A device that can control individual electrons with subnanometer and subfemtosecond precision could be the latest small electronic device developed. In vacuum nanoelectronics, where electrons travel through a vacuum from a nanoscale emitter to the target electrode, tremendous progress has been made in the control of ultrafast electronic processes over the past few decades.
Electronic Structure of Solids
Hirofumi Yanagisawa of the Japan Science and Technology Agency and colleagues have made significant progress towards achieving optimum spatial control by using orbitals of a single molecule to control electron emission. While this method gives the chance to create highly ordered electron emitters, it also improves our knowledge of the function of molecular orbitals in the electronic structure of solids.
Studies on Electron Emission
Determining the point at which electrons emanate from the emitter is crucial to achieving extraordinary control over the electron emission. One method is to physically mold the emitter's material to create the desired dot pattern. But doing this at the sub-nanometer scale would present significant material and manufacturing challenges. Instead, Yanagisawa and colleagues have shown that it is a brilliant idea to redirect electrons for emission using a molecule's innate electronic structure. The emission pattern is essentially spatially filtered by molecular orbitals.
The team's research stems from two main areas where significant progress has been made in recent years. One focuses on the investigation of femto- and attosecond electron dynamics and the development of ultrafast electron sources, as evidenced by the 2006 demonstration of precise spatial control over femtosecond electron pulses via emission from a nanoscale metallic tip. The second is the investigation of electron emission patterns from molecular and nanoscale structures. Examples include designs representing nanotube and nanowire tip architectures that change as the tip evolves during nanotube growth. Yanagisawa and colleagues combined ultrafast emission and emission microscopy techniques, showing that emission patterns can be closely related to certain chemical orbitals.
Molecular Orbital Studies
The scientists coated a tungsten tip with a layer of C60 molecules (also known as “buckyballs”). Individual C60 molecules emerged from the layer and emitted electrons from the underlying tungsten reservoir when exposed to a strong electric field. When the emission pattern was projected onto a phosphor screen, patterns such as dots, rings, crosses, or "two-leaf" patterns appeared that nearly mirrored the spatial symmetries of the empty molecular orbitals of C60 through which electrons passed in the emission path. Based on calculations using density functional theory (DFT), the researchers were able to reproduce these patterns.
Yanagisawa and colleagues first observed emission patterns under a strong electric field, then significantly weakened the field to avoid field-induced emission, and instead used laser pulses to illuminate the tip to create two-photon photoelectron emission. When photoelectron emission occurred, the two-photon excitation energy pushed the electrons above the Fermi level as they passed through an empty C orbital near the Fermi level of the tungsten reservoir, in contrast to the field-induced emission. Electrons have been given an additional boost, allowing them to pass through a new, higher energy vacant molecular orbital. The researchers discovered that the emission pattern changes accordingly when switching between field emission and photoemission.
These findings therefore provide strong support for the idea that excitation energy determines which chemical orbitals electrons will pass through before leaving the C60 molecule. Furthermore, in line with this finding, they showed that even spontaneous changes in the emission pattern during pure field emission can be attributed to the opening of new molecular orbitals to be used as emission pathways as a result of small displacements of the molecule caused by thermal energy.
The underlying electronic structure of a molecule has been used to demonstrate sub-nanometer spatial control of electron emission, paving the way for a fascinating new method for creating electron sources with precise and desirable emission properties. Contributing to the development of the electronics industry in the first half of the 20th century, vacuum devices are still very important in high-speed and high-power applications, as electrons move freely in vacuum. Nanoscale vacuum electronic devices are becoming more and more desirable as solid-state technology becomes harder to scale down. It will be fascinating to observe how the current work is progressing. Will emission timing and noise in the emission stream be affected by orientation over certain chemical orbitals? What effects will this have on electron-optical properties?
Can similar results be achieved with other molecular structures—carbon nanotubes come to mind right away? Many applications are possible, including vacuum transistors, highly cohesive electron-emitter arrays, and desktop accelerators.
Regardless of these potential future effects, the work itself is fascinating. We are aware of the strong interactions between electrons. The multibody wave function is the cornerstone for the accurate representation of any system, whether it's a molecule, a nanoscale element, or a macroscopic mass. Molecular orbitals, which are single-electron functions, are used in DFT because their validity is approximate, but because they are mathematically and intuitively convenient.
It is fortunate that DFT and even simple single-electron models have succeeded in accurately describing a wide variety of material systems, but this success should not be taken for granted. For example, strongly correlated materials sometimes need more complex manipulations or at least corrections to single-electron model results. The work of Yanagisawa et al gives us direct visual access to DFT orbitals in a molecule, further supporting the idea of single-electron wave functions empirically.
Source: physics.aps.org/articles/v16/35
Günceleme: 09/03/2023 11:06