Time-resolved energy transduction in a quantum capacitor

Abstract

The capability to deposit charge and energy quantum-by-quantum into a specific atomic site could lead to many previously unidentified applications. Here we report on the quantum capacitor formed by a strongly localized field possessing such capability. We investigated the charging dynamics of such a capacitor by using the unique scanning tunneling microscopy that combines nanosecond temporal and subangstrom spatial resolutions, and by using Si(001) as the electrode as well as the detector for excitations produced by the charging transitions. We show that sudden switching of a localized field induces a transiently empty quantum dot at the surface and that the dot acts as a tunable excitation source with subangstrom site selectivity. The timescale in the deexcitation of the dot suggests the formation of long-lived, excited states. Our study illustrates that a quantum capacitor has serious implications not only for the bottom-up nanotechnology but also for future switching devices.field switching | silicon | soliton C harge and energy transfers play a key role in the chemical and physical processes in materials. Manipulating the flow of energy and charge quantum-by-quantum into a specific site with the subangstrom precision would not only shed light on excitation dynamics, but also provide the capability to control the interactions at the atomic and molecular level. Manipulating their flows typically takes a form of atomically localized, dynamically switchable external field. Understanding the interaction between charge and such field may help us to handle the challenges of bottom-up nanotechnology. Scanning tunneling microscopy (STM) is capable of providing the necessary spatial resolution and highly localized field (1, 2). It had been demonstrated that the atomistic control of direct deposition of charge and energy is possible in many applications such as desorption (3), single molecular synthesis (4), dissociation (5, 6), and configuration switching (7) just to name a few, by using STM.Unexplored thus far, another route one may exploit to deposit charge and energy site selectively is the quantum dot. In a capacitor whose lateral dimension is in the subnanoscale, the strongly confined electric field between the electrodes would not only induce extra charge on their surfaces, but also place the charge in a quantum dot. Dynamic discharging and charging of such a quantum dot may serve as a unique excitation source. Conversely, energy dissipation through a quantum capacitor has serious implications for future switching devices by raising the unavoidable minimum energy cost for the logic operation (8). Such a subnanosize capacitor can be realized between the STM tip and the conducting counterelectrode. On semiconductor surfaces without conducting surface states such as InAs (110), a zero-dimensional quantum dot is formed in the bulk by tip-induced localized band bending (9). Typical STM geometry produces capacitance on the order of 10 −18 F (10), and thus the dot may be charged with only a few electrons per volt.Ordin...

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