Journal of Electroanalytical Chemistry 662 (2011) 12–16

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Journal of Electroanalytical Chemistry

journal homepage: www.elsevier .com/locate / je lechem

Locally enhanced cathodoluminescence of electrochemically fabricated goldnanostructures

Xinzhou Ma, Rolf Schuster ⇑Karlsruhe Institute of Technology and DFG Center for Functional Nanostructures, Kaiserstr. 12, 76131 Karlsruhe, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Available online 12 March 2011

Keywords:Gold microstructuresCathodoluminescenceElectrochemical micromachiningLocal EM field enhancement

1572-6657/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jelechem.2011.01.046

⇑ Corresponding author. Tel.: +49 72160842102.E-mail address: rolf.schuster@kit.edu (R. Schuster)

Three dimensional structures of Au with submicron sizes and high aspect ratios were fabricated by elec-trochemical micromachining with short voltage pulses. Due to the use of LiCl/dimethyl sulfoxide electro-lytes, submicrometer machining-precision was achieved with voltage pulses of moderate duration of theorder of 10 ns. Careful adjustment of the machining parameters like rest potentials of tool and workpieceand pulse amplitude allowed for the routine fabrication of Au microstructures. The optical properties ofsuch structures were investigated by measuring their cathodoluminescence upon electron irradiation in ascanning electron microscope. Excitation of localized plasmons in small structures led to the enhance-ment of the light emission by a factor of up to 2.5, if compared to the bare Au surface.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Due to their optical properties, nanostructures of Au or Ag withdimensions of the order of the wavelength of light are veryattractive as optical components for sub-wavelength opticalsystems [1–3]. Ag and Au are susceptible to the excitation of collec-tive oscillations of the conduction electrons [4,5] which can eitherpropagate along a smooth surface as so called surface plasmonspolaritons (SPP) or can be confined in a small space by nanostruc-tures or particles as localized surface plasmons (LSP). By properdesign of hole or slit patterns in a Au or Ag film the propagationof light can be manipulated on sub-wavelength scale [1–3]. Thestrong enhancement of the electromagnetic field in localizedsurface plasmons on Au nanoparticles [6] or at a Au tip of a scan-ning tunneling microscope [7–9] can be exploited for conductingsurface enhanced Raman spectroscopy. Also single molecularspectroscopy [10,11] and plasmonic nanosensors [12] became pos-sible because of the strongly enhanced local field.

Surface plasmons can be excited by light and imaged by scan-ning near field optical microscopy (SNOM) [13–15]. However, thesize of the optical tip limits the resolution of SNOM and the tipmodifies the local field [16]. Alternatively, surface plasmons canalso be locally excited by a focused electron beam, for examplein a scanning transmission microscope [4,17,18] or a scanning elec-tron microscope (SEM) [19]. The excitation of plasmons can beprobed either by the specific energy loss of the primary electronbeam upon penetrating the sample [20–22] or by measuring theemitted light, which originates from the conversion of plasmons

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to photons, e.g., at sharp edges of the irradiated structures[16,23,24]. Measuring the emitted light, i.e., the cathodolumines-cence of the specimen has some advantages over electron energyloss spectroscopy (EELS) [19], in particular for the investigationof extended and thick structures, which are of interest in the pres-ent contribution. The electron beam does not need to penetrate thesample and the emitted light can be collected above the samplesurface, which renders cathodoluminescence particularly usefulfor the investigation of thick, supported samples. Furthermore,detecting the emitted light is in general more routine than per-forming high resolution EELS. For example, the propagation lengthof surface plasmon polaritons on a smooth surface was investi-gated via cathodoluminescence [25–27]. Recently also localizedplasmon modes of artificial nanostructures or particles becameaccessible to local excitation, e.g., local resonant modes on a Aunanowire [28] or a ridge [23]. However, the investigated structuresare mostly limited to two dimensional holes or slit patterns, whichwere often lithographically produced in thin metal films. Thick,three dimensional structures like small columns with high aspectratio or small cones were only rarely investigated so far. In thepresent paper we study the cathodoluminescence of such threedimensional structures, which were directly machined into a thick(100 lm) Au sheet. This work was triggered by recent investiga-tions of tip enhanced Raman scattering, where Au-STM tips wereused as local ‘field amplifiers’ [7,8]. We study the cathodolumines-cence of ‘model tips’, i.e., simple geometrical structures like roundand square columns or pyramids, in order to derive information ofappropriate tip shapes and surface properties for local fieldenhancement.

There exist only few methods to fabricate three dimensionalmetal structures on the submicrometer scale. Focused ion beam

X. Ma, R. Schuster / Journal of Electroanalytical Chemistry 662 (2011) 12–16 13

milling (FIB) [16,23] and electron beam lithography (EBL) [28] arecommonly used methods, which achieve spatial resolution in thelower nanometer range. However, among those methods only FIBis suited to generate three dimensional structures of freely design-able shapes. Recently, Wegener et al. [29] introduced a threedimensional optical lithographic process, which they used for theproduction of 3D photonic crystals with structural dimensionsdown to the 100 nm scale. Also metal structures could be producedby filling the cavities of a 3D polymer mask with metal. Herein, weprepared gold structures using an electrochemical micromachiningmethod, where the electrochemical reactions are locally confinedby the application of short voltage pulses [30,31]. Spatial resolu-tions down to a few 10 nm could be achieved. Electrochemicalmicromachining is a relatively low-cost, mask-less and one stepmethod. Sharp contours can be routinely achieved. In the currentpaper we will first describe the particular machining procedurefor gold structures, employing an optimized electrolyte for highresolution machining. Afterwards we present cathodolumines-cence results of such structures, where panchromatic light emis-sion maps and secondary electron images were obtainedsimultaneously.

20

2. Experimental details

2.1. Electrochemical machining

Polycrystalline gold foils (99.995%), mechanically polished with3 lm and 1 lm diamond paste, were used as workpiece electrodes.In order to retain the mechanical strength of the Au substrates,they were not flame annealed. Mechanically flattened carbon fibers(Goodfellow) and electrochemically etched carbon fiber tips wereused as tools. Tool preparation is described in detail in Ref. [32].Counter and pseudo reference electrodes were prepared from Ptwires. All potentials are referenced to the potential of the Pt pseu-do reference electrode. Electrochemical machining was conductedin a home-built apparatus, where the workpiece electrode wasincorporated in a small electrochemical cell. The cell was mountedon a piezo-driven manipulator, by which the relative movement oftool and workpiece could be controlled in three dimensions. Highfrequency voltage pulses with a pulse-to-pause ratio of 1–9 weregenerated by a HP 8130A pulse generator and applied to the carbonfiber via a pulse amplifier, mounted close to the tool. LiCl solutionsin dimethyl sulfoxide (DMSO) were used as electrolytes. Chemicalswere of analytical grade and supplied by Merck. The electrochem-ical cell was open to air, which resulted in uptake of water from theenvironment during machining. However, as investigated in [32],this is of minor influence for the machining of Au. Further detailson the experimental setup can be found in [30,31].

-1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2

-10

-5

0

5

10

15

j / m

A c

m-2

E / V vs. Pt wire

Fig. 1. Cyclic voltammogram of Au in 1 M LiCl/DMSO. The potential scan started atOPC (ca. 0.1 V versus the Pt pseudo reference electrode) in the positive directionwith 50 mV/s scan rate.

2.2. Cathodoluminescence measurements

The cathodoluminescent measurements were carried out in anSEM (Hitachi S570), equipped with a LaB6 cathode. An aluminumparabolic mirror (Linos Optics) was used for light collection. Its col-lection angle was about 1.5p. The mirror was attached to a 3Dmicromanipulator and placed ca. 0.5 mm above the specimen.The electron beam passed through a 1 mm diameter hole drilledinto the mirror. Emitted light was collected by the mirror andpassed through an optical window to the cathode of a photomulti-plier (R 649), by which single photons were counted. For recordinga cathodoluminescence map, which was usually 256 pixel � 256pixel wide, the electron beam was slowly scanned across the sur-face with a residing time of 10 ms/pixel. Secondary electron imagesand CL images were recorded simultaneously.

3. Results and discussion

3.1. Electrochemical machining of Au microstructures

Gold microstructures were electrochemically machined employ-ing ultrashort voltage pulses, by which the electrochemical reactionsare local confined. The principle of the method is described indetail in [30,31]. Briefly, it is based on local charging of the electricdouble layer (DL) upon the application of short voltage pulses to asmall tool electrode. Due to the finite electrolyte resistance the localcharging time constant of the DL on the electrodes is cruciallydependent on the locally varying distance between the electrodes.Areas of the DL on workpiece and tool, where both electrodes arein close vicinity, are charged fast, because of the low electrolyteresistance along the current path through the electrolyte. Vice versa,areas where the electrodes are further apart are charged slower.Upon application of short enough voltage pulses only those areasof the DLs on the electrodes are charged significantly, where thedistance of the electrodes is sufficiently small. Electrochemicalreactions, whose rates are typically exponentially dependent onthe overpotential, are strongly confined to the charged areas. Asestimated from the charging time constant, for typical aqueouselectrolytes with a specific resistance of the order of 3 X cm (1 MHCl) and typical DL capacitances of 10 lF/cm2, the electrodes haveto become closer than about 10 lm in order to significantly chargethe DL upon application of about 30 ns pulses [31]. Since in thecurrent studies structure sizes of submicrometer dimensions arerequested, this would imply pulse durations of the order of a few100 ps for machining such structures in aqueous electrolytes.Achieving pulses of such short durations in electrochemical environ-ment is far from routine.

As an alternative approach to lowering the pulse duration forachieving higher machining resolution, the specific electrolyteresistance could be increased. This would result in a longer timeconstant for the DL charging at constant electrode separation. Inother words, longer pulses could be applied for achieving the samemachining resolution. For this reason we already studied the appli-cability of LiCl solutions in dimethyl sulfoxide (DMSO) [32]. InDMSO based electrolytes the specific electrolyte conductivity islowered by a factor of 10 in comparison with the correspondingaqueous electrolyte by increasing the viscosity of the solvent.Hence the ionic strength of the electrolyte remains unchangedupon lowering the conductivity, which is advantageous to achievefast Au dissolution. A typical CV of Au in 1 M LiCl/DMSO electrolyteis shown in Fig. 1. Strong Au dissolution occurs at potentials above

14 X. Ma, R. Schuster / Journal of Electroanalytical Chemistry 662 (2011) 12–16

0.4 V versus a Pt pseudo reference electrode. The redeposition ofAu is strongly suppressed down to a potential of about �0.3 V indi-cating the strong irreversibility of Au dissolution in LiCl/DMSObased electrolytes. Therefore, by choosing a rest potential of theworkpiece around 0 V the redeposition of dissolved Au can be eas-ily avoided without having significant corrosion of the surface. Thelatter gives an additional advantage above aqueous electrolytes forAu machining: In aqueous chloride containing electrolytes, the Audissolution is highly reversible and the rest potential of the work-piece had to be precisely controlled near the Nernst potential of Auin this solution in order to avoid redeposition. In addition, thestrong exchange current density would lead to significant corro-sion of the workpiece. Further details on the electrochemicalbehavior of Au in LiCl/DMSO can be found in Ref. [32].

Although the general applicability of LiCl/DMSO for themachining of submicron structures was demonstrated in Ref.[32], for routinely fabricating such structures machining parame-ters have to be optimized with respect to spatial resolution,machining speed and absence of corrosion of the workpiece. Foroptimum machining speed, i.e., full control of the spatial resolu-tion by the pulse duration in combination with high machiningspeed, the dissolution rate of Au should be limited by the chargetransfer step of the electrochemical reaction rather than by masstransport of dissolved Au in the small tool–workpiece gap. Hence,the amplitude of the applied pulses should be carefully chosen inorder to adjust for slow enough Au dissolution also in the case ofcomplete charging of the DL during the potential pulse. Similarlythe rest potential of the workpiece is a critical parameter. In orderto avoid corrosion, it should be adjusted to the lowest possiblevalue, before Au redeposition would occur. However, since uponapplication of the potential pulses the workpiece DL is chargedpositively with respect to the workpiece rest potential, the max-imum polarization of the workpiece electrode and, therefore, alsothe Au dissolution rate is coupled to the variation of the rest po-tential. This is demonstrated in Fig. 2a, where SEM images ofholes are shown, which were drilled into a Au sheet with a cylin-drical carbon fiber tool of 7 lm diameter with varying rest poten-tials of the workpiece. The other parameters like pulse duration(100 ns), repetition frequency (1 MHz), pulse amplitude (4.2 V),

5 m

-50mV 0mV 50mV 100mV 150mV

-50 0 50 100 1500.0

0.4

0.8

1.2

1.6

2.0

gap

wid

th/

m

rest potential

a

b

Fig. 2. (a) SEM images of ca. 10 lm deep holes drilled in Au in 1 M LiCl/DMSO with4.2 V 100 ns pulses. The rest potential of the Au workpiece was varied from �50 mVto 150 mV as indicated. (b) Dependence of the gap width between tool and the edgeof the hole versus workpiece rest potential.

and feed rate of the tool (45 min for 10 lm depth) were kept con-stant. In Fig. 2b the gap width between the tool and the edge ofthe hole is plotted versus the workpiece rest potential. It shouldbe noted that upon drilling the hole with the lowest rest potentialof the workpiece (left hole of Fig. 2a), the feed rate of the tool washigher than the etching speed, resulting in frequent electric con-tact between tool and workpiece. The contact between tool andworkpiece was electronically detected during drilling. Upon con-tact the tool was slightly retracted, before the drilling processwas continued. The frequent contacts signaled that the Au disso-lution became very slow under those machining conditions. Weconclude that at a workpiece rest potential of �50 mV the actualpolarization of the DL on the workpiece inside the tool–workpiecegap was only slightly exceeding the threshold for Au dissolution.Upon increasing the workpiece potential the maximum polariza-tion of the DL during the pulses and subsequently also the Au dis-solution rate increased, however, at the expense of a wider gapwidth, i.e., worse machining resolution. It should be mentionedthat apparently only a small part of the applied potential pulseis actually polarizing the workpiece DL. As discussed in detail in[32], due to the small DL capacitance of the carbon tool in com-parison with the Au workpiece most of the pulse amplitude willdrop in the tool’s DL. Furthermore, the conductivity of the carbonfiber is fairly small compared to a metallic conductor. Therefore,in order to avoid excessive voltage drop along the shaft of thetool, the carbon fiber tool should be kept as short as possible.

For machining the structures for the cathodoluminescencestudies we chose a compromise between machining speed andmachining resolution. The rest potential was set to about 50 mV,which suppresses significant corrosion of the Au surface and al-lowed for reasonable machining speed. For achieving machiningresolution on the submicrometer scale pulses with durations be-tween 10 ns and 20 ns were employed. The pulse-to-pause ratiowas set to 1:9. In order to machine small troughs, we used electro-chemically etched carbon fiber tip with tip diameters of the orderof 50 nm–100 nm. Resulting structures are shown in Fig. 3 with de-tailed machining parameters given in the figure caption. For shap-ing the structures we first drilled a hole vertically into the goldsubstrate. Then the tool was moved sideways similar to a minia-ture milling cutter in conventional milling.

3.2. Cathodoluminescence of electrochemically fabricated goldstructures

We studied the cathodoluminescence of the structures in Fig. 3upon irradiation with a focused 15 keV electron beam. The beamcurrent was approximately 9 nA. Fig. 4 shows the correspondingpanchromatic cathodoluminescence images of the structures inFig. 3. The sensitivity of the photomultiplier extended from about300 nm to the near infrared at about 850 nm. By inserting light fil-ters into the optical path, strongest emission was found around500 nm to 600 nm with considerable light intensity extending intothe infrared. It should be noted that the cathodoluminescencemaps rather indicate the origin of the excitation of the light emit-ting processes than the spatial origin of the light, since the wholemicron-sized image area is within the focal point of the mirror.Only for very extended scan areas exceeding about 100 lm �100 lm, significant intensity variations occur, due to insufficientlight collection by the mirror.

As can already be seen by rough inspection of Fig. 4, light emis-sion of the structures was strongly enhanced, when the electronbeam hit small structures. For example, upon scanning the squaretop of the columns in Fig. 3, strongest light emission was found,when the electron beam hit near the rims of the square columns.A few 100 nm inside the flat tops of the columns the light intensitywas enhanced up to a factor of 2, compared with the flat Au

a b

c dFig. 3. SEM images of microstructures machined on Au in 1 M LiCl/DMSO solution.The applied machining parameters are: (a) 4.2 V, 20 ns pulses, UAu = 0.05V; (b) 5.6 V, 10 ns pulses, UAu = 0.05 V; (c) 5.6 V, 20 ns pulses, UAu = 0 V; (d) 5.6 V,11 ns pulses, UAu = 0 V. The depth of structures is indicated in the figures.

X. Ma, R. Schuster / Journal of Electroanalytical Chemistry 662 (2011) 12–16 15

surface. Though the intensity obtained upon irradiation of thecenter of the columns was lower than that found at the rims, itwas still slightly higher than that obtained at the unstructured

Fig. 4. Panchromatic cathodoluminescence images obtained from the structures shownthe sample was used (image size: 256 pixel � 256 pixel, irradiation time: 10 ms/pixel). Threferences to color in this figure legend, the reader is referred to the web version of thi

surface, surrounding the microstructure. When the size of thestructures was reduced to submicron sizes, strongest intensitywas usually found, when the beam hit the middle of the structures(Fig. 4b–d). For example for the finger-like structures in Fig. 4b,where each finger had a size of about 5 lm by 1 lm, light emissionwas enhanced by a factor of 2 along the finger, compared to the flatsurface. The intensity at the end of the finger was slightly higherthan at the other side, where the finger was connected to the flatsurface. This is in line with the observation, that in general theenhancement of the light emission was strongest for enclosedstructures for all investigated structures in Fig. 4a–d. Emissionupon electron bombardment of the outer edges of the troughs, nextto the extended flat surface, was comparatively small. Furtherreducing the size of the structures (Fig. 4c and d) led to enhance-ment factors for the emitted light up to about 2.5 compared to lightemission from the flat surface. Lowest light emission was foundupon irradiation of the bottoms of the troughs.

There are in principle several contributions to the total lightintensity upon electron irradiation of a Au sample [17,19,33],which are Cherenkov radiation, Bremsstrahlung, transition radia-tion stemming from recombination of the incident electrons withtheir image charge, and radiative decay of electronically excitedsurface plasmons. In addition, light might be generated by scat-tered electrons, striking the mirror or the vacuum chamber. How-ever, since the focal area of the mirror was rather small, straylight from the chamber walls will not significantly contribute tothe measured light intensity. Furthermore, the observed enhancedlight emission was not correlated with the intensity distributionin the secondary electron image. Therefore, we believe that thelight emission maps correspond to light generated at the samplesurface. Since Bremsstrahlung and Cherenkov light will be of min-or importance, due to the relatively low electron energy of15 keV, the main contributions to cathodoluminescence can be

in Fig. 3. For irradiation a 15 keV electron beam with approximately 9 nA current ate color scale gives the respective photon counts per pixel. (For interpretation of the

s article.)

16 X. Ma, R. Schuster / Journal of Electroanalytical Chemistry 662 (2011) 12–16

attributed to transition radiation and radiative decay of surfaceplasmons [33]. In detail, two kinds of surface plasmons couldradiate light, i.e., propagating surface plasmons, which need edgesor surface roughness in order to conserve their momentum uponconversion to light, and localized surface plasmons, i.e., plasmonicmodes of a microstructure, which are bound to that structure andwhich can radiate light similar to a microantenna. We believethat in accordance with the studies of light emission of smallannular Ag and Au nanoresonators [24] or nanoholes in a thinAu film [34], light emission from our structures can be attributedto the excitation of localized surface plasmons by the incomingelectrons. This is substantiated by the observation that light emis-sion was strongest for enclosed structures. Apparently localizedplasmons could be excited also in thick structures as investigatedin the present contribution. At those trough edges, which wereoriented towards the flat surface, only small enhancement wasobserved, probably due to excitation of propagating surface plas-mons, which can radiate light only very ineffectively upon prop-agating along the surface. It should be noted that also theunstructured surface areas are expected to be slightly rougheneddue to corrosion during machining. This might also lead to con-version of propagating surface plasmons into light [27]. However,from the current measurements we cannot discriminate, whetherthe light, which was excited at the flat surface area was stem-ming from transition radiation or propagating plasmons con-verted to light by surface roughness. Spectral investigationsmight help for solving this question.

4. Conclusions

Electrochemical micromachining with short voltage pulses wasshown to be an alternative method for the fabrication of opticallyactive Au structures of submicrometer dimensions. In comparisonwith FIB and EBL, this method allowed the straight forward fabri-cation of thick structures with high aspect ratios, which extendedseveral microns deep into the substrate. Optical activity of thestructures was demonstrated by cathodoluminescence maps uponirradiation with electrons in a SEM. Up to now only a few artificialnanostructures, which were mostly machined by conventional FIBor EBL methods, were investigated by cathodoluminescence[16,23,24,28]. Those structures usually had dimensions muchsmaller than the wavelength of light. Herein we showed that alsoin structures, which are intimately connected to the bulk of thesubstrate, localized plasmons could be excited. Although the struc-tures studied here rather had the dimensions of the wavelength ofthe detected light, considerable enhancement of the light emissionup to a factor of 2.5 compared with the bare surface was detectedupon electron irradiation. Such three dimensional structures, ase.g., the pyramid in Figs. 3d and 4d might deal as model systemsto study field enhancement near Au tips, as e.g., used in tipenhanced Raman spectroscopy. In this respect, besides thespectrum of the emitted light, also the emission direction may beparticularly interesting to investigate.

Acknowledgements

We gratefully acknowledge funding by the DFG Center for Func-tional Nanostructures (CFN) and the Fonds der Chemischen Indust-rie. We would also like to thank D. Waltz and his colleagues fromthe mechanics workshop for their help in constructing and build-ing the experimental setups.

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