b. Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 211816, China;
c. Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, China;
d. University of Chinese Academy of Sciences, Beijing 100049, China
The fast development of semiconductor technology in recent decades has dramatically changed the world and improved our life. Semiconductor devices' increasing performance requires to be continually smaller and faster. New materials, such as 2D materials with outstanding electronic and optoelectronic properties, are also pursued to construct new devices with improved characteristics [1-6]. The transport and relaxation of carriers at semiconductor devices' heterojunction, which take place on the nanometer spatial scale and the picosecond to femtosecond time scales, play the key role in determining their functions. These critical small-scale details have attracted a great deal of attention from both experimentalists and theoreticians in both scientific and industrial fields over the past decades. The time-resolved photoemission technique has been very successful in investigating the dynamics of excited electrons on ultrafast time scales. The development of angle-resolved photoemission electron spectroscopy (ARPES) combined with the time-resolved technique has offered additional momentum information, allowing the further understanding of ultrafast electron dynamics [7, 8]. However, due to the lack of spatial resolution in the time-resolved photoelectron spectroscopy, the ultrafast electron dynamics of small samples in micrometer scale has been rarely measured with this method [9-11]. Although the sub-micrometer and even tens of nanometer-scale ARPES has been established by focusing synchrotron lights with Fresnel Zone Plates , time-resolved spectroscopy measurements are difficult and achieved only with tens of picoseconds time resolution. In recent years, the development of the time-resolved photoemission electron microscopy (TR-PEEM) technique, which combines ultrafast optics and electron microscopy, provides access to the data needed to make movies of spatiotemporal electron dynamics with high resolution at material interfaces [13-15]. The early applications of TR-PEEM with single-color lasers focused on studies of surface plasmon dynamics [13, 16-18]. This topic is continued to be studied by two-color TR-PEEM [19, 20], which was also used to observe the photo-generated carrier dynamics in GaAs [15, 21]. The PEEM equipped with the energy analyzer offers one more capability to observe details of charge carrier dynamics: the simultaneous evolution of electron density in spatial, temporal and energy domains. Very recently, by using such a 4D spectro-microscopy, Man et al. reported imaging the motion of electrons across the semiconductor heterojunction with unprecedented access to their dynamics .
In this work, the setup of the time-resolved spectroscopic PEEM, which we have built up recently, is presented in detail. Its time resolution, electron kinetic energy resolution, and spatial resolution abilities are documented. The results of time-resolved micro-area photoelectron spectra and energy-resolved TR-PEEM images on Pb/Si(111) demonstrate that this setup is a powerful tool to investigate many interesting phenomena in heterojunctions. Examples include the charge transfer and dynamic processes of free carriers, and characterization of novel materials such as size-dependent 2D materials and their heterojunctions.Ⅱ. INSTRUMENT CONFIGURATION
The aberration-corrected spectroscopic photoemission and low energy electron microscopy system (SPELEEM III, ELMITEC GmbH) [23, 24] we used here is located at the XPEEM (X-ray photoemission electron microscopy) endstation of the Dreamline (BL09U) in the Shanghai Synchrotron Radiation Facility (SSRF). FIG. 1(a) shows the electron-optical schematic of the SPELEEM. As a cathode lens electron microscopy, the sample tested in the SPELEEM is held at a high negative potential (
We very recently combined the ultrafast laser with the SPELEEM to achieve the femtosecond time-resolved spectroscopic PEEM test results. FIG. 1(b) shows a schematic of the pump-probe laser path for the TR-PEEM. An oscillator (FLINT, Light Conversion) delivers 6 W with a pulse width of less than 100 fs, at the wavelength of 1030 nm and at the repetition rate of 76 MHz. A true zero-order half-wave plate and a thin film polarizer are used to distribute the laser power to the pump and probe lasers paths. The pump laser, i.e. the second order harmonic (SH) 515 nm (2.4 eV) is generated by focusing the fundamental harmonic within a lithium triborate (LBO) crystal, and the third order harmonic 343 nm (3.6 eV) is generated by mixing the fundamental and SH within a
The temporal resolution was measured by the cross-correlation of the second harmonic (515 nm) and the fourth harmonic (257.5 nm) pulses. A boron doped Si(111) substrate (
The TR-PEEM images were recorded using a delay step of 13 fs. FIG. 2(a)-(c) show TR-PEEM images for
In FIG. 2(a)-(c), there are some bright spots with higher photoelectron yield than surrounding areas. These are assumed to be nanoscale structures with lower work functions or more electronic states . One of them is marked out with green square in FIG. 2(c). This green square zone is magnified to show pixel-resolvable (about 150 nm/pixel) images of different delay times in FIG. 3(a)-(c) with the same gray scaling. As shown in FIG. 3(b) of zero delay time, pixel-1 (P1) has the highest intensity of the bright spot. Four other pixels, P2 to P5, are also chosen in both horizontal (H) and vertical (V) directions respectively, which are at different distances from P1. Time-resolved PEEM intensity profiles collected from these pixels are plotted in FIG. 3(d) and (e). Apparently, the peaks of the intensity curves for the pixel H-P5 in FIG. 3(d) and V-P5 in FIG. 3(e), far away from the pixel P1, appear earlier than that for the pixels closer to P1. This must be attributed to the specific electron dynamics of such an nanoscale structure distinct from that of the surrounding areas. Among all the experiments in this work, we have not tested the spatial resolution limit of our instrument in time-resolved measurements. The above result shows that at least the intensity/dynamics discrepancies in space as fine as 150 nm can be discriminated. Further time-resolved measurements with high spatial resolution will be done in future work.Ⅳ. RESULTS WITH PB/SI(111)
The structures and energetics of the interfaces between metals and semiconductors are crucial in both fundamental and technological fields. In semiconductor devices, interfaces with metal electrodes play an important role in the functionality of the devices. Examples include integrated circuits, photovoltaic cells, and light-emitting diodes. Silicon, as the most commonly used semiconductor, has been thoroughly investigated with various metal adsorbates. These form a variety of interface structures. The quantum well states of epitaxial Pb thin films on Si(111), a nearly ideal quasi-2D model system, have been intensively studied in the electron structures and ultrafast electron dynamics of all the occupied, unoccupied and interface states in the past [26-30]. Here, the electron dynamics of Pb islands in the small spatial scale on Si(111) as a test example has been studied on the ultrafast time scale. The functions of our new setup were fully demonstrated with this system.
The Pb islands were grown in situ on the Si(111)-7
Time-resolved spectro-microscopy images of the Pb island were recorded by selecting electrons within the energy window of 0.3 eV as marked with the gray bar in FIG. 4(c). FIG. 5(a-e) correspond to the obtained images at different time delays of
We constructed an ultrafast time-resolved spectroscopic PEEM for studying the carrier dynamics of the heterojunctions. This instrument is composed of a femtosecond time-resolved laser system and a spectroscopic PEEM. It allows us to study the evolution of electrons in space, time, and energy, representing 4D spectro-microscopy. The temporal resolution was determined to be 184 fs, by measuring the cross-correlation of the pump (515 nm) and the probe (257.5 nm) pulses on the SiO
This work was conducted at the XPEEM endstation (BL09U, Dreamline) in the Shanghai Synchrotron Radiation Facility. This work was supported by the National Key R & D Program (No.2018YFA0208700 and No.2016YFA0200602), the National Natural Science Foundation of China (No.21688102 and No.21403222), the Strategic Priority Research Program of the Chinese Academy of Sciences (No.XDB17000000), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (No.2017224).
C. H. Lee, G. H. Lee, A.M. van der Zande, W. Chen, Y. Li, M. Han, X. Cui, G. Arefe, C. Nuckolls, T. F. Heinz, J. Guo, J. Hone, and P. Kim, Nat. Nanotechnol. 9, 676(2014). DOI:10.1038/nnano.2014.150
K. S. Novoselov, A. Mishchenko, A. Carvalho, and A. H. Castro Neto, Science 353, aac9439 (2016).
W. Zhang, Q. Wang, Y. Chen, Z. Wang, and A. T. S. Wee, 2D Mater. 3, 022001(2016). DOI:10.1088/2053-1583/3/2/022001
Q. Wang, J. Lai, and D. Sun, Opt. Mater. Express 6, 2313(2016). DOI:10.1364/OME.6.002313
A. Nourbakhsh, A. Zubair, M. S. Dresselhaus, and T. Palacios, Nano Lett. 16, 1359(2016). DOI:10.1021/acs.nanolett.5b04791
X. Hong, J. Kim, S. F. Shi, Y. Zhang, C. Jin, Y. Sun, S. Tongay, J. Wu, Y. Zhang, and F. Wang, Nat. Nanotechnol. 9, 682(2014). DOI:10.1038/nnano.2014.167
H. S. Karlsson, G. Chiaia, and U. O. Karlsson, Rev. Sci. Instrum. 67, 3610(1996). DOI:10.1063/1.1147067
S. Mathias, L. Miaja-Avila, M. M. Murnane, H. Kapteyn, M. Aeschlimann, and M. Bauer, Rev. Sci. Instrum. 78, 083105(2007). DOI:10.1063/1.2773783
M. M. Gabriel, J. R. Kirschbrown, J. D. Christesen, C. W. Pinion, D. F. Zigler, E. M. Grumstrup, B. P. Mehl, E. E. M. Cating, J. F. Cahoon, and J. M. Papanikolas, Nano Lett. 13, 1336(2013). DOI:10.1021/nl400265b
O. F. Mohammed, D. S. Yang, S. K. Pal, and A. H. Zewail, J. Am. Chem. Soc. 133, 7708(2011). DOI:10.1021/ja2031322
M. Wagner, Z. Fei, A. S. McLeod, A. S. Rodin, W. Bao, E. G. Iwinski, Z. Zhao, M. Goldflam, M. Liu, G. Dominguez, M. Thiemens, M. M. Fogler, A.H. Castro Neto, C. N. Lau, S. Amarie, F. Keilmann, and D. N. Basov, Nano Lett. 14, 894(2014). DOI:10.1021/nl4042577
J. Avila, and M. C. Asensio, Synchrotron Radiat. News 27, 24(2014).
A. Kubo, K. Onda, H. Petek, Z. Sun, Y. S. Jung, and H. K. Kim, Nano Lett. 5, 1123(2005). DOI:10.1021/nl0506655
M. Cinchetti, A. Gloskovskii, S. A. Nepjiko, G. Schonhense, H. Rochholz, and M. Kreiter, Phys. Rev. Lett. 95, 047601(2005). DOI:10.1103/PhysRevLett.95.047601
K. Fukumoto, K. Onda, Y. Yamada, T. Matsuki, T. Mukuta, S. I. Tanaka, and S. Y. Koshihara, Rev. Sci. Instrum. 85, 083705(2014). DOI:10.1063/1.4893484
A. Kubo, N. Pontius, and H. Petek, Nano Lett. 7, 470(2007). DOI:10.1021/nl0627846
M. Bauer, C. Wiemann, J. Lange, D. Bayer, M. Rohmer, and M. Aeschlimann, Appl. Phys. A 88, 473(2007). DOI:10.1007/s00339-007-4056-z
A. Kubo, Y. S. Jung, H. K. Kim, and H. Petek, J. Phys. B 40, S259 (2007).
M. Shibuta, T. Eguchi, and A. Nakajima, Plasmonics 8, 1411(2013). DOI:10.1007/s11468-013-9554-6
K. Yamagiwa, M. Shibuta, and A. Nakajima, Phys. Chem. Chem. Phys. 19, 13455(2017). DOI:10.1039/C7CP01693J
K. Fukumoto, Y. Yamada, S. Y. Koshihara, and K. Onda, Appl. Phys. Express 8, 101201(2015). DOI:10.7567/APEX.8.101201
M. K. Man, A. Margiolakis, S. Deckoff-Jones, T. Harada, E. L. Wong, M. B. Krishna, J. Madeo, A. Winchester, S. Lei, R. Vajtai, P. M. Ajayan, and K. M. Dani, Nat. Nanotechnol. 12, 36(2017). DOI:10.1038/nnano.2016.183
E. Bauer, C. Koziol, G. Lilienkamp, and T. Schmidt, J. Electron. Spectrosc. Relat. Phenom. 84, 201(1997). DOI:10.1016/S0368-2048(97)00007-8
E. Bauer, J. Electron. Spectrosc. Relat. Phenom. 114-116, 975 (2001).
T. O. Menteș, and A. Locatelli, J. Electron. Spectrosc. Relat. Phenom. 185, 323(2012). DOI:10.1016/j.elspec.2012.07.007
P. S. Kirchmann, L. Rettig, X. Zubizarreta, V. M. Silkin, E. V. Chulkov, and U. Bovensiepen, Nat. Phys. 6, 782(2010). DOI:10.1038/nphys1735
Y. F. Zhang, J. F. Jia, T. Z. Han, Z. Tang, Q. T. Shen, Y. Guo, Z. Q. Qiu, and Q. K. Xue, Phys. Rev. Lett. 95, 096802(2005). DOI:10.1103/PhysRevLett.95.096802
T. Zhang, P. Cheng, W. J. Li, Y. J. Sun, G. Wang, X. G. Zhu, K. He, L. Wang, X. Ma, X. Chen, Y. Wang, Y. Liu, H. Q. Lin, J. F. Jia, and Q. K. Xue, Nat. Phys. 6, 104(2010). DOI:10.1038/nphys1499
P. S. Kirchmann, and U. Bovensiepen, Phys. Rev. B 78, 035437(2008). DOI:10.1103/PhysRevB.78.035437
M. Ligges, M. Sandhofer, I. Sklyadneva, R. Heid, K. P. Bohnen, S. Freutel, L. Rettig, P. Zhou, P. M. Echenique, E. V. Chulkov, and U. Bovensiepen, J. Phys.: Condens. Mater. 26, 352001(2014). DOI:10.1088/0953-8984/26/35/352001
T. Kiss, T. Shimojima, K. Ishizaka, A. Chainani, T. Togashi, T. Kanai, X. Y. Wang, C. T. Chen, S. Watanabe, and S. Shin, Rev. Sci. Instrum. 79, 023106(2008). DOI:10.1063/1.2839010
L. I. Chelaru, M. Horn-von Hoegen, D. Thien, and F. J. Meyer zu Heringdorf, Phys. Rev. B 73, 115416 (2006).
G. H. Zhang, J. L. Sun, Y. L. Jin, K. Zang, F. Z. Guo, and X. M. Yang, Chin. J. Chem. Phys. 26, 369(2013). DOI:10.1063/1674-0068/26/04/369-373
L. Rettig, P. S. Kirchmann, and U. Bovensiepen, New J. Phys. 14, 023407(2012).
b. 南京工业大学，江苏省国家先进材料协同创新中心，柔性电子重点实验室和先进材料研究所，南京 211816;
c. 南方科技大学化学系，深圳 518055;
d. 中国科学院大学，北京 100049