductance at the single-atom level measurements, an additional voltage in the millivolt Figure 4 shows a sequence of reproducible switching range is applied between the two gold electrodes. To events between an insulation “off-state” and a quantifabricate the atomic transistor, silver is deposited on zed conducting “on-state” (at 1 G0), where the each of the two working electrodes, until finally two sil- quantum conductance (red curves) of the switch is ver crystals meet, forming an atomic scale contact controlled by the gate potential (blue curves), as which is bridging the gap. commonly observed in transistors. As calculations While silver islands are deposited in the junction we have shown [ 10 ], for atomic-scale silver contacts a monitor the conductance between the two electrodes. quantized conducting “on-state” of 1 G0 corresponds to a single-atom contact. When we set the gate potential to an intermediate “hold” level between the “on” and the “off ” potentials, the currently existing state of the atomic switch remains stable, and no further switching takes place. is is demonstrated in Fig. 5 both for the “on-state” of the switch (le arrow) and for the “off-state” of the switch (right arrow). us, the switch can be reproducibly operated by the use of three values of the gate potential for “switching on”, “switching off ” and “hold”. ese results give clear evidence of a hysteresis when switching between the two quantized states of the switch. It can be explained by an energy barrier which has to be overcome when performing the structural changes within the contact when switching from the conducting to the non-conducting state of the switch and vice versa. e results indicate that switching occurs by a reversibly rearrangement of the contacting group of atoms between two different stable configurations with a potential barrier between them. For silver the observed quantum conductance levels appear to coincide with integer multiples of the conductance quantum [ 1,10 ]. e observed integer conductance levels of the switch are determined by the available bistable junction conformations, similar to the observation of preferential atomic configurations in metallic clusters corresponding to “magic numbers” [12]. By snapping into ‘magic’ bistable conformations, such energetically preferred junctions configurations are mechanically and thermally stable at room temperature, and they are reproducibly retained even during long sequences of switching cycles. Mulitlevel switching Reproducible switching in the above cases was always performed by opening and closing a quantum point contact, i.e. by switching between a quantized conducting state and a non-conducting state. However, it was not clear if this kind of gate-electrode controlled switching is also possible between two different conducting states of one and the same contact. Such kind of switching would involve two different stable contact configurations on the atomic scale, between which reversible switching would occur even without ever breaking the contact. Such multi-level logics and storage devices on the atomic scale would be of great interest as they allow a more efficient data storage and processing with a smaller number of logical gates. By developing a modified procedure of fabrication, a multi-level atomic quantum transistor was obtained, allowing the gate-controlled switching between different conducting states. Instead of setting the lower threshold where the dissolution process is stopped by the computer, to a value near 0 Go, the lower threshold was set at a value above the desired quantized conductance of the lower of the two “on-state” levels [ 13 ]. Figure 6 demonstrates the operation of such a twolevel transistor: A controlled change of the gate potential UG leads to a controlled switching of the conductance of the quantum point contact between two different quantized conducting states, exhibiting conductance levels of 1 G0 and 3 G0, respectively. Sharp transitions are observed between the two levels. No intermediate steps or staircase-like structures in conductance are observed in the diagram. e transitions are instantaneous within the time resolution of the diagram of Fig. 6 (50 ms). Conclusions and perspectives e development of the single-atom transistor represents a first demonstration of the functionality of a transistor on the atomic scale. is is of great interest, as there were many previous demonstrations of passive devices such as atomic-scale and molecular resistors. However, there was a lack of an actively switching device such as a transistor on the atomic scale. e atomic transistor as an • ey are a first demonstration of an all-metal transistor actively controllable device, which reproducibly operates without the use of any semiconductor, the lack of a at room temperature, is filling this gap. band gap allowing operation at very low voltages. Atomic transistors represent a new class of devices Such devices provide a number of advantages: ey poswhich show remarkable properties: sess extremely nonlinear current-voltage characteristics, • ey allow the switching of an electrical current by the desirable in many applications, and they can be manugeometrical relocation of individual atoms rather than factured using conventional, abundant, inexpensive and by locally changing electronic properties as done in non-toxic materials. At the same time, the devices open conventional transistors. perspectives for electronic switching at ultrafast frequen• ey represent quantum switches, the levels between cies: although the switching time in our current which the switching occurs being given by funda- investigations is limited by the response time of the elecmental laws of quantum mechanics. tronic measuring setup (3-5 microseconds), the intrinsic III fe Taur eS FiG. 5: Demonstration of quantum conductance switching between a nonconducting “off-state” and a preselected quantized “ on-state” at 4 G0. A conductance level can be kept stable, if UG is kept at a “hold” level (see arrows). (cf. [ 1 ]). III operation time is expected to be limited by the atomicscale rearrangement within the junction (picoseconds), opening perspectives for ultra-high frequency operation. Because the switching process is achieved with very small gate potentials in the millivolt range, the power consumption of such devices is by orders of magnitude lower than that of conventional semiconductor-based electronics. Although the development of the single-atom transistor just marks the beginning of actively controlled electronics on the atomic scale, it opens fascinating perspectives for quantum electronics and logics based on individual atoms. e development of a first, simple integrated circuit [ 1,14 ] and a multilevel quantum transistor [13] are first encouraging steps in this direction. Acknowledgment We thank Robert Maul, Wolfgang Wenzel and Gerd Schön for stimulating discussions, for the excellent FiG. 6: Example of a two level quantum transistor switch ing between the conductance levels of 1 G0 and 3 G0, respectively. The source-drain conductance (GSD) of the atomic switch (red curves) is directly controlled by the gate potential (UG) (blue curves). cooperation and for corresponding theoretical work. is work was supported by the DFG-Center for Functional Nanostructures and by the Baden-Württemberg Foundation within the Network of Excellence on Functional Nanostructures, Baden-Württemberg. About the authors Prof. omas Schimmel is Professor of Physics and Joint Institute Director at the Institute of Applied Physics, University of Karlsruhe. He is at the same time heading a research group at the Institute of Nanotechnology at the Research Center Karlsruhe, Karlsruhe Institute of Technology (KIT). Prof. Schimmel is Scientific Director of the Network of Excellence “Functional Nanostructures” and Editor in Chief of the Beilstein Journal of Nanotechnology. Dr. Christian Obermair and Dr. Fang-Qing Xie are Senior Scientists in the same group, their research focusing on quantum transport and atomic-scale electronics. Dr. Obermair is Administrative Director of the above research network. For further information see Correspondence should be addressed to . Schimmel (e-mail: ) [1] F.-Q. Xie , R. Maul , A. Augenstein , Ch. Obermair, E.B. Starikov , G. Schön, Th. Schimmel, W. Wenzel, Nano Lett. 8 ( 12 ), 4493 ( 2008 ). [2] N. Agraїt , A. L. Yeyati , J. M. van Ruitenbeek , Phys. Rep . 377 , 81 ( 2003 ). [3] E. Scheer , N. Agraıt , JC. Cuevas, A. Levy Yeyati , B. Ludoph , A. Martin-Rodero , G. Rubio Bollinger , J. M. van Ruitenbeek , and C. Urbina , Nature 394 , 154 ( 1998 ). [4] A. I. Mares , J. M. van Ruitenbeek , Phys. Rev. B 72 , 205402 ( 2005 ). [5] C. Z. Li , A. Bogozi , W. Huang , N. J. Tao , Nanotechnology 10 , 221 ( 1999 ). [6] F.-Q. Xie , L. Nittler, Ch. Obermair, Th. Schimmel, Phys. Rev. Lett . 93 , 128303 ( 2004 ). [7] F.-Q. Xie , Ch. Obermair and Th. Schimmel, Solid State Communications 132 , 437 ( 2004 ). [8] D.P.E. Smith , Science 269 , 371 ( 1995 ). [9] K. Terabe , T. Hasegawa , T. Nakayama , and M. Aono , Nature 433 , 47 ( 2005 ). [10] F.-Q. Xie , R. Maul , S. Brendelberger , Ch. Obermair, E. Starikow , W. Wenzel, G. Schön, Th. Schimmel, Appl. Phys. Lett . 93 , 043103 ( 2008 ). [11] F.-Q. Xie , Ch. Obermair and Th. Schimmel, in: R. Gross et al. (eds.), Nanoscale Devices - Fundamentals and Applications . Springer, 153 - 162 , 2006 . [12] M. N. Huda and A. K. Ray , Phys. Rev. A 67 , 013201 ( 2003 ). [13] F.-Q. Xie , R. Maul , Ch. Obermair, G. Schön, W. Wenzel, Th. Schimmel, Advanced Materials 22 , 2033 ( 2010 ). [14] Th. Schimmel , F.-Q. Xie , Ch. Obermair, Patent pending, US 2009195300 .

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Ch. Obermair, F.-Q. Xie, Th. Schimmel. The Single-Atom Transistor: perspectives for quantum electronics on the atomic-scale, Europhysics News, 25-28, DOI: 10.1051/epn/2010403