A new study led by Professor DAI Qing’s and Professor LI Chi’s team from the National Center for Nanoscience and Technology (NCNST) of the Chinese Academy of Sciences (CAS) has demonstrated the coherent ultrafast photoemission from a single quantized energy level of a carbon nanotube. Collaborators include Professor MENG Sheng's team from the Institute of Physics, Chinese Academy of Sciences, Professor LIU Kaihui's team from Peking University, Professor WAN Xiangang's team from Nanjing University, and Professor DAI Jiayu's team from the National University of Defense Technology.
The work was published in Science Advances on Oct. 12.
Exploring dynamical processes at extreme spatiotemporal scales is pivotal for scientific and technological advancements. This is particularly true in the microscopic realm, where most movements are ultrafast, especially at the atomic spatial scale where ultrafast processes can reach durations of a few femtoseconds or even attoseconds. Consequently, ultrafast characterization techniques are fundamental for scientific and technological progress. In recognition of this, the Nobel Prize in Physics this year was awarded for research pertaining to attosecond light pulses. Ultrafast electron pulses, in comparison to ultrafast light pulses, offer both high temporal and spatial resolution. This positions them as a promising next-generation ultrafast characterization technology that could potentially exceed attosecond light pulses.
The monochromaticity of the electron source is vital for achieving high spatial resolution. However, the strong interaction between electrons and the optical field results in excited electrons occupying a wide range of energy levels. This leads to significant energy dispersion (>600meV) in ultrafast electron sources that rely on traditional metal nanostructures. To address this issue, Dai Qing's team proposed the use of carbon nanotubes as ultrafast electron source materials, replacing conventional metal nanostructures. In their previous work, by leveraging the size effect and quantum effect of carbon nanotubes, they achieved a low energy dispersion of 0.25eV (Advanced Materials, 2017, 29(30): 1701580) and unprecedented extreme nonlinear ultrafast electron emission (Nature Communications, 2019, 10(1): 4891). Building on this achievement, Dai Qing's team successfully constructed a double-barrier structure at the tip of the carbon nanotube. This unique structure supports both resonant tunneling and single-electron emission (Advanced Materials, 2023, 3, 2300185).
In the latest research, DAI’s team used single-walled carbon nanotubes with a diameter of approximately 2nm as emitters, successfully achieving ultrafast resonant tunneling single-electron emission (as shown in Figure 1). Initially, they employed Time-Dependent Density Functional Theory (TDDFT) for simulation and discovered that a depletion layer barrier could form between the carbon nanotube’s cap and its body. This, in conjunction with the vacuum barrier, forms a double barrier structure, enabling the zero-dimensional cap to serve as an electron resonant cavity, supporting both resonant tunneling and Coulomb blockade effects.
Figure 1. (a) Schematic diagram of ultrafast electron emission from carbon nanotubes. (b) TDDFT calculation results show that a depletion layer barrier can be formed at the tip of the carbon nanotube.
Subsequently, they finely tuned the double barrier structure at the tip by controlling the carrier concentration through operating the local temperature (as shown in Figure 2). As a result, they successfully observed the phenomenon of laser-induced Negative Differential Resistance (NDR), proving the effect of resonant tunneling. The adjustable peak distance of the negative resistance peak also suggested the presence of energy level renormalization in the cap, hence supporting the Coulomb blockade-controlled single-electron emission mechanism.
Figure 2. (a) Experimental observation of negative differential resistance in ultrafast electron emission. (b) The dependence of the peak-to-peak distance of the negative resistance peak on temperature.
Furthermore, they observed the splitting phenomenon of the NDR peak (as shown in Figure 3). TDDFT simulations confirmed that this phenomenon is due to Stark splitting of two degenerate quantum states caused by the combined effect of the static field and laser field. This indicates that quantum energy levels can be further fine-tuned to achieve more controlled electron emission. By assessing the degree of energy level splitting and combining it with time-dependent first-principles calculations, it was estimated that the electron emission energy spread is approximately 57meV, which is an order of magnitude lower than that of metals.
Figure 3. (a) Splitting phenomenon of the negative resistance peak. (b) Using TDDFT calculation, the energy value corresponding to the splitting is estimated to be about 110meV (corresponding to about 11.6V bias), and the electron emission energy spread is estimated to be about 57meV (corresponding to about 6V bias).
"By utilizing the unique atomic structure of carbon nanotubes, it is possible to achieve an ultrafast coherent electron source close to the time-energy uncertainty principle limit. This could enable electron probes to have sub-angstrom spatial resolution and femtosecond time resolution, which is of great significance for many scientific and technological applications, including attosecond electron microscopy." said Professor DAI.
Contact:
Professor DAI Qing
National Center for Nanoscience and Technology
