Direct Measurements of Ultrafast Structural Dynamics in Graphene by Low Energy Femtosecond Electron Diffraction Duan Luo1,2, Xing Wang1，Jinshou Tian1, Wenlong Wen1 1 Key laboratory of Ultrafast Photoelectric Diagnostics Technology, Xi’an Institute of Optics and Precision mechanics(XIOPM), Chinese academy of Sciences(CAS), Xi’an, 710119, China 2 The University of Chinese Academy of Sciences (UCAS), Beijing 100049, China 1
Motivation
Photocathode
“If you want to understand function, study structure.”--(Francis Crick) …but to know structure is not enough! “If you want to understand function, study time-dependent structures.”
Table 2: Current capabilities of UED system in our lab
Magnetic lens X-Y Deflector
Anode
Graphene is a promising material for future applications in optoelectronics, ultracapacitors, and biosensors. Despite its impressive development in recent years, there are still many challenges on the way to these applications. For example, the high-quality and reproducibility to obtain large area epitaxial graphene films, the necessary to induce a bandgap at the Dirac point. A microscopic understanding of the ultrafast structural dynamics (femtosecond time scale) is crucial for designing and engineering graphene-based devices. However, many important aspects of experimental data have not yet been fully complemented with theoretical studies on a microscopic footing. Time-resolved electron or x-ray diffraction based on pump-probe techniques have become a promising tool with sufficient temporal precision to directly deliver insights into ultrafast phenomena at the atomic level.
Figure 4: Schematic of ultrafast electron gun.
Femtosecond laser Acceleration energy Electron beam spot size Lattice spatial resolution Temporal resolution Sample temperature
60fs, 1mJ/pulse@2kHz 50keV 100μm 0.00026𝑛𝑚 494fs ~ 8 K - 325 K
4D sample holder
Double-MCP detector Ultrafast electron gun
Photocathode
CCD Electrons
Probe laser
UHV sample chamber Figure 5: Femtosecond Electron Diffractometer in our lab
3 Figure 1: Some phenomena in terms of relevant spatial and temporal resolutions (left) [1] and Laser induced lattice structural changes and the corresponding Bragg peak responses in diffraction patterns ( right) [2]. (a) Uniform expansion. (b) Inhomogeneity. (c) Disorder.
Compared with ultrafast X-ray diffraction, there are some advantages of using ultrafast electron diffraction, which is shown in table 1 [3]. Table 1: Comparison of properties of electron and x-ray probes[2] Ratio of inelastic/elastic scatering events Energy deposited per inelastic event Scattering cross section Penetration depth Focusing and manipulation Spatial resolution(Δx/x) complexity and expense Challenges
2
Electrons (50-500 kV)
X-rays (0.15 nm)
3 20 eV 1 × 10−18 𝑐𝑚2 ≈ 10−9 𝑚 fairly easy 10−4 (in diffraction) UED: table-top system space charge broadening
10 8000 eV 1 × 10−24 𝑐𝑚2 − 1 × 10−23 𝑐𝑚2 ≈ 10−4 𝑚 difficult 10−2 (in diffraction) X-ray: Synchrotron Radiation Facility photon fluxes
Results
On the first step, the ordered-to-disordered phase transitions of polycrystalline Al and Au under strong driving conditions were investigated. Figure 6(a) is a diffraction pattern of polycrystalline freestanding thin-film aluminum of 20 nm thickness obtained by the UED system, where four ring patterns can be observed. Figure 6(c) is the Bragg peak intensities and their assignments corresponding to the lattice structure. Currently, we are focusing on the time-resolved study of the vibration of lattice in monolayer and multilayer graphene induced by optical excitation. Figure 7(a) shows the epitaxial graphene film on the TEM grid and the diffraction pattern is shown in figure 7(b). More work on this project is on going.
Ultrafast electron diffraction system in XIOPM
(110) (200)
Pump-probe technique+ Electron diffraction ultrafast temporal resolution & ultrahigh spatial resolution
Magnetic lens
Double-MCP-detector
Electron pulse δ3 UV pulse δ1
(311)
Figure 6: Diffraction pattern of polycrystalline free-standing thin-film aluminum of 20-nm thickness (Left Panel) and its corresponding radial averaged intensity curve (Right Panel). It is recorded with 30 keV beam energy.
cryostat
Probe Electron pulse δ2
(220)
graphene
Pinhole CCD
266nm
Sample X-Y Deflector 5mm 800nm
photocathode
V1
Anode
V2 V3
Pump Laser pulse δ4
Figure 2: Schematic of working principle of UED.
The main components of our ultrafast electron diffraction system are: 1) a femtosecond laser system; 2) an optical pump probe setup; 3) an ultrafast electron gun (including photocathode, anode, magnetic lens and X-Y Deflector, refer to figure 4); 4) an ultrahigh vacuum sample chamber; 5) an ultra low vibration closed cycle refrigerator system; 6) a detection system (including double microchannel plates, phosphor screen and CCD). In Figure 3 an overview of the UED experiment in our laboratory is shown.
Figure 7: 20X micrograph and diffraction pattern of graphene sample
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An ultrafast photo-electron diffractometer has been described in detail. This device depends upon an ultrafast laser to deliver optical pulses to “pump” a sample, as well as to “probe” the structure by means of photoelectrons. Experiments have been conducted in transmission mode with unsupported polycrystalline thin films of aluminium and graphene. These experiments show that it is possible to probe dynamically for structure changes caused by intense but nondestructive laser pulses at the sub-picosecond time scale.
UV Generation 800nm M12 𝝎
266nm THG-FS
𝟑𝝎
Probe pulse
Attenuator
BS (5:5) M4 M1
Laser 800nm/1.55eV 1mJ/pulse@2kHz
M3
UHV sample chamber
Deflector Photocathode Magnetic lens Anode Cryostat L1
Attenuator M2
Diffraction pattern
M13
M14
Pinhole MgF2 Window
Periscope
M5
5mm
Double-MCP detector ICCD
Sample
Electron probe Optical Pump L2 M10
Variable Delay Line
M11
800nm M8
M9 Periscope
Pump pulse M6
M7
Figure 3: Schematic of our FED setup.
Conclusion
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References
1. J. Appl. Phys. 97, 111101 (2005); doi: 10.1063/1.1927699. 2. Ziegler A. Ultrafast materials science and 4D imaging with atomic resolution both in space and time[J]. MRS bulletin, 2011, 36(02): 121-131. 3. Rousse A, Rischel C, Gauthier J C 2001 Rev. Mod. Phys. 73 17 This work is supported by The National Natural Science Foundation of China (NSFC) under grant No. 11304374 corresponding author:
[email protected] [email protected]
