Tip-enhanced Electron Emission Microscopy Bull. Korean Chem. Soc. 2014, Vol. 35, No. 3 891

http://dx.doi.org/10.5012/bkcs.2014.35.3.891

Tip-enhanced Electron Emission Microscopy Coupled with the

Femtosecond Laser Pulse†

Dahyi Jeong, Ki Young Yeon, and Sang Kyu Kim*

Department of Chemistry, KAIST, Daejeon 305-701, Korea. *E-mail: skkim1230@kaist.ac.kr

Received October 2, 2013, Accepted October 11, 2013

The ultrashort electron pulse, laser-emitted from the metal tip apex has been characterized and used as a

probing source for a new electron microscope to visualize the morphology of the gold-mesh in the nanometric

resolution. As the gap between the tungsten tip and Au-surface is approached within a few nm, the large

electromagnetic field enhancement for the incident P-polarized laser pulse with respect to the tip-sample axis

is strongly observed. Here, we demonstrate that the time-resolved tip-enhanced electron emission microscope

(TEEM) can be implemented on the laboratory table top to give the two-dimensional image, opening lots of

challenges and opportunities in the near future.

Key Words : Ultrashort laser pulse, Nano tip, Gold mesh, Microscopy

Introduction

Nano science and technology1 have been blossomed withthe advent of various electron microscopes with which onecan nowadays get the spatial resolution of ~1 Å.2,3 Materialsof specially-designed nanostructures are vastly producedthese days, and their optical, physical, or functional proper-ties in the sub-nanometer spatial resolution are stronglydemanded. Compared to the optical microscopes of whichthe spatial resolution is often diffraction-limited, the electronmicroscope provides a much better opportunity to investi-gate the material at the molecular level.

Here, we report an electron microscope with which, inprinciple, one can visualize the nano-scale morphology ofthe sample with femtosecond time resolution. The time-resolved electron microscope has been realized by theZewail group4,5 through the combination of the conventionaltransmission electron microscope (TEM) with the femto-second laser pulse. In the setup, they utilize the photoelectriceffect induced by the femtosecond laser pulse to generate theultrashort electron pulse, which is then further acceleratedand focused to give the highly-resolved TEM image as afunction of the reaction time. This 4D microscopy has beenenormously successful, providing unprecedented informationabout the ultrafast structural changes occurring on thesurface. The technique, however, is costly, and the widerapplication is rather limited.

A quite different approach for attaining the same goal canbe implemented on the laboratory table-top. In this ap-proach, the tungsten tip is irradiated by the femtosecondlaser pulse. Due to the lightening rod effect,6-11 the laser-induced field enhancement is peaked at the apex of the tip,resulting in the emission of electron in the tip-pointingdirection. Many research groups have recently investigated

the detailed mechanism of photoemission from the nano-sized metallic tip quite intensively, and the photo-inducedelectron emission from the metal tip seems to be wellcharacterized.12-17 When the tip is very near to the samplebeing probed, the dispersion of the emitted electron will beconfined in nanometer scaled space whereas the timeduration of the electron pulse remains within a few tens offemtosecond. If one uses this ultrashort and yet spatiallyconfined electron pulse, it may be possible to investigate thespatiotemporal property of surface material, Figure 1. Thefirst demonstration of the electron microscope based on thisnovel concept actually had been reported by Ropers et al.17

to give the local electromagnetic field distribution as theilluminated tungsten tip is linearly scanned over the nano-metrically grooved gold structure, showing the great promiseof this technique as a time-resolved electron microscope.Since this pioneering work was reported, however, thedevelopment of this tool as a practical electron microscopehas rather slowly progressed.

Here, we report the image of a gold mesh, providing thefirst realistic TEEM image which reflects the two-dimen-sional morphology of the surface structure. The effect of thebias voltage, laser intensity, tip-sample distance, and laserpolarization have been systematically investigated as anendeavour toward the realization and wide application ofthis new technique as a table-top time-resolved electronmicroscopic tool, eventually for the study of the molecularlevel mechanism of photo-catalysis, solar cell, or photo-chemical energy conversion.

Experimental

Materials. Tungsten wires (300 um-thick, 99.95%, NilacoCorporation) Potassium hydroxide (85%, JUNSEI), Goldmesh (MG-47, Precision Eforming), Carbon Tape (Okenshoji),Tuning fork (32.768 kHz, Microquartz Electronics).

Electrochemical Etching. A tungsten tip was manu-

†This paper is to commemorate Professor Myung Soo Kim's honourable

retirement.

892 Bull. Korean Chem. Soc. 2014, Vol. 35, No. 3 Dahyi Jeong et al.

factured by an electrochemical etching technique using atungsten rod with 300 µm-thickness in 3 N KOH solution. Atungsten rod was etched by two steps. In the first step, weapplied 10 V of constant voltage to process the electro-chemical reaction. When the cuurent was decreased to about0.09 Ampere, the first step is finished. In the second thevoltage was 3 V until the electrochemical etching wascompleted. Cut-off time was determined by a homemadeprogram using LabView software (National Instruments,LabView2009). Produced tungsten tips are dipped into 48%HF and rinsed in de-ionized water to remove the layer ofelectrolyte. In our electrochemical etching setup, tungstentips are reproduced with a curvature of tens of nanometer.

Shear Force Microscopy. An electrochemically etchedtungsten tip was mounted on the side of a tuning fork usinginstant glue and sine wave produced by a function generator(DS345, Stanford Research Systems) is supplied to a tuningfork and a pre-amplifier. At the resonance frequency, thetuning fork is mechanically excited. Then, the amplitude atthe resonance frequency was amplified by home-made elec-tronic circuit and detected by lock-in amplifier (SR810,Stanford Research Systems).

Tip-enhanced Electron Emission Microscopy (TEEM).

Figure 1 shows the TEEM experimental setup schematically.The solid-state diode-pumped Nd:YVO4 laser (Verdi V-5,Coherent, 532 nm single-frequency output) was used to pumpthe cavity dumped Ti:sapphire oscillator (KM Lab.). Braggcell was used for cavity dumping by acousto-optic modu-lation from a RF driver (NEOS). We tuned laser pulses sothat the output power was around 24 mW at a central wave-length of 810 nm and 15 fs pulse width at 800 kHz repetitionrate. A prism pair compensates the group delay dispersion(GDD) of the output pulse. This pulse is separated by two.One acts as pump pulse and the other is served as probe

pulse. These pulses are focused on the tungsten tip using 75mm-focal length planoconvex lens which can travel 25.4mm-forward and backward using x-axis picomotor stage.And electron pulse is generated by a multi-photon ionizationprocess. Because of the phase interference of two laserpulses, we have observed the strong temporal modulation inthe electron signal of which the Fourier-transformed fre-quency corresponds to the central wavelength of 810 nm.This pulse interacts with surface. After then, these electronsare detected by a home-made microchannel plate detectorand integrated by a photon-counter (SR400, Stanford ResearchSystems). All data acquisition was obtained by home-madeprogram using LabView software.

Results and Discussion

The resulting two dimensional TEEM images, taken at thesame position after shear force microscopy, is shown inFigure 2(c)-(h). At zero bias voltage, the electron signal isenhanced at the hole whereas it gives the much less electronsignal when the tip is at the gold surface. This experimentalobservation may indicate that the electron emitted along thetip-pointing direction is reflected when the tip is positioned

Figure 1. Experimental setup of the tip-enhanced electron emissionmicroscope. The femtosecond laser pulses with the repetition rateof 800 kHz are focused on the tip for the emission of the electron.The electron signal is temporally modulated by the interferometricautocorrelation of two time-delayed coherent femtosecond laserpulses, giving the corresponding pulse width of ~25 fs, as shown inthe inet. (PLCX: plano convex lens, WP: λ/2 waveplate, BS:beamsplitter).

Figure 2. Gold mesh images taken by (a) scanning electronmicroscope and (b) shear force microscope (SFM). The tip-enhanced electron emission microscope (TEEM) images takenwith the P-polarized femtosecond laser pulse at the tungsten tipbias voltages of (c) 0, (d) −50, (e), −100, (f) −200, (g) −300, and (h)−500 V are shown.

Tip-enhanced Electron Emission Microscopy Bull. Korean Chem. Soc. 2014, Vol. 35, No. 3 893

at holes as the base-plate for the gold mesh is made of quartz,whereas it is absorbed when the tip is at the gold surface.The TEEM image taken at the zero bias voltage, however,does not reflect the realistic shape of the gold-mesh. The sizeof the hole is much smaller than that of the real one, and theelectron signal from the upper-half part of the image is muchstronger than that from the lower-half part. In the lower-halfpart, only the left hole gives the weak electron signal. Thisasymmetry should come from the tilt of the sample withrespect to the plane perpendicular to the tip-pointing direc-tion. Interestingly, the TEEM image becomes inverted whenthe small negative bias voltage is applied to the tungsten tip,Figure 2(d)-(h). Since the sample is electrically floated, theapplication of this bias voltage to the tip gives the poorlydefined electric field. Nonetheless, the effect of the negativebias voltage is found to be dramatic. The electron signal ismuch enhanced when the negatively biased tip is positionedat the gold surface, whereas it remains more or less constantwhen the tungsten tip is positioned at the hole of the goldmesh. The TEEM images taken at the bias voltages between−50 and −200 V are very similar to the SFM image, indicatingthat the TEEM technique can be employed as a practicalhigh-resolution electron microscope. As the bias voltage isfurther increased, it is found that the TEEM images becomeblurred.

It should be noted that no electron is detected when thefemtosecond laser pulse is irradiated solely on the gold-surface. Secondly, the gap between the tip and sample shouldbe very tiny to observe the significant enhancement of theelectron signal, as previously reported in the study byRopers et al..17 Thirdly, no electron is detected without thelaser irradiation. Therefore, the laser pulse, the tungsten tip,and gold sample should be at right positions in order to getthe electron signal enhancement. Another requirement is thepolarization of the laser pulse. The P-polarization is essentialfor obtaining the TEEM image. Both the lightning rod (LR)and surface plasmon polariton (SPP)18-24 effects seem to beresponsible for these phenomena. The LR effect comes fromthe tip apex. The laser irradiation on the tungsten tip inducesthe free-electron oscillation along the tip surface and itsamplitude becomes maximized at the apex, so that the elec-tron emission gets efficient only at the tip apex. The appli-cation of the bias voltage to the tungsten tip then lowers thetunneling barrier of the ionization from tungsten tip. Anotherfactor mainly contributing to the TEEM image of the gold-mesh should come from the SPP effect.25 In this case, theelectron on the gold surface oscillates with the irradiatedlaser pulse, and this oscillation induces the local electro-magnetic field enhancement26-28 at the small gap between thetip and sample.

Calculations are carried out using the finite-difference intime-domain (FDTD)29-32 technique for the current experi-mental condition. The curvature radius of the tungsten tipand its cone angle are assumed to be 50 nm and 35º, respec-tively. The gap between the tungsten tip and the gold surfaceis fixed at 2 nm. In the simulation, the Yee cell size is set tobe 2 × 2 × 2 nm3 in all regions. The broadband P-polarized

light with the central wavelength of 810 nm is shone in thesimulation at the incidence angle of 10º with respect to theplane of the gold substrate. Calculated electric field distri-butions generated around the tungsten tip with and withoutthe gold substrate nearby are shown in Figure 3(a) and (b),respectively. The electric field intensity is found to beapproximately 30 times enhanced as the tungsten tip is inclose proximity to the gold substrate, confirming the essen-tial role of SPP in the electron signal enhancement in thepresent work. The application of the negative bias voltage tothe tungsten tip automatically induces the strong local elec-tric field enhancement at the gap between the tungsten tipand sample, giving the synergistic effect to the SPP excita-tion. Therefore, the electron signal is strongly enhancedwhen the tip is close to the gold surface whereas it is onlyweakly detected when the tip is distant from the gold surfaceat the hole of the gold mesh, giving the realistic TEEMimage. The TEEM images in Figure 2 show somewhat in-homogeneous intensity distribution especially along the x-coordinate.

This inhomogeneity is most likely due to the tilt of thesample, resulting in the change of the tip-sample distancealong the z-direction, which is consistent with the TEEMimage taken at the zero bias voltage (vide supra). This sug-gests that the TEEM image could be much more sensitivethan the SFM image along the z-direction, giving the plausi-bility of TEEM as a tool for the three-dimensional mappingof the surface morphology. For testing the possibility of thetime-resolved image recording, the femtosecond laser pulseis divided into two parts using a beam-splitter, and theresulting pump and probe laser pulses are optically delayedto give the surface image as a function of the delay time.

Actually, because of the phase interference of two laserpulses, we have observed the strong temporal modulation inthe electron signal of which the Fourier-transformed fre-quency corresponds to the central wavelength of 810 nm.This reflects that the actual pulse width of the emittedelectron is also in the femtosecond time scale, giving thebright future of TEEM as a time-resolved nano-probing tool.This also verifies again that the TEEM image reported in

Figure 3. FDTD calculations of the electric field distribution for(a) a single tungsten tip and (b) a tungsten tip on the gold surface.The distance between the tungsten tip and the gold surface is set tobe 2 nm. The polarization E and wave vector k of the propagatinglight are illustrated as arrows. One can notice that the electric fieldintensity has been increased by ~30 fold as the Tungsten tipapproaches to the gold surface.

894 Bull. Korean Chem. Soc. 2014, Vol. 35, No. 3 Dahyi Jeong et al.

this work is produced by the ultrashort electron pulsegenerated by the femtosecond laser pulse irradiation. Itshould be noted that our first two-dimensional TEEM imageis rather preliminary in terms of quantitative measures, andthere is much room for this TEEM technique to be muchimproved and refined as a microscopic tool in terms of bothspatial and temporal resolutions.

Conclusion

In conclusion, we have demonstrated that the tip-enhancedelectron emission microscope can be implemented on thelaboratory table top to give the two-dimensional image ofthe surface structure. The ultrashort time duration of theelectron pulse from the tip, which is being used as a nano-probe in TEEM, should be ideal for the real time-resolvednano electron microscope. The lightning rod effect inducedby the tip apex with the aid of the surface plasmon polaritoneffect maximized at the tiny tip-sample gap gives the clearpicture of the surface structure of the gold-mesh. The imageshould depend on the surface plasmon resonant property ofthe material, and thus TEEM may also be quite useful tostudy the structure of alloy materials with different dielectricconstants. Moreover, since the time duration of the electronpulse is in the femtosecond time scale, it should be plausibleto see the nanometer confined structural change of thesurface including the morphology, electric property, orvibrational structure in real time.

Acknowledgments. This work was supported by NationalResearch Foundation (2009-008247, 2010-0000068, 2010-0015031). The support from the center for space-timemolecular dynamics (2010-0001635) is also appreciated.

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