行政院國家科學委員會補助專題研究計畫 V 成 果 報 告 □期中進度報告

結合同步輻射與掃描穿隧顯微術來測量奈米尺寸下表面化學成份之分佈

計畫類別：V 個別型計畫 □ 整合型計畫

計畫編號：NSC － － － － －

執行期間： 96 年 8 月 1 日至 97 年 12 月 31 日

計畫主持人：陸大安、羅夢凡

共同主持人：許瑤真、魏德新、陳家浩

計畫參與人員：林楹樟、邱清源、陳悅來

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執行單位：國家同步輻射研究中心

中 華 民 國 98 年 12 月 17 日

計畫名稱 結合同步輻射與掃描穿隧顯微術來測量奈米尺寸下表面化學成份

之分佈

計畫編號 5 執行期間 2007/08/01 ~ 2008/12/31

報告類別 研究成果報告(完整版) 計畫類別 新世代用戶研究計畫

目錄

報告內容 o 前言 1 o 研究目的 1 o 研究方法 1 o 結果與討論 5

計畫成果自評 9 附件 10

1

報告內容

前言

Characterization of surface properties, such as surface morphology, electronic

properties, and chemical composition, is always the main task in the experiments of

surface science research. As the era of nanotechnology is arriving, many devices of

nanometer sizes are constructed with novel material, and characterization of surface

properties at a nanometer scale has become extremely important to fully explore the

potential of these newly constructed nanodevices. To fulfill a variety of demands, it is

expected that future material used in building nanodevices with specific functional

properties will likely consist of several chemical elements, and the ability to perform

chemical mapping at a nanometer scale becomes increasingly important.

Unfortunately, there is no such tool available currently, and our proposed experiment

is aimed to address the issue.

研究目的

For surface characterization, the photoelectron spectroscopy (PES) and the scanning

tunneling microscopy (STM) are two of the most powerful techniques, and both have

unique advantages over other techniques. PES can provide information on the

chemical composition on surface, but without spatial resolution; STM can explore

surface with atomic resolution with little information on chemical composition. These

two techniques complement each other. We propose to develop a novel instrument to

take advantages of both techniques simultaneously, and our goal is to gain the ability

to acquire surface chemical mapping at a nanometer scale. This ability is expected to

be a key step to construct and study novel nanometer-size devices with multiple

chemical elements.

研究方法

The photoelectron spectroscopy (PES) and the scanning tunneling microscopy (STM)

are two powerful techniques for surface characterization. PES provides a direct probe

into surface electronic states. In its angle-resolved mode, it measures both the kinetic

energy and momentum of photoelectrons, and yields a three-dimensional band

2

structure. PES can be very surface sensitive if the kinetic energy of photoelectrons

from interested electronic states is set properly, which can be accomplished by using

photons of suitable energy from synchrotron radiation. PES can identify different

chemical species easily by measuring photoelectrons excited from core levels. PES

can also study crystal structures on and below the surface through photoelectron

diffraction. One major disadvantage of PES is its lack of spatial resolution because of

the limitation on the size of the incoming photon beam. Even with the most advanced

and very costly scanning photoemission microscope (SPEM), the best spatial

resolution is roughly 100 nm, which is far worse than what the techniques utilizing

scanning probes can achieve. The lack of spatial resolution severely limits the

possibility to utilize PES in studying nanostructures.

STM, one of the techniques utilizing scanning probes, perhaps is the most

powerful tool to explore surface with atomic resolution, and it is especially useful

when the surface lacks of a macroscopic order. The major limitation of STM is that it

cannot provide information on chemical composition easily. To avoid disturbing the

surface, the tip bias must be limited, and the tip can only pick up electrons from

conduction bands or valence bands with low binding energy. These electrons possess

system-wide properties due to stronger interaction among them, and it is difficult to

determine the atomic species that the electrons originate from. Some techniques, such

as the scanning tunneling spectroscopy (STS), have been developed to identify

chemical composition on surface from these electrons of low binding energy, but

yield little success.

To take advantages of both STM and PES simultaneously, we propose to

develop the synchrotron radiation assisted scanning tunneling microscopy (SR-STM).

The key operation mechanism of SR-STM is to collect the photoelectrons using the

STM tip as a function of the bias applied. Figure 1 shows the major difference

between standard STM and SR-STM. The tip bias typically used in STM

measurements is limited (less than a few volts) to avoid disturbing surfaces, so only

the electrons in conduction bands and valence bands of low binding energy contribute

to the measured tunneling current, shown as the shaded area in Figure 1(b). Little

species information can be extracted in this kind of measurements. Therefore, atoms

of different chemical species cannot be distinguished in STM images, as indicated in

3

the drawing in Figure 1(a). However, if the electrons of the core-levels can be excited

by incoming photons and gain enough kinetic energy to tunnel into the tip, the

variation in the tunneling current should contain information on chemical composition

on the surface, as shown in Figure 1(c). Figure 1(d) shows a curve of photoelectron

density as a function of kinetic energy. The curve contains many common features of

measured photoelectron intensity, including the Fermi edge, valence bands, a core

level, vacuum cutoff, and a secondary electron background. The tunneling current in a

STM measurement is proportional to the integration of all electrons available for

tunneling. If the tip is biased positively, all photoelectrons, including the huge

secondary electron background, will contribute to the tunneling current. This scenario

is not desirable because the core level signals, the major signals used for species

determination, will be buried among a big secondary electron background. However,

if the tip is biased negatively, it will act as a discriminator to repel most of the

electrons with kinetic energy lower than the bias voltage. In this case, the background

of the tunneling current will be greatly reduced and the signals from core-levels can

be detected as a function of the bias. The electron state density can also be measured

by performing the measurement of the conductance, dVdI , as a function of the bias

voltage, V.

The electron-collecting surface of the tip apex is small and also designable.

We expect that SR-STM reaches a high spatial resolution (~10 nanometers) in

determining the chemical compositions and electronic states of the surface. In

addition, by employing a tip with a multi-layer coating, the SR-STM collect only the

photoelectrons coming from the surface right below the tip, which is expected to

improve the spatial resolution further to a range of a few nanometers.

4

Figure 1: Standard STM vs. SR-STM

e-

+V

Standard STM

e-

+V

e-

+V

Standard STM(a)

(b)

hv-V

e-

Lock-in

Electron density of state: dI/dV

SR-STM

hv-V

e-

hv-V

e-

Lock-in

Electron density of state: dI/dV

SR-STM(c)

(d)

5

結果與討論

This project is collaboration among five principle investigators: Dr. Y.-J. Hsu

(NSRRC), Dr. W.-D. Wei (NSRRC), Dr. C.-H. Chen (NSRRC), Dr. M.-F. Luo

(NCU), and Dr. D.-A. Luh (NCU). Despite limited funding and manpower, the result

generated from the project has been fruitful, including an end system, one journal

paper, and international and domestic presentations. A summary of the research

accomplishments and ongoing activities related to the current project is in the

following.

1. Acquisition of the SR-STM

Because of the limited funding, it was quite an effort to acquire a STM, a major part

of the project. After months of negotiations and discussions, a customized STM was

ordered from Unisoku Co., Ltd., Japan in the spring of 2008, and the STM was

delivered six months later. In addition to common functionalities, the customized

STM was designed to work with the synchrotron radiation. Beam ports were add to

allow irradiating sample surface during the operation of STM. In addition, the tip

holder was specifically designed to hold a tip with a multi-layer coating. Tests of

performance were carried out during the delivery of the STM. The tests were

performed near the beamline 09A1. Although the STM was installed in a noisy

environment and without external suspension, we successfully acquired atomically

resolved images of the surfaces of graphite and Si(111)-(7×7), which indicates the

good performance of the STM.

2. Construction of the SR-STM endstaion

An endstation is a necessity to install the STM on a beamline. In order to perform

studies on clean surfaces under an UHV condition, the endstation needs to outfit many

expensive accessories, such as ion pumps, turbo pumps, ion gauges, and controllers,

etc. The limited budget could not cover the expense of a STM, let alone the

construction of an endstation. To put together an endstation, the NSRRC collaborators

have contributed their own research funding and equipments that were supposed to be

used for their own researches. Special credits go to Dr. Wei for his great effort to

design the endstation.

6

The endstation was built with combining the STM with vacuum pumps,

loadlock, and a supporting frame with the mechanism for alignment, as shown in

Figure 2. The endstation was installed on 09A2 for commissioning from

3/26/2009~3/29/2009 and 4/7/2009~4/9/2009. During the commissioning, many tests

were carried out to answer the questions, such as how to align the system and the

beam efficiently, how the feedback electronics of the STM reacts under the influence

of photocurrent, and how the image quality changes with the tip under irradiation, etc.

Further commissioning of the endstation is scheduled in the end of this year, and a

preparation chamber for the growth of advanced samples is planned for construction

in the coming year.

3. Development of tips with a multi-layer coating

In this project, we plan to collect photoelectrons with a STM tip to improve the spatial

resolution of chemical mapping. To estimate to the spatial resolution, simulations of

electron trajectory during the operation of SR-STM with varied tip sizes and positions

were performed, and the results were published in one journal paper (Appl. Phys. Lett.

92, 103101 (2008)) and many conference papers. According to the result of

simulation, the tips with coating are needed to achieve an ultimate spatial resolution.

Methods are under investigations to manufacture tips with an insulated coating. We

have tried to coat the W tip with glass. A re-etching method, as shown in Figure 3,

produces a glass-coated W tip, but with too much exposed surface of the bare tip

(Figure 4). We also tried to coat the W tip with a thin layer of Al2O3. With atomic

layer deposition, we are be able to coat a W tip with a smooth thin layer of Al2O3

(Figure 5). Removing of the Al2O3 coating on the tip apex with focus ion beam is now

in progress.

7

Figure 2. The endstation of the SR-STM. The customized STM and the loadlock are on the left and the right in the photo.

5mm glass tube

Fix → pull

torch

5mm glass tube

Fix → pull

torch

Re-etching

Figure 3. Sketches to illustrate the re-etching method. Glass is coated on a pre-etched W wire at high temperature. The glass coating is then broken, and the wire is re-etched to form a tip.

8

Glass Coated

50 µm

Figure 4. A SEM image of a glass-coated tip made with the re-etching method.

Figure 5. SEM images of an Al2O3-coated tip made with atomic layer deposition. The W tip was coated with an Al2O3 film of 140Å.

9

計畫成果自評

With very limited budget and manpower, we have managed to construct a prototype

to utilize the SR-STM, a novel technique for surface analysis that we believe to play

an essential role in the era of nanotechnology. During the execution of the project, we

set up the main instrument, an ultrahigh-vacuum variable-temperature STM with the

capabilities to acquire atomically resolved images in the noisy environment of the

storage ring, to irradiate the sample surface during the operation of the STM, and to

hold a tip with a multi-layer coating. An endstation with basic vacuum and a limited

ability of sample preparation was constructed, and its commissioning is in progress.

Researches on the tip with a multi-layer coating are active, seeking to achieve an

ultimate spatial resolution of SR-STM. In addition, an efficient collaboration among

researchers of the NSSRC and NCU was established, and the research results from the

collaboration have laid a solid foundation for future collaboration. By building and

operating the main instrument, the researchers who participate this project have

gained hands-on experience on many useful techniques, including UHV, STM, PES,

SEM, surface analysis tools, and data acquisition and analysis, etc. The training that

the researchers receive in this project was proven very valuable. Overall, we believe

that this project was executed well as proposed, and we appreciate greatly the

financial support of the NSRRC.

Collecting photoelectrons with a scanning tunneling microscope nanotipChing-Yuan Chiu, Yuet-Loy Chan, Y. J. Hsu, and D. H. Weia�

National Synchrotron Radiation Research Center, Hsinchu, Taiwan 30076, Republic of China

�Received 16 January 2008; accepted 13 February 2008; published online 10 March 2008�

The collection of photoelectrons excited with a synchrotron via a nanotip placed near the surface ofa sample is studied. Simulating the electron trajectory, we found that photoelectrons escaping fromthe surface are too weak to be the only source of electrons contributing to a photocurrent detectedwith a scanning tunneling microscope tip, as reported recently. The tunneling of low-energyelectrons generated with synchrotron irradiation is suggested as an additional channel contributingto the photocurrent at a small separation between tip and sample. An image based on x-rayabsorption is expected to attain a resolution comparable to a topographical image. © 2008 AmericanInstitute of Physics. �DOI: 10.1063/1.2894186�

Microscopy and spectroscopy are two highly popularmethods employed in modern research. With a dedicated mi-croscope and spectrometer, scientists are able to examine finetopographical geometries and dilute chemical speciesthrough independent measurements. Among modern micro-scopes, the scanning tunneling microscope �STM� is knownfor its resolving capability on an atomic scale via monitoringthe small variation in tunneling current when a metallic tipscans across the surface. Photoelectron spectroscopy �PES� isrecognized for its superior chemical sensitivity when imple-mented with synchrotron radiation �SR�. As both STM andPES involve the detection of electrons, these two techniquescan be combined into one. Such an idea for amalgamatingmicroscopy and spectroscopy is not new. The developmentof instruments with an aim to acquire spectral informationfrom a finite area began in the late 1980s with the name“spectromicroscope” or “microspectrometer.”1–3 After almosttwo decades of effort, a state-of-the-art instrument promisesto deliver a chemical image with a resolution better than5 nm through a sophisticated design of electron optics.4,5 Theunion of STM with SR illumination discussed here is analternative approach toward nanospectromicroscopy. Takingadvantage of a well established microscope and a bright pho-ton source provided by a synchrotron facility, Saito et al.6

and Eguchi et al.7 reported the element-specific images ac-quired by a STM tip raster over a SR-illuminated surface.While the photocurrent was found weak �approximately pi-coampere� in comparison with the tunneling current �ap-proximately nanoampere�, both Saito et al. and Eguchi et al.demonstrated the feasibility of performing a localized spec-tral analysis and acquiring a two-dimensional chemical im-age through a STM tip. To examine the potential of thisnanotip approach, we simulated electron trajectories to studythe collection of photoelectrons emitted from a surface onplacing a tip of nanometer size at several distances above themetallic surface under various bias voltages. Besides themagnitude of the photocurrent, we investigated the dimen-sions of the effective probed area defined by the presence ofa metallic tip and an acceleration field.

In our initial test, the STM tip approached from 200 nmabove the surface to a separation of 1 nm. The SR-inducedemission of photoelectrons and their collection via a STM

nanotip are treated as independent events. For a STM toacquire an image conventionally, its metallic tip must be heldnear a surface under a voltage bias so that electrons cantunnel through the energy barrier between the STM tip andthe surface during the scan; the magnitude of the tunnelingcurrent is generally a function of voltage bias �Vbias� andseparation �z� between tip and surface. When SR illumina-tion is activated, the emission of photoelectrons produces anadditional contribution to the overall tip current, but unlikethe tunneling electrons for which the path of travel is con-fined to the tip apex, the available area for detection of pho-toelectrons to register includes the sidewall of the STM tip. Itis, therefore, understandable that a bare STM tip would havea large effective probing area on the surface. To decrease theeffective probing area without interfering with the conven-tional STM operation, it was suggested that the area for col-lecting electrons on the metallic tip be redefined by coatingmost of it, except for a tiny region about the apex, with aninsulating layer.8 Questions arise as to what area a reasonablebare tip region should be and how it can be fabricated and towhat extent the photocurrent is diminished after distant pho-toelectrons are discriminated by the modified tip.

The simulation program used in the current work is SI-

MION 8.9 By defining the electrodes �i.e., the tip and the me-tallic surface�, we constructed the field potential betweenelectrodes by solving the Laplace equation with finite-difference methods. Based on that field, electron trajectorieswith various initial conditions were then calculated and plot-ted. The inset of Fig. 1 illustrates the geometric arrangementof the tip surface, in which the bare tip region acts as anelectron detector of dimension defined by Ltip. Because thesystem exhibits rotational symmetry about the tip axis�z-axis�, all results presented here are based on simulationsperformed on a two-dimensional plane. Furthermore, consid-ering that the low-energy secondary electrons are the mostpopulous upon SR irradiation, we focused on the impactscaused by secondary electrons. The emission of secondaryelectrons was approximated by letting each grid representinga metallic surface emits an electron with a brightness distri-bution described by Henke et al.10 With the emission prob-ability per unit angle and unit energy written as P�Ek ,������Ek / �Ek+W�4�cos �, in which ��, Ek, W, and � representthe photon absorption coefficient, kinetic energy of the pho-toelectron, work function of the surface, and emission anglea�Electronic mail: dhw@nsrrc.org.tw.

APPLIED PHYSICS LETTERS 92, 103101 �2008�

0003-6951/2008/92�10�/103101/3/$23.00 © 2008 American Institute of Physics92, 103101-1Downloaded 17 Dec 2009 to 140.115.30.198. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

measured from the surface normal, respectively, the trajec-tory simulation involves the emission of 18 000 electronsfrom each surface grid. The 18 000 trajectories are the resultof emission angles 0°–180° and an energy window of0–10 eV under steps 1° and 0.1 eV. Calculating the finalposition of each electron trajectory emitted from every sur-face grid under the influence of an accelerating field, weobtain the normalized photocurrent on summing those trajec-tories that reach the bare tip region.

The quantity used to evaluate the resolving power in ourwork is the effective probing diameter �Dprob�. Assuming aconstant photon flux to be homogeneously delivered to anarea Aph on the metallic surface, each surface grid locatedwithin the illumination area would then produce the samenumber of emission events. For the nanotip placed at a dis-tance z above the surface, the negative surface bias �−Vbias�pushes electrons toward the tip and results in a reading ofphotocurrent �Itip�. As shown in Fig. 1, Dprob is defined as thediameter of a circular region on the surface within whichemitted electrons contribute 90% of Itip. According to thisdefinition, the absolute value of Itip is estimated from thesample current �Isample�,

Itip = ��10

9

Dprob

2�2 Isample

Aph. �1�

As the near-edge x-ray absorption spectra �NEXAS� revealchemical information through Isample, the STM tip collectingphotoelectrons can acquire the same information from anarea defined by Dprob. In Fig. 2, the dependences of Dprob onthe three parameters z, Ltip, and Vbias are illustrated. As ex-pected, decreasing the probing area necessitates a smalldistance between the tip and the sample �small z�, aminimized area of detection �small Ltip�, and a strong accel-erating field �large Vbias�. According to Fig. 2, Dprob can bedecreased to about 40 nm at best, even when the system isrunning at a tunneling condition such that �z ,Vbias ,Ltip�= �1 nm,1 V,10 nm�. Applying a greater bias or decreasingfurther the separation between the tip and the sample is prac-ticable, but neither approach significantly improves Dprobwithout raising a concern about surface modification. An-

other option to improve Dprob is to shrink the bare tip region,but a small detector is generally accompanied by a weaksignal. We seek here to estimate the photocurrent detected bythe nanotip. The photocurrent collected by a tip at Dprob=40 nm is estimated on inserting 50�300 �m2 and10−8 A—typical values delivered by the U5 undulator atNSRRC—into Eq. �1� as Aph and Isample respectively, and aphotocurrent 10−15 A is returned for Itip. Compared with thephotocurrent of 10−13 A or larger experimentally measuredvalues at the Ni L edge �KEK BL13C�,7 even when employ-ing a state-of-the-art photon source, our estimate based oncollecting electrons flying through free space under the in-fluence of an accelerating electric field is clearly at most 0.01times the experimental values.

Such a large photocurrent found at a tunneling distanceis not limited to synchrotron illumination. In a measurementexcited with a laser, Gray found a similar enhancement ofcurrent at a small separation between the tip and the surfaceand suggested that the tunneling of photoemission electronsafter their ejection from the surface is the origin of the largetip-surface conductivity.11 Applying a similar concept to thecase of synchrotron illumination, we argue that the photoex-cited electrons are detectable with the STM tip through tun-neling, as illustrated in Fig. 3. The impinging of energeticx-rays would basically generate “primary” photoelectrons,

FIG. 1. �Color online� Geometric arrangement of the tip and surface used inour simulation and definition of the effective probing diameter. In simula-tion, only the tip-apex region �Ltip� can receive photoelectrons; most of thetip is considered nonconductive. With the work function of the metal surfaceset at 4 eV and the emission probability as ���Ek / �Ek+4�4�cos �, this plotshows the relative extent of photoelectrons capable of reaching the bare tipregion as a function of their emission positions. According to this plot, theeffective probing diameter is defined as the region producing 90% of thephotocurrent collected by the tip. The dip of photocurrent appearing atz=0 is due to the finite grid numbers used to describe the tip apex.

FIG. 2. �Color online� Dependences of Dprob on parameters z, Ltip, and Vbias.�a� Probing diameter �Dprob� vs separation between tip and surface �z� isexamined for several values of Ltip. �b� Probing diameter �Dprob� vs voltagebias �Vbias� at z=50 nm. The relevant dimension Ltip appearing in both casesclearly demonstrates how distant photoelectrons affect Dprob.

103101-2 Chiu et al. Appl. Phys. Lett. 92, 103101 �2008�

Downloaded 17 Dec 2009 to 140.115.30.198. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

including core electrons and Auger electrons. Although someprimary electrons can transport to, and escape from, the solidsurface, a large proportion of them lose their large kineticenergy to surrounding atoms or the lattice through inelasticcollisions. During such collisions, more excited electronscarrying less kinetic energy are generated. This cascadingprocess results in many electrons of small energy of whichthe population scales inversely with their kinetic energy. Thesituation is that before an excited electron can be detectedwith an instrument such as an electron-energy analyzer, itmust first break away from the surface; hence, only thoseexcited electrons carrying sufficient kinetic energy to over-come the work function of the material and reaching thesurface from the solid side with an angle of incidence smallerthan a critical angle of total reflection ��c� have the chance tobecome detected. With the critical angle expressed in termsof the surface work function �W� and the kinetic energy ofthe electron outside the surface �Ek�, �c=sin−1�Ek /Ek+w�1/2,the equation indicates that the smaller the Ek is that an elec-tron possesses, the smaller is the proportion of electrons ofits kind that can leave the surface.10,12 The constraint im-posed by �c hence strongly suppresses the emission of pho-toexcited electrons with small Ek. This situation alters sig-nificantly when a positively biased metallic tip approaches asurface to a tunneling distance. Because the lifetime13–15 andthe tunneling period16 of an electron of small energy arecomparable �both in the order of femtoseconds�,17 the ap-proach of a nanotip opens a gateway for those weakly ener-getic electrons that did not escape to reach the tip throughtunneling. As depicted in Fig. 3, when a nanotip is placednear a surface, the highly energetic electrons breaking freefrom the surface �group A� reach the STM tip without effectof �c. For electrons carrying less energy, in groups B and C,the relevant parameters are both the effective work functions��� and �c. With the tunneling current described as I�� exp�−2�z�, in which � is the electron density and �

=�2me� / we estimate that a 100-fold increase in photocur-rent is attainable when the tunneling of low-energy electrons

in a large population is included; in addition to enhancing thecurrent, the inclusion of a tunneling mechanism for SR-excited electrons implies a possibility of acquiring a chemi-cal image with resolution comparable to those recorded withtopographic contrast.

In conclusion, by simulating electron trajectories for thecollection of photoelectrons through an insulator-coatedSTM tip, we find that at a large separation between the tipand sample for which there is no tunneling, photoelectronsemitted from the surface are the only source contributing tothe photocurrent collected at the tip. At this stage, both thephotocurrent and the effective probing diameter are functionsof bias voltage, separation between the tip and sample, andbare tip dimension. For a tip moved nearer the surface, oursimulation indicates that photoelectrons emitted from thesurface can neither provide a highly resolved chemical imagenor generate a photocurrent adequate to account for the ex-perimental values reported. To explain that large photocur-rent, tunneling of excited electrons must be included in thescenario. Because the tunneling mechanism is highly local-ized and because the population of excited electrons is pro-portional to the cross section for x-ray absorption, NEXAS-based nanospectromicroscopy is practicable through acombination of STM and SR.

National Synchrotron Radiation Research Center andNational Science Council of Taiwan, Republic of China sup-ported this work.

1H. Ade, J. Kirz, S. Hulbert, E. Johnson, E. Anderson, and D. Kern, Phys.Scr. 41, 737 �1990�.

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6A. Saito, J. Maruyama, K. Manabe, K. Kitamoto, K. Takahashi, K.Takami, M. Yabashi, Y. Tanaka, D. Miwa, M. Ishii, Y. Takagi, M. Akai-Kasaya, S. Shin, T. Ishikawa, Y. Kuwahara, and M. Aono, J. SynchrotronRadiat. 13, 216 �2006�.

7T. Eguchi, T. Okuda, T. Matsushima, A. Kataoka, A. Harasawa, K. Ak-iyama, T. Kinoshita, Y. Hasegawa, M. Kawamori, Y. Haruyama, and S.Matsui, Appl. Phys. Lett. 89, 243119 �2006�.

8K. Akiyama, T. Eguchi, T. An, Y. Hasegawa, T. Okuda, A. Harasawa, andT. Kinoshita, Rev. Sci. Instrum. 76, 083711 �2006�.

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�1977�.11S. M. Gray, J. Electron Spectrosc. Relat. Phenom. 109, 183 �2000�.12For a solid with W=4 eV, electrons with Ek=0.1 and 4 eV give �c=9° and

45°, respectively. Such numbers correspond to 1% and 25% of electronsescaping for those electrons carrying kinetic energies 4.1 and 8 eV insidethe solid.

13J. J. Quinn, Phys. Rev. 126, 1453 �1962�.14J. J. Quinn, Appl. Phys. Lett. 2, 167 �1963�.15E. Zarate, P. Apell, and P. M. Echenique, Phys. Rev. B 60, 2326 �1999�.16S. K. Sekatskii and V. S. Letokhov, Phys. Rev. B 64, 233311 �2001�.17The lifetime of a low-energy electron is estimated from the relation

= l*�me /2E0�1/2. For �E0 , l�= �1 eV,10 �, the electron has a lifetime ofabout 5.4 fs. In a metal, a low-energy electron can have a mean free pathof hundreds of angstroms, as reported in Refs. 13 and 14.

FIG. 3. �Color online� Radiation from a synchrotron incident on a metalsurface generates a wide spectrum of excited electrons, but not all leave thesurface like those in group A. A metallic tip placed near a surface eliminatesa constraint posted by a critical angle of total reflection and allows excitedelectrons marked as groups B and C to reach the tip via tunneling under theeffective work functions �b and �c.

103101-3 Chiu et al. Appl. Phys. Lett. 92, 103101 �2008�

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