Compendium of Surface and Interface Analysis

431424_Print.inddCompendium of Surface and Interface Analysis
Compendium of Surface and Interface Analysis
The Surface Science Society of Japan Editor
Compendium of Surface and Interface Analysis
123
ISBN 978-981-10-6155-4 ISBN 978-981-10-6156-1 (eBook) https://doi.org/10.1007/978-981-10-6156-1
Library of Congress Control Number: 2017949165
© Springer Nature Singapore Pte Ltd. 2018 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Printed on acid-free paper
This Springer imprint is published by Springer Nature The registered company is Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore
List of the Editorial Staff
Editor-in-Chief
Takuya Masuda Hitoshi Abe Takahiro Kondo Kan Nakatsuji Toru Shimada
Associate Editors
Shuji Hasegawa Yoshikazu Homma Akiko N. Itakura Ryohei Kokawa Atsushi Kubo Fumihiko Matsui Tomonobu Nakayama Tetsuya Narushima Hidenori Noguchi Kazuyuki Sakamoto Kaoru Sasakawa Ryugo Tero
v
Preface
Surfaces and interfaces are the places where the rotation/inversion symmetry of the crystal is broken and therefore the electronic and geometric structures significantly differ from those in bulk, leading to the unique electric, magnetic, catalytic and optical properties. In addition, various interesting processes such as adsorption/desorption, etching, deposition, corrosion, and electron transfer and catalytic reactions take place at the surfaces and interfaces. Since the electronic, geometric and molecular structures of surfaces and interfaces play crucial roles in those interfacial processes, it is important to understand such structures as well as the elemental composition.
Despite the importance of surface and interface analysis, it is generally more difficult than bulk analysis because the very small number of atoms is the subject of investigations and the signals from the surface species are often buried within those from bulk. Therefore, tremendous effort has been dedicated to the development of analysis techniques which can extract the information of the surface species from those of bulk with a high sensitivity and selectivity.
This book covers various surface analysis techniques to investigate the morphology, atomic structure, electronic structure and properties of the surfaces and interfaces. In each chapter, experts of the corresponding techniques briefly describe their principle, features and instrumentation together with a few examples of related works, so that readers can understand the capabilities of the techniques and requirements for the use in their own researches. The list of techniques summarized in this book is available from http://extras.springer.com. We hope that this book is useful to a wide range of scientists and students who study in this research field or start to do.
Finally, we thank Springer Publishing, especially Dr. Shin’ichi Koizumi, Ms. Risa Takizawa, for giving us an opportunity to edit such a book and all the authors for accepting to contribute to this book, and we hope that the readers will find this book both useful and delightful.
Tokyo, Japan Manabu Kiguchi March 2017 Takuya Masuda
Hitoshi Abe Kan Nakatsuji
Takahiro Kondo Toru Shimada
2 Action Spectroscopy with STM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Kenta Motobayashi
3 Ambient Pressure X-Ray Photoelectron Spectroscopy . . . . . . . . . . 15 Hiroshi Kondoh
4 Angle-Resolved Ultraviolet Photoelectron Spectroscopy . . . . . . . . . 21 Takafumi Sato
5 Atom Probe Field Ion Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Masahiko Tomitori
6 Atomic Force Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Shintaro Fujii
7 Auger Electron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Fumihiko Matsui
8 Cathodoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Takashi Sekiguchi
9 Conductive Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . 51 Risa Fuji
10 Differential Interference Contrast Microscopy/Phase-Contrast Microscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Hiroshi Komatsu and Gen Sazaki
11 Dynamic Secondary Ion Mass Spectrometry . . . . . . . . . . . . . . . . . . 61 Mitsuhiro Tomita
12 Elastic Recoil Detection Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Daiichiro Sekiba
ix
13 Electrochemical Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . 73 Toru Utsunomiya, Yasuyuki Yokota and Ken-ichi Fukui
14 Electrochemical Infrared Spectroscopy . . . . . . . . . . . . . . . . . . . . . . 79 Shen Ye
15 Electrochemical Scanning Tunneling Microscopy . . . . . . . . . . . . . . 87 Tomoaki Nishino
16 Electrochemical Second Harmonic Generation . . . . . . . . . . . . . . . . 91 Ichizo Yagi
17 Electrochemical Sum Frequency Generation . . . . . . . . . . . . . . . . . . 97 Hidenori Noguchi
18 Electrochemical Surface X-Ray Scattering . . . . . . . . . . . . . . . . . . . 103 Toshihiro Kondo
19 Electrochemical Transmission Electron Microscopy . . . . . . . . . . . . 109 Yoshifumi Oshima
20 Electrochemical X-Ray Absorption Fine Structure . . . . . . . . . . . . . 113 Takuya Masuda
21 Electrochemical X-Ray Photoelectron Spectroscopy . . . . . . . . . . . . 119 Takuya Masuda
22 Electron Backscatter Diffraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Rika Yoda
23 Electron Energy-Loss Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 133 Tadaaki Nagao
24 Electron Probe Microanalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Hiroshi Sakamae
25 Electron-Stimulated Desorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Naoya Miyauchi
26 Electron-Beam-Induced Current . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Jun Chen and Takashi Sekiguchi
27 Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Toshihide Tsuru
28 Environmental SEM (Atmospheric SEM) . . . . . . . . . . . . . . . . . . . . 165 Yusuke Ominami
29 Environmental Transmission Electron Microscopy. . . . . . . . . . . . . 171 Tadahiro Kawasaki
30 Extended X-Ray Absorption Fine Structure . . . . . . . . . . . . . . . . . . 177 Hitoshi Abe
x Contents
31 Focused Ion Beam Scanning Electron Microscope . . . . . . . . . . . . . 181 Tetsuo Sakamoto
32 Force Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Akinori Kogure
33 Force Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 Christina Puckert and Michael J. Higgins
34 Frequency-Modulation Atomic Force Microscopy . . . . . . . . . . . . . 201 Masayuki Abe
35 Gap-Mode Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 Katsuyoshi Ikeda
36 Glow Discharge Mass Spectrometry. . . . . . . . . . . . . . . . . . . . . . . . . 211 Takashi Saka
37 Glow Discharge Optical Emission Spectrometry . . . . . . . . . . . . . . . 219 Patrick Chapon, Sofia Gaiaschi and Kenichi Shimizu
38 Hard X-Ray Photoelectron Spectroscopy. . . . . . . . . . . . . . . . . . . . . 229 Akira Sekiyama
39 Helium Atom Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 Takahiro Kondo
40 High-Resolution Elastic Recoil Detection Analysis . . . . . . . . . . . . . 247 Kaoru Nakajima
41 High-Resolution Electron Energy Loss Spectroscopy . . . . . . . . . . . 253 Hiroshi Okuyama
42 High-Resolution Rutherford Backscattering Spectrometry. . . . . . . 259 Kaoru Nakajima
43 High-Speed Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . 263 Takayuki Uchihashi
44 Imaging Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 Akiko N. Itakura
45 Impact Collision Ion Scattering Spectroscopy . . . . . . . . . . . . . . . . . 275 Masakazu Aono and Mitsuhiro Katayama
46 Inelastic Electron Tunneling Spectroscopy . . . . . . . . . . . . . . . . . . . 283 Akitoshi Shiotari
47 Infrared External-Reflection Spectroscopy . . . . . . . . . . . . . . . . . . . 289 Takeshi Hasegawa
48 Infrared Reflection–Absorption Spectroscopy . . . . . . . . . . . . . . . . . 295 Jun Yoshinobu
Contents xi
51 Kelvin Probe Force Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 Risa Fuji
52 Laser Ionization Secondary Neutral Mass Spectrometry . . . . . . . . 319 Tetsuo Sakamoto
53 Laser Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 325 Ryuichi Arafune
54 Lateral Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 Shiho Moriguchi
55 Liquid SPM/AFM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 Akinori Kogure
56 Low-Energy Ion Scattering Spectroscopy . . . . . . . . . . . . . . . . . . . . 343 Kenji Umezawa
57 Low-Energy Electron Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . 349 Yoshimi Horio
58 Low-Energy Electron Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . 355 H. Hibino
59 Magnetic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361 Masato Hirade
60 Matrix-Assisted Laser Desorption/Ionization. . . . . . . . . . . . . . . . . . 365 Takaya Satoh
61 Medium-Energy Ion Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 Tomoaki Nishimura
62 Micro-Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Katsumasa Fujita
63 Microprobe Reflection High-Energy Electron Diffraction. . . . . . . . 381 Masakazu Ichikawa
64 Multiple-Probe Scanning Probe Microscope . . . . . . . . . . . . . . . . . . 387 Tomonobu Nakayama
65 Nanoscale Angle-Resolved Photoelectron Spectroscopy . . . . . . . . . 395 Koji Horiba
66 Nonlinear Spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 Shoichi Yamaguchi
xii Contents
67 Nuclear Reaction Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 Markus Wilde and Katsuyuki Fukutani
68 Optical Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413 Kazuya Kabayama and Ryugo Tero
69 Optical Second-Harmonic Generation Spectroscopy and Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 Khuat Thi Thu Hien and Goro Mizutani
70 Particle-Induced X-Ray Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 Koichiro Sera
71 Penning Ionization Electron Spectroscopy . . . . . . . . . . . . . . . . . . . . 435 Takuya Hosokai
72 Phase Mode SPM/AFM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441 Hideo Nakajima
73 Photoelectron Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 Fumihiko Matsui and Tomohiro Matsushita
74 Photoelectron Holography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 Tomohiro Matsushita and Fumihiko Matsui
75 Photoelectron Yield Spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . 457 Hisao Ishii
76 Photoemission Electron Microscope . . . . . . . . . . . . . . . . . . . . . . . . . 465 Toyohiko Kinoshita
77 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471 Yuhei Miyauchi
78 Photon Emission from the Scanning Tunneling Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477 Makoto Sakurai
79 Photo-Stimulated Desorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 Akihiko Ikeda and Katsuyuki Fukutani
80 Piezoresponse Force Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 Masato Hirade
81 Positron-Annihilation-Induced Desorption Spectroscopy . . . . . . . . 497 Takayuki Tachibana and Yasuyuki Nagashima
82 p-Polarized Multiple-angle Incidence Resolution Spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501 Takeshi Hasegawa
Contents xiii
83 Quartz Crystal Microbalance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509 Yuji Teramura and Madoka Takai
84 Reflectance Difference Spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . 521 Ken-ichi Shudo and Shin-ya Ohno
85 Reflection High-Energy Electron Diffraction . . . . . . . . . . . . . . . . . . 527 Yoshimi Horio
86 Resonant Inelastic X-Ray Scattering . . . . . . . . . . . . . . . . . . . . . . . . 531 Yoshihisa Harada
87 Rutherford Backscattering Spectrometry . . . . . . . . . . . . . . . . . . . . 539 Daiichiro Sekiba
88 Scanning Capacitance Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 545 Nobuyuki Nakagiri
89 Scanning Electrochemical Microscopy . . . . . . . . . . . . . . . . . . . . . . . 551 Yasufumi Takahashi
90 Scanning Electron Microscope Energy Dispersive X-Ray Spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557 Masaki Morita
91 Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563 Yasuyuki Okano
92 Scanning Helium Ion Microscope. . . . . . . . . . . . . . . . . . . . . . . . . . . 571 Keiko Onishi and Daisuke Fujita
93 Scanning Near-Field Optical Microscopy/Near-Field Scanning Optical Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577 Tetsuya Narushima
94 Scanning Probe Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583 Ken Nakajima
95 Scanning Transmission Electron Microscopy . . . . . . . . . . . . . . . . . 587 Koji Kimoto
96 Scanning Transmission X-Ray Microscopy . . . . . . . . . . . . . . . . . . . 593 Yasuo Takeichi
97 Scanning Tunneling Microscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . 599 Yukio Hasegawa
98 Scanning Tunneling Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 605 Keisuke Sagisaka
99 Soft X-Ray Absorption Fine Structure. . . . . . . . . . . . . . . . . . . . . . . 611 Kenta Amemiya
xiv Contents
101 Spin- and Angle-Resolved Photoelectron Spectroscopy. . . . . . . . . . 623 Taichi Okuda
102 Spin-Polarized Scanning Electron Microscopy . . . . . . . . . . . . . . . . 631 Teruo Kohashi
103 Spin-Polarized Scanning Tunneling Microscopy . . . . . . . . . . . . . . . 637 Toyo Kazu Yamada
104 Spin-Resolved Photoemission Electron Microscopy. . . . . . . . . . . . . 643 Keiki Fukumoto
105 Super-Resolution Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 651 Kazuya Kabayama and Ryugo Tero
106 Surface Acoustic Wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657 Shinya Sasaki
107 Surface Enhanced Raman Scattering . . . . . . . . . . . . . . . . . . . . . . . . 661 Katsuyoshi Ikeda
108 Surface Magneto-Optic Kerr Effect . . . . . . . . . . . . . . . . . . . . . . . . . 667 Takeshi Nakagawa
109 Surface Plasmon Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673 Kaoru Tamada
110 Surface Profilometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 Masahiro Tosa
111 Surface Sensitive Scanning Electron Microscopy . . . . . . . . . . . . . . 683 Yoshikazu Homma
112 Surface X-Ray Diffraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 689 Etsuo Arakawa
113 Surface-Enhanced Infrared Absorption Spectroscopy . . . . . . . . . . 697 Masatoshi Osawa
114 Synchrotron Radiation Photoelectron Spectroscopy . . . . . . . . . . . . 707 Jun Fujii
115 Synchrotron Scanning Tunneling Microscope . . . . . . . . . . . . . . . . . 713 Toyoaki Eguchi
116 Thermal Desorption Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 719 Shohei Ogura and Katsuyuki Fukutani
117 Time-of-Flight Secondary Ion Mass Spectrometry . . . . . . . . . . . . . 725 Satoka Aoyagi
Contents xv
119 Time-Resolved Photoemission Electron Microscopy . . . . . . . . . . . . 741 Atsushi Kubo
120 Time-Resolved Scanning Tunneling Microscopy . . . . . . . . . . . . . . . 749 Hidemi Shigekawa
121 Tip-Enhanced Raman Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . 755 Norihiko Hayazawa
122 Total Reflection X-Ray Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . 763 Jun Kawai
123 Transmission Electron Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . 769 Yoshio Matsui
124 Transmission Electron Microscope. . . . . . . . . . . . . . . . . . . . . . . . . . 775 Masanori Mitome
125 Ultraviolet Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . 783 Kenichi Ozawa
126 Ultraviolet–Visible Spectrophotometry. . . . . . . . . . . . . . . . . . . . . . . 791 Hiro Amekura
127 Vibrational Sum Frequency Generation Spectroscopy . . . . . . . . . . 801 Satoshi Nihonyanagi and Tahei Tahara
128 X-Ray Absorption Near Edge Structure . . . . . . . . . . . . . . . . . . . . . 809 Hitoshi Abe
129 X-Ray-Aided Noncontact Atomic Force Microscopy. . . . . . . . . . . . 815 Shushi Suzuki, Wang-Jae Chun, Masaharu Nomura and Kiyotaka Asakura
130 X-Ray Crystal Truncation Rod Scattering . . . . . . . . . . . . . . . . . . . 821 Tetsuroh Shirasawa
131 X-Ray Magnetic Circular Dichroism . . . . . . . . . . . . . . . . . . . . . . . . 827 Kenta Amemiya
132 X-Ray Photoelectron Spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . 833 Makoto Nakamura
133 X-Ray Reflectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843 Wolfgang Voegeli
134 X-Ray Standing Wave Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849 Akira Saito
xvi Contents
1.1 Principles
1.1.1 Observation of Soft Biological Matters
Biological ultrasonic microscopy, also known as biological scanning acoustic microscope, provides quantitative acoustic parameters like sound speed and characteristic acoustic impedance that are relevant to elastic properties. As it needs no staining process, the observation can be performed rapidly without introducing any chemical or biological damage to the specimen. Figure 1.1 illustrates two types of acoustic microscopes for soft objects: sound speed mode and acoustic impedance mode. Either pulse or tone burst signal is supplied to the acoustic transducer to produce a strongly focused ultrasound.
If the soft tissue can be treated as fluid-like, we may assume that only pressure wave (longitudinal wave) can propagates through the specimen. The sound speed of a pressure wave is given as c ¼ ffiffiffiffiffiffiffiffiffi
K=q p
, where K is the elastic bulk modulus and q is the specific gravity. In order to measure the local sound speed, we compare the reflections from the front and rear surfaces of a tissue slice, which is placed on a glass substrate and dipped into a coupling medium (Fig. 1.1a). The sound speed c can be calculated as c ¼ d=Dt, where d is the thickness of the tissue and Dt is the time lag between the reflections from front and rear surfaces. In most cases, however, it is not easy to precisely measure d at the point where the sound beam is focused. Therefore, both the thickness and sound speed are simultaneously assessed by referring the sound speed of the coupling medium (mostly pure water), and the sound speed is obtained as
N. Hozumi (&) Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, Toyohashi, Japan e-mail: [email protected]
© Springer Nature Singapore Pte Ltd. 2018 The Surface Science Society of Japan (ed.), Compendium of Surface and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_1
1
/m
4pfmd
1
; ð1:1Þ
where c0 is the sound speed of the water, fm is one of the maximum points in the intensity spectrum, and /m is the corresponding phase angle (see Appendix for details). Direct reflection from the place where no specimen exists is taken for the reference signal c0.
In some cases, it is required that the observation is performed without slicing the tissue. In such a case, characteristic acoustic impedance Z ¼ ffiffiffiffiffiffiffi
Kq p
is alternatively determined. The acoustic impedance reflects the elasticity and therefore is basically equivalent to the sound speed. The acoustic impedance of the target specimen (Ztarget) is obtained as
Ztarget ¼ 1 A 1þA
Zsub; ð1:2Þ
;
S represents the signal component at an arbitrary frequency, and subscripts target, sub and ref indicate the target specimen, substrate and reference material, respectively (See Appendix for details). In case water is used as the reference, its acoustic impedance is assumed to be 1.5 106 Ns/m3.
Fig. 1.1 Two typical modes for observation of soft matters. a Sound speed mode using thin slice as a specimen. b Acoustic impedance mode for cross-sectional observation. The transducer is supplied with 50–500 MHz (or higher) in frequency depending on the required spatial resolution
2 N. Hozumi
1.1.2 Observation of Solid Matters
When an object is a solid material like semiconductor or metal, a strong leaky surface acoustic wave (LSAW) is excited when defocused acoustic beam is irradiated onto the surface. Propagation of LSAW is strongly affected by surface conditions; thus, the observation method making use of LSAW is very powerful to know physical properties of a surface. The basic concept is illustrated in Fig. 1.2. The focal point of the acoustic beam (focal length ‘) is set a bit under the surface (z). The directly reflected signal is detected at the beam component represented as D. LSAW is produced at the critical incident angle hR, and its beam component is represented as R. These two reflections (D and R) interfere; therefore, periodical change in the received signal (V(z)) is observed when degree of defocusing (z) is continuously varied. Speed of LSAW (cR) is obtained from V(z) as follows:
cR ¼ c0 1 1 c0 2fDz
2 ( )1=2
; ð1:3Þ
where c0 is the sound speed of the coupling medium (water is used most com- monly), f is frequency of the ultrasound, and z is the interval between minima in V(z) (See Appendix for details). cR is sensitive to surface morphology and physical properties. Therefore, the observation by means of cR is useful to detect very small defect on the surface, and to characterize the surface conditions and film thickness.
1.2 Features
without staining processes.
Fig. 1.2 Schematic illustration of surface observation by means of leaky surface acoustic wave (LSAW). ‘ and z represent focal length and defocusing distance, D and R represent direct reflection and the reflection through LSAW, respectively. In addition to 2-D (x–y) scanning, scanning along z direction is needed to acquire V(z) signal
1 Acoustic Microscopy 3
1.3 Applications
1.3.1 Sound Speed Profile
An example of two-dimensional sound speed profiles is shown in Fig. 1.3 [1]. Pulmonary structures such as alveoli, bronchial trees, blood vessels, and stromal connective tissues are recognized corresponding to those visualized with an optical microscope. The sound speed increases in the following order: normal alveoli, bronchial walls, congested alveoli, fibrosis with organizing pneumonia, cartilage, vascular smooth muscles, and blood. The mean speed through each component is significantly different from that through normal alveoli.
1.3.2 Acoustic Impedance Profiles
Figure 1.4a shows cross-sectional acoustic impedance profile of the cerebellar cortex tissue of a rat at mature stage [2]. The granular layer (GL), which is composed of small neuronal cell bodies, molecular layer (ML), which is composed of
Fig. 1.3 Sound speed profile of congested lung with dilated pulmonary vein and blood-filled alveoli. a The optical microscope image with hematoxylin and eosin (HE) staining (left) and b its corresponding sound speed image. The speed of sound is greater through vascular smooth muscle and blood-filled alveoli than through normal alveoli. 2.4 2.4 mm in field of view is indicated with 300 300 of lateral resolution. The images are reprinted from Ref. [1] with permission from Nature Publishing Group
4 N. Hozumi
elongated axon (neurite) called parallel fibers, and Purkinje layer (PL), which is composed of Purkinje cells as the origin of dendrites, are clearly distinguished. The correspondence with immunohistological observation is also obtained [2].
Figure 1.4b shows an acoustic impedance profile of glial cells cultured on a film substrate [3]. Nucleus is seen at the center of each cell. The nucleus has a round shape, and its acoustic impedance is as high as 1.6 MNs/m3. The nucleus is sur- rounded by the portion of which acoustic impedance is as high as 1.65–1.7 MNs/m3. This portion has the highest acoustic impedance in the cell. It is con- sidered that the region close to the nucleus is filled with fibrous cytoskeleton, microtubules, which have higher density compared with the other part of the cell.
Appendix
Derivation of Eq. (1.1)
Sound speed of the soft tissue can be assessed by either time domain or frequency domain analysis. An example of frequency domain analysis is as follows: The spectrum of the reflection from the object slice is normalized by the reference waveform. Assuming fm as one of the minimum and maximum points in the intensity spectrum, and /m as the corresponding phase angle, the phase difference between the two reflections at the minimum point is (2n − 1)p, giving
Fig. 1.4 Acoustic impedance profiles of biological tissue and cells. a Cerebellum tissue, rat. PL Purkinje layer, ML Molecular layer and GL Granular layer. b Glial cells cultured on film substrate. The images are reprinted from Refs. [2] and [3] with modification with permission from Elsevier
1 Acoustic Microscopy 5
¼ /m þð2n 1Þp; ð1:4Þ
where d, c0, and n are the tissue thickness, sound speed of the water, and a nonnegative integer, respectively. The phase difference at the maximum point is 2np, giving
2pfm 2d c0
The phase angle /m can be expressed by
2pfm 2d 1 c0
¼ /m; ð1:6Þ
since /m is the phase difference between the wave passed through the distance 2d with sound speed c and that passed though the corresponding distance with sound speed c0. Equation (1.4) gives
d ¼ c0 4pfm
/m þð2n 1Þpf g ð1:7Þ
for the minimum point. For the maximum point, Eq. (1.5) gives
d ¼ c0 4pfm
Sound speed is finally calculated as
c ¼ 1 c0
Hereafter, the signal component at an arbitrary frequency will be symbolized by S. Considering the reflection coefficient, the target signal Starget can be described as
Starget ¼ Ztarget Zsub Ztarget þ Zsub
S0; ð1:9Þ
6 N. Hozumi
where S0 is the transmitted signal and Ztarget and Zsub are the acoustic impedances of the target and substrate, respectively. On the other hand, the reference signal can be described as
Sref ¼ Zref Zsub Zref þ Zsub
S0; ð1:10Þ
where Zref is the acoustic impedance of the reference material. In case of using water as the reference, its acoustic impedance was assumed to be 1.5 106 Ns/m3. One can measure Starget and Zref; however, S0 cannot be directly measured. The acoustic impedance of the target is subsequently calculated as a solution of the simultaneous equations for Ztarget and S0, as
Ztarget ¼ 1þ Starget
1þ Starget Sref
Phases of the two reflection signals (D and R) are represented as
/D ¼ 2ð‘ zÞk0;/R ¼ 2 ‘ z cos hR
k0 þ 2zkR tan hR; ð1:12Þ
where wave numbers k0 and kR are defined as k0 ¼ 2pf =c0 and kR ¼ 2pf =cR, respectively. Relative phase shift per unit distance of z is represented as
/ðzÞ ¼ ð/DðzÞ /RðzÞÞ=z ¼ 2 k0ð1 1= cos hR þ kR tan hRf g: ð1:13Þ
Two signals emphasize together when /DðzÞ /RðzÞð Þ is 2p. Hence, periodical change appears on the V(z) curve.
The interval Dz between minima can be represented by
2p Dz
þ 2pf cR
1 Dz
; ð1:15Þ
thus
: ð1:16Þ
Speed of LSAW is determined as
cR ¼ c0= sin hR ¼ c0 1 1 c0 2fDz
2 ( )1=2
References
1. Miura, K., Yamamoto, S.: Pulmonary imaging with a scanning acoustic microscope discriminates speed-of-sound and shows structural characteristics of disease. Lab. Invest. 92, 1760–1765 (2012)
2. Gunawan, A.I., Hozumi, N., Yoshida, S., Saijo, Y., Kobayashi, K., Yamamoto, S.: Numerical analysis of ultrasound propagation and reflection intensity for biological acoustic impedance microscope. Ultrasonics 61, 79–87 (2015)
3. Gunawan, A.I., Hozumi, N., Takahashi, K., Yoshida, S., Saijo, Y., Kobayashi, K., Yamamoto, S.: Numerical analysis of acoustic impedance microscope utilizing acoustic lens transducer to examine cultured cells. Ultrasonics 63, 102–110 (2015)
8 N. Hozumi
Kenta Motobayashi
2.1 Principle
STM-AS is a spectroscopic method capable of vibrational analysis of individual adsorbates on surfaces [1]. It is well known that injection of tunneling electrons from an STM tip into adsorbates can excite their vibrational states via inelastic electron tunneling (IET) process, which is used as STM-IETS. Some of these excitations can induce motion and reactions of the adsorbates [2], such as lateral hopping, rotation, desorption, isomerization, and bond formation/scission (Fig. 2.1a). STM-AS measures the motion/reaction probability as a function of applied bias voltage showing remarkable increases near the bias voltages corresponding to the vibrational energies (Fig. 2.1c). Requiring the dynamics of adsorbates, STM-AS is complementary to STM-IETS which requires static behavior. STM-AS can also detect electronic excitations as well, but here the focus is on the more frequently employed usage of vibrational spectroscopy.
In STM-AS, the reaction yield Y (reaction rate per electron) as a function of applied bias V is measured as follows. The STM tip is positioned over a target analyte at a fixed tunneling gap followed by feedback loop off. The tunneling electrons are then injected, and the time required for a single reaction event is read out from an I-t plot in which a sudden change in current takes place at the moment of reaction (Fig. 2.1b). Y(V) is statistically determined from multiple trials of this procedure. The Y–V plot can be fitted by the formula representing Y(V) to determine the vibrational energies as well as the vibrational broadening, rate constant, and reaction order (number of electrons required for the reaction) [3]. The vibrational
K. Motobayashi (&) Department of Physical Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, Japan e-mail: [email protected]
9
energies enable identification of the functional group and orientation of the adsorbate, and evaluation of the bond strength, in the same manner with other vibrational spectroscopies. Other parameters allow us to gain deeper insights into the microscopic elementary processes behind the reactions induced by IET.
2.2 Features
• Vibrational modes (or electronic states) of a single molecule can be detected. • Vibrational modes inducing dynamic motion of adsorbates can be detected. • Selection rule is different from IR, Raman, and STM-IETS. • Sufficiently low temperature for stabilizing adsorbates is required. • Insights into elementary processes behind the dynamics can be obtained.
2.3 Instrumentation
For stable measurement of STM-AS, thermal diffusion and desorption of adsorbates, and unintended additional adsorption of any species must be suppressed. Thermal drifting must also be suppressed so that the STM tip can be stably positioned over the analyte. Therefore, the STM system must be equipped with an UHV chamber and cooled to sufficiently low temperature (depending on the analyte). The tunneling current flowing at constant applied bias voltage is recorded by an STM controller or an external oscilloscope to detect the moment of reaction; no additional equipment is required for acquiring the spectra.
vibrational excitatione-
t motion/ reactions
(a) (b) (c)
Fig. 2.1 Schematics of (a) vibrationally induced motion/reactions of adsorbates under the STM tip, (b) tunneling current as a function of time indicating motion/reactions of the adsorbates, and (c) resulting reaction yield as a function of sample bias voltage in which increases in reaction yield indicate vibrational energies
10 K. Motobayashi
2.4.1 Fundamental Structure of an Isolated Water Dimer on Pt(111)
The potential of STM-AS as a vibrational spectroscopy has been demonstrated in a structural study of a water dimer on Pt(111) [4]. This simplest building block of water clusters is a useful model for exploring the nature of the hydrogen bond (H-bond) and water–solid interactions. STM-AS is suitable for analyzing this mobile molecule even at low temperatures, and for obtaining cluster-size-specific vibrational information from mixture of differently sized clusters.
Lateral hopping of a water dimer (Fig. 2.2a, b) was detected as a sudden decrease of tunneling current (Fig. 2.2c) at a fixed tunneling gap, enabling us to obtain the STM-AS (Fig. 2.2e). A theoretical fit represents the experimental data very well, which results in a reasonable and precise assignment of each vibrational signal (marked by arrows in Fig. 2.2e). In particular, the O–H stretch mode observed at 375 mV was absent in previously reported IRAS and HREELS studies, indicating the advantage of STM-AS in its sensitivity and selectivity.
Sample bias voltage (mV)
2
4
6
δ
Fig. 2.2 STM image of a H2O dimer on Pt(111) (a) and that recorded before and after an intentionally induced lateral hopping (b). (c) Tunneling current measured over a H2O dimer at fixed tunneling gap showing sudden decrease corresponding to lateral hopping. (d) A three-quarter view of the optimized “H-down” structure of a water dimer on Pt(111) obtained from a DFT calculation. (e) STM-AS of lateral hopping of H2O and D2O dimers on Pt(111). Red circles and blue squares represent the experimental results of STM-AS for H2O and D2O, respectively. Thick solid curves are best-fit spectra. Reprinted with the permission from Ref. [4]. Copyright 2014 American Chemical Society
2 Action Spectroscopy with STM 11
The vibrational energies enable us to deduce the internal structure of the water dimer. Two DFT studies have concluded different optimized structures and different vibrational energies, and STM-AS shows good agreement in vibrational energies with one of the models. The structure of the water dimer observed by STM was thus concluded to be the so-called H-down model (Fig. 2.2d), where one of the water molecules interacts with the Pt substrate not through the oxygen lone pair but through an OH–Pt hydrogen bond. It was shown here that STM-AS has the capability of determining the internal structure of an isolated molecular cluster at the sub-nm scale, which is difficult even with simple STM imaging.
2.4.2 Dissociation Pathways of a Single Dimethyl Disulfide on Cu(111)
The elementary process of S-S bond dissociation of dimethyl disulfide (DMDS, Fig. 2.3a, b) was revealed [5]. STM-AS (Fig. 2.3d) shows that the reaction is induced by excitation of the C–H stretch mode m(C–H), or the combination of m(C– H) and S–S stretch mode m(S–S). For the excitation of m(C–H), the reaction order N was found to be 2 (Fig. 2.3c), which is usually interpreted as the reaction induced by double-quanta excitation of m(C–H). However, the fitting analysis of STM-AS shows that this scenario cannot explain the STM-AS signal and the transition from N = 2 to N = 1 for the combination mode excitation. Instead, the bias dependence of N and Y can be consistently explained by assuming that the reaction is induced when one electron excites m(C–H) and the other excites m(S–S). Thus, the STM-AS measurement and fitting analysis reveal not only the vibrational modes that trigger
300 400 500 600
curve A (n1,1 = 2) curve B (n1,1 = n1,2 = 1) experimental
396: νCH
10 -6(d)
DMDS DMDS-d6
1
2
400
R ea
ct io
n yi
el d
Fig. 2.3 STM images of DMDS on Cu(111) before (a) and after (b) inducing the dissociation. (c) Reaction order N as a function of V for the DMDS dissociation determined by measuring the reaction rate as a function of tunneling current. (d) STM-AS for the DMDS dissociation. Red circles are experimental data. Curve B is a best fit of a fitting function Y(V); curve A with different parameters is shown for comparison. The inset is an STM-AS at negative sample bias. Reprinted with permission from Ref. [5]. Copyright 2014, American Institute of Physics
12 K. Motobayashi
the reaction, but also deeper insight into the mechanism of vibrationally induced reactions via IET, leading to finding a novel mechanism; specifically, a reaction is induced only when two vibrational modes are concurrently at excited states, whether excited by one or two electron(s).
References
1. Kim, Y., Motobayashi, K., Frederiksen, T., Ueba, H., Kawai, M.: Action spectroscopy for single-molecule reactions—experiments and theory. Prog. Surf. Sci. 90, 85–143 (2015)
2. Stipe, B.C., Rezaei, M.A., Ho, W.: Inducing and viewing the rotational motion of a single molecule. Science 279, 1907–1909 (1998)
3. Motobayashi, K., Kim, Y., Ueba, H., Kawai, M.: Insight into action spectroscopy for single molecule motion and reactions through inelastic electron tunneling. Phys. Rev. Lett. 105, 076101/1-076101/4 (2010)
4. Motobayashi, K., Arnadottir, L., Matsumoto, C., Stuve, E.M., Jonsson, H., Kim, Y., Kawai, M.: Adsorption of water dimer on Platinum(111): identification of the -OHPt Hydrogen Bond. ACS Nano 8, 11583–11590 (2014)
5. Motobayashi, K., Kim, Y., Arafune, R., Ohara, M., Ueba, H., Kawai, M.: Dissociation pathways of a single dimethyl disulfide on Cu(111): reaction induced by simultaneous excitation of two vibrational modes. J. Chem. Phys. 140, 194705/1-194705/8 (2014)
2 Action Spectroscopy with STM 13
Chapter 3 Ambient Pressure X-Ray Photoelectron Spectroscopy
Hiroshi Kondoh
3.1 Principle
X-ray photoelectron spectroscopy (XPS) is one of most thoroughly used surface science techniques, which provides information on chemical states of both adsorbates and substrates on the basis of core-level shifts of photoelectrons excited primarily with X-rays. Since the photoelectrons are significantly attenuated by inelastic scattering with gas-phase species, the XPS measurements require high-vacuum conditions. In recent years, however, XPS measurements under near ambient pressure conditions have attracted much attention due to an increasing demand for understanding of surface phenomena under realistic conditions. To meet the demand, the ambient pressure (AP) XPS technique was developed by a combination of focused X-rays and differential pumping electrostatic lens system [1]. An example for AP-XPS system is shown in Fig. 3.1, where a focused X-ray beam is shined on a sample that is closely located to an aperture of the difference pumping system to reduce the attenuation effect on the photoelectrons by gas-phase species. A large part of emitted photoelectrons travel a short distance and enter the differential pumping system without inelastic scattering by the gas molecules. The photoelectrons further travel and reach an entrance of a hemispherical analyzer, while the gas-phase species are evacuated in the differential pumping system. With this kind of system, photoelectrons emitted from a sample in a high-pressure cell under near ambient pressure conditions can be detected and their kinetic energy is precisely analyzed, which yields XP spectra from samples under gas pressures typically up to several Torr.
H. Kondoh (&) Department of Chemistry, Keio University, Yokohama, Japan e-mail: [email protected]
15
hard X-ray regions.
3.3 Instrumentation
The key technology of AP-XPS is focusing of X-ray source on a narrow space of a short sample-aperture distance and differential pumping for electrostatic lenses as shown in Fig. 3.2. The distance d is typically 1 mm. To avoid inhomogeneous pressure distribution, the radius of hole of aperture R should be as small as less than d/2 [1], which means that R should be less than 500 lm. Therefore in order to use the X-ray source efficiently, the beam size should be of the order of several hun- dreds microns. In this sense, the synchrotron radiation (SR) has an advantage in efficient use of X-ray source. The shorter d and the smaller R enable to increase the upper limit of gas pressure; 100 Torr has been achieved with d = 200 lm and R = 50 lm at a kinetic energy of photoelectrons of 930 eV [2]. The photoelectron kinetic energy is another important factor to determine the pressure limit. If a higher photoelectron kinetic energy is used by photon source in a hard X-ray region, the pressure limit is significantly increased; recently, XP spectra were successfully measured using photon energy of ca. 8000 eV under the presence of N2 gas at 760 Torr.
Fig. 3.1 Example for actual AP-XPS system
16 H. Kondoh
After the photoelectrons enter the differential pumping system, they are focused by electrostatic lenses to pass through the holes of the differential pumping walls efficiently, while the gas molecules are evacuated as schematically illustrated in Fig. 3.2. The electron lens system and the electron detector have been further modified to obtain spatial and temporal resolutions. Note that not only SR-based AP-XPS systems but also laboratory-source-based systems are commercially available [3]. Although the laboratory sources (21.2 eV for HeI, 1253.6 eV for Mg Ka, and 1486.6 eV for Al Ka) give relatively low cross section and low surface sensitivity, the AP-XPS systems using these laboratory sources will definitely contribute to expansion of the user community of photoelectron spectroscopy.
3.4 Applications
3.4.1 Operando Observation of Chemical Reaction
The high-pressure cell of AP-XPS is modified to apply this technique to a broad range of application studies such as operando observations of heterogeneous catalysts and fuel cells, XPS measurements of liquids and solid–liquid interfaces under electrochemical environments. Here, two application studies concerning operando observations of (1) an Ir catalyst for exhaust gas and (2) a polymer electrolyte fuel cell are introduced.
• Ir catalyst for exhaust gas [4] Ir is known as a good catalyst for automobile exhaust gas, particularly NO reduction with a high N2 selectivity, although it is inactive at low temperatures
Fig. 3.2 Schematics for AP-XPS apparatus
3 Ambient Pressure X-Ray Photoelectron Spectroscopy 17
below 250 °C. To understand the high N2 selectivity and the absence of low-temperature activity, operando observation was performed for Ir(111) with AP-XPS as shown in Fig. 3.3. From the XP spectra, the surface coverages under reaction conditions are deduced as a function of temperature together with catalytic activity monitored by N2 and CO2 mass intensity as shown in Fig. 3.4. Below 250 °C, the surface is dominated by CO, indicating of CO poisoning. Above 250 °C, the Ir surface is activated where NO dissociation takes place resulting in light-off of N + NO and N + N reactions. At this moment, the N + NO reaction does not yield N2O but N2, which gives rise to the high N2
selectivity. • Polymer electrolyte fuel cell [5]
Ambient pressure hard X-ray photoemission spectroscopy (AP-HAXPES) was applied to in situ observation of Pt catalysts in the cathode of polymer electrolyte fuel cell under the presence of water vapor. Pt 3d XP spectra were successfully obtained, and potential-dependent oxidation and reduction of the Pt catalysts could be observed. This approach allows us to follow changes in chemical state at the Pt–water interfaces under working conditions as well as electrochemical control.
Fig. 3.3 Operando observation with AP-XPS for NO reduction by CO on Ir(111) under 50 mTorr NO and 10 mTorr CO with increasing temperature. Reprinted with the permission from Ref. [4]. Copyright 2017 American Chemical Society
18 H. Kondoh
References
1. Ogletree, D.F., Bluhm, H., Lebedev, G., Fadley, C.S., Hussain, Z., Salmeron, M.: A differentially pumped electrostatic lens system for photoemission studies in the millibar range. Rev. Sci. Instrum. 73, 3872–3877 (2002)
2. Kaya, S., Ogasawara, H., Näslund, L.-Å., Forsell, J.-O., Casalonguea, H.S., Miller, D.J., Nilsson, A.: Ambient-pressure photoelectron spectroscopy for heterogeneous catalysis and electrochemistry. Catal. Today 205, 101–105 (2013)
3. Tao, F.: Design of an in-house ambient pressure AP-XPS using a bench-top X-ray source and the surface chemistry of ceria under reaction conditions. Chem. Commun. 48, 3812–3814 (2012)
4. Ueda, K., Yoshida, M., Isegawa, K., Shirahata, N., Amemiya, K., Mase, K., Mun, B.S., Kondoh, H.: Operando observation of NO reduction by CO on Ir(111) surface using NAP-XPS and mass spectrometry: dominant reaction pathway to N2 formation under near realistic conditions. J. Phys. Chem. C. 121, 1763–1769 (2017)
5. Takagi, Y., Wang, H., Uemura, Y., Ikenaga, E., Sekizawa, O., Uruga, T., Ohashi, H., Senba, Y., Yumoto, H., Yamazaki, H., Goto, S., Tada, M., Iwasawa, Y., Yokoyama, T.: In situ study of an oxidation reaction on a Pt/C electrode by ambient pressure hard X-ray photoelectron spectroscopy. Appl. Phys. Lett. 105, 131602(1)–131602(5) (2014)
Fig. 3.4 Catalytic activity monitored by N2 and CO2
mass intensity (upper) and coverages of surface species under NO reduction reaction conditions on Ir(111) (lower). Adapted with the permission from Ref. [4]. Copyright 2017 American Chemical Society
3 Ambient Pressure X-Ray Photoelectron Spectroscopy 19
Chapter 4 Angle-Resolved Ultraviolet Photoelectron Spectroscopy
Takafumi Sato
4.1 Principle
ARUPS enables us to determine experimentally the electronic band structure and Fermi surface of crystals. In ARUPS experiments, as shown in Fig. 4.1, vacuum ultraviolet (VUV) photons are irradiated onto the crystal surface, and the kinetic energy of photoelectrons emitted from the crystal surface is measured as a function of angle with respect to sample normal. Figure 4.2 shows the energy conservation relation diagram in photoemission process [1, 2].
(1) An electron, which was at first located on the occupied band (Ei), is excited to the otherwise empty band (Ef) by a VUV photon with the energy of hx. The energy conservation is as follows.
hx ¼ Ef Ei: ð1Þ
(2) The excited state is assumed to be a free electron state with the bottom of its energy dispersion at E0 with respect to the Fermi level (EF). Momentum parallel and perpendicular to the crystal surface of the photo-excited electron in the crystal are defined as k// and k?, respectively. Then, the energy of electron in the excited state in the crystal is described as follows.
Ef ¼ h2ðk2== þ k2?Þ=2m E0: ð2Þ
T. Sato (&) Department of Physics, Tohoku University, Sendai, Japan e-mail: [email protected]
21
(3) We define the kinetic energy of photoelectron emitted into vacuum as EK, then we have,
Ef ¼ EK þU; ð3Þ
where U is the work function of the sample. (4) The momentum parallel to the crystal surface of photoelectron emitted into the
vacuum is described as follows.
K== ¼ ffiffiffiffiffiffiffiffiffiffiffiffi
where h is the polar angle of photoelectron (Fig. 4.1).
Fig. 4.1 Schematic view of ARUPS
Fig. 4.2 Energy conservation diagram of the photoemission process
22 T. Sato
(5) When the photoelectron is emitted from the crystal into vacuum through the surface, the momentum parallel to the surface is conserved.
k== ¼ K==: ð5Þ
Using formulae (1)–(5), we obtain the relationship between the energy and the momentum of the initial state in the crystal.
hk== ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
sin h:
By applying this formula to ARUPS data, one can map out the relationship between Ei and k==, namely the electronic band dispersion in the crystal.
4.2 Features
• Energy band dispersion of crystal can be directly observed. • Fermi surface of metals can be determined. • Energy gap related to insulating and superconducting properties can be
observed. • Many-body interactions responsible for the physical properties can be
elucidated.
4.3 Instrumentation
Figure 4.3 shows a schematic diagram of an ARUPS spectrometer. The apparatus consists of mainly four parts, (1) a large electrostatic hemispherical electron energy analyzer with the average diameter of about 40 cm, (2) a microwave-driven discharging lamp to produce high-intensity VUV light, (3) a sample preparation vacuum chamber where samples are prepared by cleaving, sputtering, evaporation, etc., and (4) an ultrahigh vacuum main chamber where the sample is irradiated by VUV light from the discharging lamp. In addition, the apparatus is equipped with a liquid-helium cryostat to cool down the sample, a sample transfer system, many vacuum pumps, etc. One can also use synchrotron radiation light to excite photoelectrons (see section Synchrotron Radiation Photoelectron Spectroscopy).
The recent progress in the energy resolution in ARUPS experiments owes mainly to the improvement of electron energy analyzer. Figure 4.4 shows the schematic diagram to explain how the energy and the polar angle of photoelectrons are measured with the hemispherical analyzer. By using the two-dimensional detecting system with a multi-channel plate (MCP) and a CCD camera, we are able
4 Angle-Resolved Ultraviolet Photoelectron Spectroscopy 23
to measure simultaneously the energy and the polar angle of photoelectrons. This 2D-detection system together with a large size of hemispherical analyzer enables a rapid and precise ARUPS measurement to lead to the ultrahigh-resolution measurement. The ARUPS technique provides us rich information with which we can discuss the relationship between the electronic structure and the novel properties of materials such as superconductivity.
4.4 Applications
4.4.1 Band Dispersion and Fermi Surface of 1T-VSe2
ARUPS technique is especially powerful for the layered materials due to the momentum conservation parallel to the surface. In layered materials, the energy dispersion perpendicular to the layer is negligibly small and therefore the obtained
Fig. 4.3 High-resolution ARUPS spectrometer
24 T. Sato
Fig. 4.4 Hemispherical analyzer with two-dimensional detector
Fig. 4.5 Experimentally determined band structure of layered compound 1T-VSe2 [3]
4 Angle-Resolved Ultraviolet Photoelectron Spectroscopy 25
band dispersions as a function of k// represent the band structure of the material. Figure 4.5 displays a set of ARUPS spectra of layered compound 1T-VSe2 and the experimental band dispersions obtained from these spectra [3]. Several characteristic bands from Se 4p and V 3d orbitals are clearly seen in the experimental band structure. One can find that the V 3d band reaches the Fermi level at midway between C and M points in the Brillouin zone, producing a metallic Fermi surface. The band structure calculation is also shown for comparison in Fig. 4.5. The good agreement between the experiment and the calculation indicates that ARUPS is able to map out the band structure of layered materials with high precision.
Fermi surface is one of important physical concepts with which we discuss the electronic properties of materials. By measuring many photoemission spectra all over the Brillouin zone and plotting the spectral intensity at EF as a function of two-dimensional momentum, one can map out experimentally the two-dimensional Fermi surface. Figure 4.6 shows an example applied for 1T-VSe2, where the characteristic two-dimensional cylindrical Fermi surface is clearly mapped out by ARUPS [3].
References
1. Cardona, M., Ley, L.: Photoemission in Solids I and II. Springer, Heidelberg (1978) 2. Hüfner, S.: Photoelectron Spectroscopy: Principles and Applications. Springer, Heidelberg
(2003) 3. Terashima, K., Sato, T., Komatsu, H., Takahashi, T., Maeda, N., Hayashi, K.: Phys. Rev. B 68,
155108 (2003)
Fig. 4.6 Fermi surface of 1T-VSe2 determined by ARUPS [3]
26 T. Sato
Masahiko Tomitori
5.1 Principle
A high electric field of greater than 0.5 V/Å can be generated over the apex of a sharpened metallic needle (tip) with a radius of less than 100 nm, by application of a voltage of higher than a few kV to the tip. Suppose that gas atoms (or molecules) of He, Ne, and so on (imaging gas) are admitted to a vacuum chamber, in which the tip is placed and biased at the positive high voltage. The gas atoms are electrically polarized under the high electric field, and attracted to the tip apex owing to the non-uniform high electric field around the tip; some of them are field adsorbed onto the tip apex. The high electric field pulls down the potential barrier between the tip and the gas atom; the barrier usually confines the electrons in the gas atom to itself. When the polarized gas atom comes in the proximity of the tip apex, the electron can quantum mechanically tunnel from the atom to the tip under the high electric field, where the barrier width becomes less than one nanometer (Fig. 5.1). Consequently, the atom is ionized to be a positively charged gas ion. This is referred to as field ionization. The gas ion is exposed to the high electric field, and then the repulsive force acts for the gas ion to travel away from the tip (Fig. 5.2). The ion flies away, being accelerated under the field, on the trajectory almost along the line of electric force, which extends radially from the tip apex to an electrically grounded plate placed at the front of the tip. In field ion microscope (FIM) [1, 2], the plate is a phosphor screen, on which the ion-incident position emits light. Accordingly, an image of the ion arrivals, geometrically expanded projection along the line of electric force from the tip to the screen, can be observed; the bright spots
M. Tomitori (&) School of Materials Science, Japan Advanced Institute of Science and Technology, Ishikawa, Japan e-mail: [email protected]
27
on the screen correspond to events of field ionization of the gas atoms over the tip apex. The probability of field ionization sensitively increases on the surface atoms and steps of the tip apex, because the electric field slightly increases over those areas atomically protruded from the averaged curvature of the tip. This is the principle of FIM, capable of observing the atomic arrangements on the tip apex.
By applying higher electric field, the protruded surface atoms on the tip apex can be ionized owing to the electric field permeation below the surface, which can make the ionized states of the tip atoms more stable than their neutral states in the electrostatic potential. Thereafter, the ionized tip atoms are evaporated, similarly to the ionized gas atoms in field ionization, referred to as field evaporation. The mass of individual tip atoms that field-evaporated by application of a fast-pulsed voltage is analyzed using a time-of-flight technique. This is the principle of atom probe (AP), which is usually used in combination with the FIM, i.e., as atom probe field ion microscope (APFIM).
Fig. 5.1 Potential energy diagram for field ionization. E the electric field, x the distance from the tip, / the work function of the tip, I the ionization potential of the gas atom
Fig. 5.2 Schematics of the FIM. A positive high voltage is applied to the tip. Imaging gas atoms of He are field-ionized under high electric fields
28 M. Tomitori
• Atom-by-atom mass analysis and atom-resolved observation of the tip apex. • Magnification M of the FIM is approximately the ratio of the tip-screen distance
R to the tip radius r; e.g., M * 106 = 10 cm/100 nm. • Layer-by-layer mass analysis is conducted by sequentially detecting the
field-evaporated ions, because the field evaporation proceeds statically atom-by-atom and layer-by-layer, in order of the electric field strength over the surface atoms, which depends on their atomic protrusions from the averaged curvature of the tip.
• In combination with a two-dimensional ion detector, three-dimensional mass analysis with atomic resolution is possible as the three-dimensional (3D) AP.
• When a negative high voltage is applied to the tip, electrons are emitted from the tip apex through the quantum tunneling (i.e., field emission). The amount of field emission of electrons changes with the work functions and protrusions on the tip apex; the electron projection image can be observed similarly to the FIM, referred to as a field emission microscope (FEM). The combination of APFIM with FEM can reveal the electronic states on the tip apex. The addition of an energy analyzer for field-emitted electrons provides detailed information on the electronic states.
5.3 Instrumentation
The tip and the phosphor screen are set face-to-face in a vacuum chamber equipped with a gas admission system. The tip is usually spot-welded to a tungsten loop for heating by passing current into the loop to clean the tip in the vacuum chamber, and it is biased at high voltages through vacuum electric feedthroughs. The tip can be cooled to suppress the thermal motion of the gas atoms for better resolution of the FIM image. For the AP mass analysis, the screen has a hole (probe hole) at its center, through which the field-evaporated tip atoms pass (Fig. 5.3). The direction of the tip can be mechanically tilted to change the analysis region of the tip apex. The trajectories of the field-ionized imaging gas ions to the screen are almost the same with those of the field-evaporated tip atoms. Thus, the atoms on the tip apex, which are imaged on the screen, can be targeted for the AP analysis by mechanically tilting the tip direction so as to fit the FIM spot of the targeted atom to the probe hole. In the AP analysis, the mass is measured atom-by-atom using the time-of-flight technique with a fast-pulsed high voltage or a fast laser pulse to promote the field evaporation of tip atoms, where the flight time from the tip to the ion detector is measured precisely; the flight time is shorter for the atoms with lower ratios of m:n, where m is the mass of the atom and n is the multiple of the elementary charge e for representing the electric charge of the ionized atom.
5 Atom Probe Field Ion Microscope 29
5.4 Applications
5.4.1 FIM Observation of a W Tip
Figure 5.4 shows FIM images of an electrochemically etched [111]-oriented W tip. Bright spots correspond to W atoms, mostly located at the step edges of the facetted tip apex. The spot patterns reflect the symmetry of the body-centered cubic (bcc) structure of W. Three atoms on the (111) facet with the threefold symmetry of the bcc structure, in Fig. 5.4a, were field-evaporated, in Fig. 5.4b.
5.4.2 AP Analysis: Alloy of Pt–Ir
Figure 5.5 shows the mass spectrum and plots of the cumulative number of the ions sequentially detected in the AP analysis for a Pt–Ir (10%) alloy tip. In the plots, the abscissa is the total number of the detected ions, and the ordinate is the number of respective detected atoms. The respective plots fot Pt and Ir show straight lines, indicating that the alloy is uniform. The ratio of It:Pr is calculated to be approximately 10.7%, in agreement with the specification of the supplied material.
Fig. 5.3 Schematic diagram of the APFIM setup
Fig. 5.4 FIM images of a W tip of the [111]-orientation. a V = +2.8 kV. b After voltage pulse application. Imaging gas: Ne of 10−5 Torr. Temperature: 50 K. c Facet indices
30 M. Tomitori
5.4.3 AP Analysis: The Interface Between Al and GaAs
A GaAs tip with an Al overlayer deposited at low temperature was AP analyzed with laser pulses. The plots of the cumulative number of the ions in Fig. 5.6 show the abruptness of the interface between the Al overlayer and the substrate of GaAs.
Fig. 5.5 (a) Mass spectrum and (b) the cumulative numbers of the ionized tip atoms sequentially detected in the AP analysis for a Pt–10% Ir alloy tip
Fig. 5.6 Cumulative numbers of the ions detected in pulsed-laser AP for an Al–GaAs tip
5 Atom Probe Field Ion Microscope 31
References
1. Tsong, T.T.: Atom-Probe Field Ion Microscopy: Field Ion Emission, and Surfaces and Interfaces at Atomic Resolution. Cambridge University Press, Cambridge (1990)
2. Miller, M.K., Smith, G.D.W.: Atom Probe Microanalysis: Principles and Applications to Materials Problems. Materials Research Society, Pittsburgh (1989)
32 M. Tomitori
Shintaro Fujii
6.1 Principle
AFM was developed to overcome a drawback of scanning tunneling microscopy (STM), which can only image conducting surfaces. The AFM has the advantage of imaging almost any type of flat surface, including polymers, ceramics, and biological samples. The AFM images topography of a sample surface by scanning a sharp tip with force sensor over a region of interest. The curvature radius of the tip is typically on the order of nanometers. A force between the tip and sample surface is used as a feedback signal. By using the feedback loop to control the height of the tip above the surface, the AFM can generate a topographic map of the surface features.
6.2 Features
• AFM operates in various environmental conditions such as in air, liquid, and vacuum.
• Nonconducting surface as well as conducting surfaces can be imaged with nano- or atomic-scale resolution.
• Typical contact-mode AFM operates in a short-range repulsive force regime, where a tip and a sample are in mechanical contact.
S. Fujii (&) Department of Chemistry, School of Science, Tokyo Institute of Technology, Tokyo, Japan e-mail: [email protected]
33
• Along with mapping of surface topography, simultaneous tunneling current mapping is possible.
• Single-molecule force spectroscopy with pico- or nano-Newton resolution can be routinely achieved.
6.3 Instrumentation
For sensing force between an AFM tip and surface, a cantilever is typically used as force sensor. The most common cantilevers in use are micromachined silicon (Si) or silicon nitride (Si3N4) cantilever with integrated tips (Fig. 6.1a) and are commercially available. The spring constants of the cantilevers are in the range from 0.01 to 100 N/m. A typical AFM instrument consists of a sample holder mounted on a piezoelectric scanner, a cantilever tip, and a position-sensitive photodetector (PSD) for detecting a laser beam reflected off the end of the cantilever beam to provide cantilever deflection feedback signal (Fig. 6.1b). The deflection signal is measured by the difference in light intensities between the upper and lower pho- todetectors. In a contact mode of AFM operations, a mechanical contact of the AFM tip and the sample surface is made and the AFM scans the tip over the sample surface with the feedback loop that enables the Z scanner to maintain the tip position at a constant force (constant cantilever deflection) above the sample surface. As the tip scans the surface of the sample, the contour of the surface with the constant tip-surface force can be obtained as an AFM topographic image. In addition to the contact mode, AFM has a variety of operational modes (Table 6.1). Some of the operational modes are described in the following chapters.
Fig. 6.1 a Scanning electron micrograph of a micromachined silicon cantilever with an integrated tip, which is placed on a test sample. Reprinted with permission from Ref [1]. b Schematic illustration of a typical AFM system
34 S. Fujii
6.4 Applications
6.4.1 Atomic Resolution Imaging of an Insulating Surface in Contact Mode
AFM can image nonconducting surfaces with atomic- or nano-scale resolution. Figure 6.2a shows the KBr (001) surface imaged in contact mode under ultrahigh vacuum (UHV) conditions at 4 K [2, 3]. The sample surface was prepared by cleaving KBr crystal in UHV along the (001) plane. A Si3N4 cantilever with a tungsten tip was used for the imaging. The spring constant of the cantilever was 0.37 N/m. Both ionic species are visible in the AFM image, because repulsive forces are used for imaging in the contact mode. On the basis of the observed size difference between the atoms, the small bumps are interpreted as K+ ions and the large bumps as Br− ions. Figure 6.2b shows another AFM image of the KBr (001) surface with a defect. At the upper left corner of the image, a linear atomic
Table 6.1 Operational modes of AFM
Mode Subcategory Measured AFM signal
Static mode Contact mode Deflection of a cantilever (normal force)
Torsion of a cantilever (lateral force)
Dynamic mode
Frequency modulation (FM) mode
Frequency shift of a oscillating cantilever
Fig. 6.2 a (Left) Atomically resolved image of KBr (001) in contact AFM mode. The small and large protrusions are attributed to K+and Br− ions, respectively. (Right) Surface structure of KBr (001). A lattice constant is 0.66 nm. The large circles represent the Br− ions (bare ion radius is 0.195 nm); the small circles represent the K− ions (bare ion radius is 0.133 nm). b Atomically resolved image of KBr (001) with a linear defect. The defect at the upper left of the image is indicated by an arrow. The red square corresponds to the surface area of 5 5 nm2. Reprinted with permission from Refs [2, 3]
6 Atomic Force Microscope 35
defect was observed. This result indicates that true atomic resolution was achieved by the contact-mode AFM and neglects the possibility that the observed atomic periodicity was due to averaged forces between atoms on the surface and atoms on the tip.
6.4.2 Force Measurement of a Single-Molecule Junction
AFM-based force spectroscopy was used to measure mechanical properties of single molecules. For example, interaction forces between single strands of DNA have been measured by covalently attaching DNA oligonucleotides to an AFM tip and surface [4]. The force spectroscopy can also measure rupture force of individual chemical bonds [5]. Figure 6.3a, b shows the force curve during breaking process of a single-molecule junction, where a 1,8-octanedithiol molecule binds to Au electrodes of an AFM cantilever tip and surface. Along with the force measurement, electronic conductance through the junction was simultaneously measured. While the conductance decreases in discrete steps, the force decreases in saw-tooth waves. The rupture force (i.e., the final step of the force curve, which is accompanied with a conductance drop to zero) is 1.5 nN. The rupture force is similar to the force required to break the Au–Au bond. Because the Au–S bond is stronger than the Au–Au bond, the Au–Au bond is responsible for the breakdown of the junction. In contrast, for the 4,4′-bipyridine (BPY) single-molecule junction, the rupture force is 0.8 nN (Fig. 6.3c), which is considerably smaller than the force required to break a
Fig. 6.3 a Schematic representation of a single-molecule junction formed between a gold-coated cantilever tip and a gold substrate. b, c Simultaneously recorded conductance and force curves of (b) 1,8-ontancedithiol and d BPY junctions during stretching. The spring constant of the AFM cantilever was 36 N/m. Reprinted with permission from Ref. [5]
36 S. Fujii
Au–Au bond. This is because BPY binds to Au electrodes via N–Au bond, which is weaker than the S–Au bond. The rupture force of the single-molecule junction reveals the bonding nature of the molecule to the electrodes.
References
1. Wolter, O., Bayer, Th, Greschner, J.: Micromachined silicon sensors for scanning force microscopy. J. Vac. Sci. Technol. B 9, 1353–1357 (1991)
2. Giessibl, F.J., Binnig, G.: Investigation of the (001) cleavage plane of potassium bromide with an atomic force microscope at 4.2 K in ultra-high vacuum. Ultramicroscopy 42–44, 281–289 (1992)
3. Giessibl, F.J.: Advances in atomic force microscopy. Rev. Mod. Phys. 75, 949–983 (2003) 4. Lee, G.U., Chrisey, L.A., Colton, R.J.: Direct measurement of the forces between
complementary strands of DNA. Science 266, 771–773 (1994) 5. Xu, B., Xiao, X., Tao, N.J.: J. Am. Chem. Soc. 125, 16164–16165 (2003)
6 Atomic Force Microscope 37
Chapter 7 Auger Electron Spectroscopy
Fumihiko Matsui
7.1 Principle
As each element carries unique values of core-level binding energies, EC, this has enabled the development of numerous elemental composition analysis methods based on core electron excitation. The excitation of a core level, C1, is accompanied by filling of the vacated core hole by an electron transition from a shallower bound state, C2. In particular cases, the excess energy released in this process can be transferred to another bound electron, C3, with a certain probability, leading to the subsequent emission of an Auger electron. The kinetic energy of an Auger electron is determined by the binding energies of the bound states involved: Ekin = EC1 − EC2 − EC3 − /; thus, it is dependent on the atomic number, Z, of the element involved, as shown in Fig. 7.1. Furthermore, the spectral features of Auger electrons are sensitive to chemical shifts in the core levels, as well as to the vari- ations in the valence band structure. The limited inelastic mean free path of electrons with kinetic energies of less than a few keV [3] makes Auger electron a relatively sensitive surface probe, with resolutions in the order of a nanometer. Auger electron spectroscopy (AES) is thus widely used for solid surface composition analysis and characterization. This phenomenon is named after Pierre Auger [4], a pioneering atomic physicist; it is noteworthy that Lise Meitner also discov- ered this phenomenon independently [5] prior to Auger.
Figure 7.2 illustrates the Auger electron emission process. Core electrons can be excited by X-rays, electrons, or ion beams, provided that they are of sufficient energy; note that Ekin is independent of the energy of the excitation source. To differentiate between particular Auger processes, the following nomenclature is used to refer to the subshells: “K” is for 1s, while “L1” is for 2s; when spin–orbital
F. Matsui (&) Graduate School of Materials Science, Nara Institute of Science and Technology, Nara, Japan e-mail: [email protected]
39
interactions are significant, the double degeneracy for the energy associated with core-hole generation for p, d, and f orbitals is removed, with “L3” and “L2” being used for 2p3/2 and 2p1/2, respectively. “V” is used when a valence electron is involved. As an example of the naming convention, KL1L2,3 stands for 1s excitation with the creation of two holes in 2s and 2p states.
Fig. 7.1 The kinetic energy of Auger electrons for various elements. Solid symbols indicate the energy of primary components, while open ones represent less intense satellite components [1, 2]. Note that H and He do not produce Auger electrons
Fig. 7.2 Schematic diagram of the Auger electron emission process. In practice, the work function of the analyzer, /ANA, is used instead of / for the sample surface, since the kinetic energy of Auger electron is measured from the vacuum level of the analyzer
40 F. Matsui
Auger electron emission and X-ray fluorescence are complementary phenomena: Fluorescent X-rays possess a large probe depth and therefore provide bulk information, whereas Auger emission yields information from the surface region. In addition, the yields of Auger electrons depend on the atomic number, as well as on the types of core level involved. For example, the Auger transition probability, aK, for 1s (K shell) excitation can be approximately expressed as
aK ¼ 1 1þ bKZ4ð Þ1, where the fitting parameter, bK 1:12 106. For Z values of less than 33, aK is larger than the corresponding transition probability for X-ray fluorescence, xK ¼ 1 aK.
7.2 Features
• Elemental composition and chemical environmental information. • Electrons and X-rays can be used for excitation. • Microscopic elemental imaging by scanning focused electron beams. • Through combination with ion sputtering, depth composition analysis is
possible.
7.3 Instrumentation
AES measurement systems for laboratory use are equipped with an electron gun for the excitation source, an electron energy analyzer, and a sample manipulator in an ultra-high vacuum chamber. Typically, electron beams with kinetic energies in the range 3–5 keV are used: This is because the ionization cross section of a core level is largest when the electron beam energy is around three to five times the binding energy [6]. In the case of electronic excitation, the energies of backscattered electrons contribute to a strong background signal following the primary elastic electron peak. Conventionally, Auger electron spectra are presented in their derivative forms, as the background intensity in the measured spectra is much higher than in the case of X-ray excitation.
Figure 7.3 shows an example of an AES measurement for an InSb surface passivated by sulfur, acquired using an electron gun and 4-grid low-energy electron diffraction (LEED) optics. The second derivative Auger electron spectra were measured using lock-in amplification to remove the background intensity. After annealing under ultra-high vacuum conditions, the intensities of the S MNN and C KLL signals were reduced, while the signals corresponding to In MNN and Sb MNN from the substrate increased. For high-sensitivity and high-energy-resolution measurements, cylindrical mirror analyzers (CMA) and concentric hemispherical analyzers (CHA) are used, respectively. For quantitative
7 Auger Electron Spectroscopy 41
analysis, refer to the ISO 18118, which describes the various effects that can alter the AES signal intensity [7].
7.4 Applications
X-ray Auger electron spectroscopy: Fig. 7.4a shows a two-dimensional measurement of Cu LMM Auger electron emission at the L3 and L3 absorption edges as a function of photon energy and photoelectron kinetic energy [8]: The diagonal lines where the kinetic energy increases with the photon energy correspond to the photoelectrons. By averaging the intensity of various photon energies at each kinetic energy, an X-ray-excited Auger electron spectrum can be obtained, as shown in Fig. 7.4b. The Cu L3M4,5M4,5 Auger electrons appear as horizontal lines with constant kinetic energies, starting at a photon energy of 933.1 eV in Fig. 7.4a. The peak with highest intensity, at a kinetic energy of 914 eV, and the second largest peak at 917 eV correspond to the two-hole final states of the 1G4 and
3F2,3,4
multiplets, respectively. For the further information about the fine structure of Auger electron spectra, refer to the review by Bambynek [9]. Depth profiling: The Auger electron probe depth can be altered by changing the polar angle of emission detection: For this purpose, a CHA with high angular resolution is adequate. Recently, various input lens/deflector combinations for CHAs have been commercialized, enabling investigations of thin films with thicknesses of a few atomic layers and surface segregation of alloys.
Sample surface sputtering by noble gas ions is a destructive depth-profile analysis method, where the composition ratio obtained from AES analysis can be plotted as a function of sputtering time. As the sputtering efficiency differs for each element and material, calibration by other methods is necessary.
Fig. 7.3 a Schematic diagram of a retarding-field and concentric-mirror analyzers. b The derivative Auger electron spectra from InSb(001) surface before and after the annealing treatment in the ultra-high vacuum condition
42 F. Matsui
Microscopy: Electron beams can be easily focused by electrostatic lenses. By adding an electron spectrometer to a scanning electron microscope system, the elemental composition of a particular point can be analyzed. As mentioned above, CHA gives access to depth information, and the sensitivity limit of AES is about 0.1–1% of surface monolayer adsorbates. Figure 7.5 shows the example of SEM observations combined with AES measurements: Scanning Auger microscopy (SAM) is achieved by mapping the intrinsic Auger electron peak intensity. As SAM image acquisition can take several hours, the region to be analyzed should be determined by SEM measurements prior to SAM imaging.
Fig. 7.4 a A two-dimensional intensity map of electrons from Cu surface. The abscissa and the ordinate are photon energy and photoelectron kinetic energy, respectively. b An X-ray-excited Auger electron spectrum is obtained by averaging the intensity for various photon energies at each kinetic energy. c An X-ray absorption spectrum is obtained by averaging the intensity for various kinetic energies of each photon energy
7 Auger Electron Spectroscopy 43
References
1. Physical Electronics, Inc. Handbook of Auger electron Spectroscopy, 3rd edn. (1996) 2. https://www.nist.gov 3. Tanuma, S.: Electron Attenuation Lengths in Surface Analysis by Auger and X-ray
Photoelectron Spectroscopy. In: Briggs, D., Grant, J.T. (eds.) IM Publications and Surface Spectra Limited, pp. 259–294 (2003)
4. Auger, P.: Sur les rayons b secondaires produits dans un gaz par des rayons X. C. r. hebd, séances Acad. Sci. 177, 169–171 (1923)
5. Meitner, L.: Über die Entstehung der b-Strahl-Spektren radioaktiver Substanzen. Zeitschrift für Physik. 9, 131–144 (1922)
6. Casnati, E., Tartari, A., Baraldi, C.: An empirical approach to K-shell ionisation cross section by electrons. J. Phys. B 15, 155–167 (1982)
7. ISO 18118: Surface Chemical Analysis—Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy—Guide to the Use of Experimentally Determined Relative Sensitivity Factors for the Quantitative Analysis of Homogeneous Materials (2004)
8. Matsui, F., Maejima, N., Matsui, H., Nishikawa, H., Daimon, H., Matsushita, T., Muntwiler, M., Stania, R., Greber, T.: Circular dichroism in Cu resonant Auger electron diffraction. Z. Phys. Chem. 230, 519–535 (2016)
9. Bambynek, W., Crasemann, B., Fink, R.W., Freund, H.-U., Mark, H., Swift, C.D., Price, R.E., Venugopara Rao, P.: X-Ray fluorescence yields, Auger, and Coster-Kronig transition probabilities. Rev. Mod. Phys. 44, 716 (1972)
Fig. 7.5 SEM observation of a semiconductor device. The magnified array consists of Fe dots with a width of a few µm. C and O KLL Auger peaks are from the coating materials
44 F. Matsui
8.1 Principle
Cathodoluminescence is a light emission according to an electron beam injection. Using this phenomenon, we can characterize not only the nature of defects or impurities but also their distributions in the material. CL is useful for the characterization of semiconducting materials such as GaN, ZnO, III–V heterostructures, as well as fluorescent materials like rare-earth-doped oxides. CL can also be applied to the characterization of nanomaterials [1] and for the failure analysis of the optical devices such as light-emitting diodes (LEDs) or lasers. CL is related to direct transition of electrons in materials, such as excitonic emission, donor (D)- or accepter (A)-related transitions, D-A pair recombination. Low-temperature observation sometimes gives useful information for CL spectroscopy.
8.2 Features
• Spectroscopy gives the energies of direct transition, which are related to the energy levels of defects or impurities.
• CL monochromatic image gives the distribution of the defects or impurities. • CL can be combined with secondary electron imaging and EBIC observation. • Low energy electron beam excitation improves spatial resolution of CL. • Low-temperature observation typically improves CL intensity as well as
sharpness of spectral feature.
45
8.3 Instrumentation
CL system is usually installed in a scanning electron microscope (SEM). As shown in Fig. 8.1, typical system is composed of light collecting unit, light detection unit as well as PC controller. Beam blanker is sometimes installed for lock-in amplification to suppress background signal of the charge-coupled device (CCD). Specimen cooling system is preferable for CL spectroscopy. Sometimes EBIC system is also attached on the specimen stage. The light collecting unit is composed of ellipsoidal mirror and optical fiber. Specimen is set at one focal point of ellipsoid so that CL emitted from the specimen is focused on another focal point, at which the end of optical fiber is fixed. The optical fiber is versatile to align the light collecting system in SEM chamber with detection system outside. Parabolic mirror is also used for light collection, but the off-axis light emitted from out focal point is difficult to collect. CL light is then guided into monochromator or optical filters to select the wavelength. The selected light is led to CCD or photomultiplier (PMT). The former is used for spectral detection (parallel), while latter for imaging (serial). Photon counting is preferable for PMT detection. Special consideration should be done for the detection of ultraviolet (UV) or infrared (IR) emissions to avoid unexpected absorption or low sensitivity. The spectra or images are recorded in the memory of personal computer (PC) to conserve quantitative information. Spectral line scan or two-dimensional mapping of CL spectra is preferable to analyze the detail of emission [2].
8.4 Applications
Nanostructures are suitable for CL because their geometry restricts the carrier diffusion to keep high spatial resolution. CL is also useful because it does not need any pretreatment like contact fabrication. Figure 8.2a, b shows the secondary
Fig. 8.1 Block diagram of CL system
46 T. Sekiguchi
electron (SE) images of ZnO nanotubes grown on a sapphire substrate by MOCVD [3]. The nanotubes have hexagonal rod features of 1 lm in diameter and several lm long with the wall thickness of 150 nm. The CL spectrum at room temperature shown in (c) has a strong emission at 3.24 eV and a weak broad one around 2.1 eV. The former corresponds to band-edge emission and the latter to defect-related emission. These CL images are shown in Fig. 8.2f, g as well as SE image (e). The band-edge emission is localized at the bottom region of nanotube while defect-related emission is uniform. At 10 K, on the other hand, the band-edge emission becomes about 100 times stronger, and the peak splits into two sharp lines at 3.316 and 3.364 eV (Fig. 8.2d). These peaks are assigned as D-A pair recombination and neutral donor bound exciton recombination (D0X), whose images are shown in Fig. 8.2h, i. The D-A pair recombination looks more localized than D0X recombination, suggesting that acceptor species are nonuniformly distributed in ZnO nanotubes. Thus, low-temperature observation gives fruitful information of luminescence centers.
GaN heterostructures are widely used for blue and/or white LEDs. They are typically grown on the sapphire substrates. The patterned substrate and GaN buffer layer are generally used to avoid high density of misfit dislocations. The cross-sectional CL observation of such structure is interesting to investigate the defect propagation. Figure 8.3 shows the cross-sectional CL images of a blue LED and line plot of spectra. The SE image (a) indicates the interface of sapphire and GaN layer. The schematic representation of this structure is shown in (b). Monochromatic CL image of 300 nm (c) is bright in the substrate region. CL at 360 nm (d) shows darker buffer GaN and brighter epitaxial GaN layers. The vertical
Fig. 8.2 ZnO nanotubes grown on sapphire. Secondary electron (SE) images (a, b), CL spectra at 300 K (c) and 10 K (d), SE and CL images at 300 K (e, f, g), and CL images at 10 K (h, i). Electron beam; 10 kV, 0.8 nA
8 Cathodoluminescence 47
dark lines in epilayer are the dislocations, maybe propagated from the buffer layer. The 430 nm CL image (e) shows the bright line at the part of surface region, which is an InGaN/GaN active layer to emit blue light. These luminescence images are overlapped with the colors such as blue, green, and orange as shown in (f). The line scan of CL spectra along yellow line of image (d) is shown in (g). The analyses of peak positions and intensity distribution give more detailed information of this device structure.
Fig. 8.3 Cross-secti

Compendium of Surface and Interface Analysis

Documents

Transcript of Compendium of Surface and Interface Analysis