Recently, Dai’s Group from the National Center for Nanoscience and Technology (NCNST) has made important progress in the research of Carbon Nanotubes based spatio-temporal coherent electron source, and it is expected to provide a new technology for "atomic manufacturing". Related research result has been published on Nature Communications (https://doi.org/10.1038/s41467-019-12797-z). Collaborators include the team of Professor Kaihui Liu from Peking University, the team of researcher Sheng Meng from the Institute of Physics of the Chinese Academy of Sciences, the team of Professor Jiayu Dai of the National University of Defense Technology, and Professor Feng Zhai of Zhejiang Normal University and Professor Zhipei Sun of the Aalto University of Finland.
"Atomic manufacturing" is expected to break through the bottleneck of Moore's Law, which will trigger the revolution of the electronics industry, and is a significant research area that China is laying out. The ultimate goal of "atomic manufacturing" is to precisely fabricate materials and control performance at the atomic level, which puts higher demands on the spatial and temporal resolution of the manufacturing method (on the order of Angstrom and Attosecond, respectively). Previous researchers used the probe tip of a scanning tunneling microscope to "handle" a single atom to initially achieve atomic-level spatial accuracy, but the temporal accuracy of the mechanical operation is much lower than that required by atomic manufacturing.
The electron wavelength is close to the atomic scale, and the electron beam probes can be used to replace the nano-tips to achieve more modulation functions and higher temporal resolution. In recent years, it has been reported that the picosecond-nano spatio-temporal resolution can be achieved by using a scanning transmission electron microscope combined with an electron pulse excited by an ultrafast laser. However, to control the position, charge state and spin state of one to hundreds of atoms on the time scale of atomic motion (the order of the femtosecond or even the attosecond), it is necessary to further improve the spatio-temporal resolution of ultrafast electron pulses, that is, its spatio-temporal coherence. Both temporal coherence and spatial coherence require that the electronic pulse have a very low energy spread. In addition, the former also requires the electronic pulse to have laser phase synchronism, while the latter also requires the electron beam to have high collimation.
There are two work mechanisms of ultrafast electron sources that have been reported: multiphoton emission (relative weak light) and optical-field emission (relatively strong light). Multiphoton emission can achieve lower energy spread (0.7 eV, Nature 521, 200, 2015), but due to the process of photon absorption and conversion, the angle and time of electron emission are relatively random, so the spatio-temporal coherence of the electron beam is difficult to be further improved. In contrast, the electron pulse emitted by opical-field-driven has the advantage of optical phase synchronization, beam alignment by optical-field, and tip emitting. In recent years, it has become an important technical path for realizing a high spatio-temporal coherent electron source. In the previous, the research group successfully realized the optical field emission by using the metal nanotip with a diameter of 20 nm. However, due to the size effect of the tip, the excitation wavelength and the energy spread of the emitted electrons are mutually constrained. It has been found that, to access optical field emission mode, laser wavelength greater than 800 nm is required to obtained sufficient ponderomotive potential. This results in a large energy spread greater than 30 eV (Nature 483, 190, 2012), which is far from the requirement of TEM (<0.7 eV). Therefore, further reducing the energy spread of optical-field emission is a key scientific issue in the construction of high temporal (attosecond) and spatial (Angstrom) coherent electron source.
In recent years, Dai's group systematically studied the effects of various factors such as chirality, surface defects, adsorption molecules, geometric morphology, arrangement density, and modulation voltage of carbon nanotubes on the performance of electron sources (Carbon 89, 1, 2015; RSC Advances 5, 105111, 2016; IEEE Electron Device Letters 35, 786, 2014); By designing boron nitride-carbon nanotube heterostructures, the negative effects of surface adsorption on emission performance are effectively avoided, the effective work function of the surface is reduced, and the probability of electron tunneling is increased (Small 11, 3710, 2015); clarifying the effect of light field on carbon nanotube field emission (Applied Physics Letters 104, 113501, 2014, ACS Applied Materials & Interfaces 7, 2452, 2015; Nanoscale 7 , 4242, 2015); combining with theory, charge transition and transport mechanism are analyzed, proved that carbon nanotubes have excellent photothermal electron emission performance (Carbon 96, 641, 2016; Small 14, 1800265, 2018) and significant local optical field Enhancement (Applied Physics Letters 110, 093105, 2017).
Based on the above research work, Dai's group proposed, for the first time, to replace the metal nanotips with carbon nanotubes to realize the optical-field driven electron pulse with low energy spread due to the advantage of the structure and energy band of carbon nanotube, and made a series of progress: Firstly, the special single-walled carbon nanotubes were prepared by an optimized growth process, and the optical field enhancement factor was improved by one order of magnitude. It broke through the bottleneck of the insufficient ponderomotive potential of the previous launching tip. For the first time, the optical field emission is access under visible laser. The emitted electrons are obtained with energy spread as low as 0.25 eV by optimizing the structure of the carbon nanotubes, which satisfy the requirement of electron beam energy spread (<0.7 eV) of atomic resolution (Advanced Materials 29, 1701580, 2017, cover story).
Compared with the metal tip, the carbon nanotube has a hollow structure, which can greatly reduce the electron backscattering effect. Besides, the quantized band structure (Van Hove singularity) also provides more possibilities for optical field emission modulation. After in-depth research, it is found that 40th order extreme nonlinear photoemission is achieved using semiconducting carbon nanotubes, and the phase modulation performance is 5 times higher than that of metal tip and approaching the theoretical limit (the phase modulation depth is 100%). The result was published online by Nature Communications on October 25, 2019 with the title "Extreme nonlinear strong-field photoemission from carbon nanotubes" (https://doi.org/10.1038/s41467-019-12797-z).
The series of research has been supported by the National Key R&D Program of China (2016YFA0202000), the National Natural Science Foundation of China (11427808), the Key Research Program of the Chinese Academy of Sciences (ZDBS-SSW-JSC002), and CAS Interdisciplinary Innovation Team (JCTD-2018-03). It provides assistance for the realization of atomic-level spatio-temporal resolution electron sources by using novel nanomaterials.
Diagram of OFE from CNT
Extremely nonlinear (40th order) OFE
Sensitive CEP modulation effect with 100% modulation depth