Growth characterization of electron-beam-induced silver deposition from liquidprecursorLeonidas E. Ocola, Alexandra Joshi-Imre, Cynthia Kessel, Brian Chen, Jonathan Park, David Gosztola, and

Ralu Divan

Citation: Journal of Vacuum Science & Technology B 30, 06FF08 (2012); doi: 10.1116/1.4765629 View online: http://dx.doi.org/10.1116/1.4765629 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/30/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Photocatalytic and antibacterial properties of Au-TiO2 nanocomposite on monolayer graphene: From experimentto theory J. Appl. Phys. 114, 204701 (2013); 10.1063/1.4836875 Control of the structure and density of silver nanoparticles obtained by laser-induced chemical deposition fromliquids J. Vac. Sci. Technol. B 31, 06F303 (2013); 10.1116/1.4824328 Preparation and characterization of nanostructured zinc oxide thin films AIP Conf. Proc. 1482, 539 (2012); 10.1063/1.4757530 Electron beam induced deposition of cobalt for use as single- and multiwalled carbon nanotube growth catalyst J. Vac. Sci. Technol. B 27, 2982 (2009); 10.1116/1.3250259 Mechanics of hydrogenated amorphous carbon deposits from electron-beam-induced deposition of a paraffinprecursor J. Appl. Phys. 98, 014905 (2005); 10.1063/1.1940138

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Growth characterization of electron-beam-induced silver depositionfrom liquid precursor

Leonidas E. Ocolaa) and Alexandra Joshi-ImreCenter for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439

Cynthia KesselGeorge Washington Middle School, Lyons, Illinois 60534

Brian ChenIllinois Mathematics and Science Academy, Aurora, Illinois 60506

Jonathan ParkDepartment of Molecular Biosciences, Northwestern University, Evanston, Illinois 60208

David Gosztola and Ralu DivanCenter for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439

(Received 29 June 2012; accepted 18 October 2012; published 8 November 2012)

Deposits of aggregated silver particles were grown from aqueous silver-nitrate solution by electron-beam

induced deposition in a liquid cell in a scanning electron microscope. Electron energies of 2, 5, and

20 keV were evaluated and found to produce distinguishably different deposits. Optimal energies exist

for maximum growth rate and for minimum feature sizes. The physical structure of the deposits was

found to evolve with increasing thickness as silver particles grow larger and develop more structure.

Optical absorbance measurements and Raman spectroscopy on adsorbed dye suggest appropriate silver

purity for using this fabrication method in nano-optics applications. VC 2012 American Vacuum Society.

[http://dx.doi.org/10.1116/1.4765629]

I. INTRODUCTION

Electron beam induced deposition (EBID) by direct decom-

position of gas phase precursor molecules1 or ionic liquids2

have been extensively studied as a means to directly synthesize

metal clusters using a focused electron beam in a scanning elec-

tron microscope. Liquid phase metal deposition in a fluid cell is

also an alternative and has recently been shown to achieve

higher purity levels than traditional gas phase deposition3,4 and

is currently being pursued as a viable nanofabrication tech-

nique.5,6 The ability to create high-resolution metal deposits

from a liquid solution enables a wide variety of materials to be

studied such as gold,6 platinum,7 silver,8,9 and nickel and

chrome.8 Liquid based deposition also offers other advantages

including the use of less toxic and less expensive chemistry, the

ready dissipation of charge in the conductive solution, and

potentially increased deposition rates. Metal deposits from liq-

uid source differ morphologically from similar deposits using

gaseous phase EBID, and from metal deposits via lift-off or

etching, which may open up different applications in chemical

sensing and energy transfer via plasmons.

II. EXPERIMENT

The scanning electron microscope capability of an FEI

Nova 600 NanoLab instrument was employed for electron-

beam induced silver deposition inside liquid capsules. We

have investigated the effect of incident electron energy, at 2,

5, and 20 keV, on the growth of deposits from the liquid

phase. An aqueous solution of 0.001 M concentration of

AgNO3 was filled into a 15 ll volume Quantomix 102

capsule that was built with a 3 mm diameter size and 150 nm

thick, metal-grid-supported polyimide membrane cover, Figs.

1(a) and 1(b). The electron beam penetrates the silver-salt so-

lution through this membrane and reduces silver ions at the

membrane–liquid interface as well as deeper inside the solu-

tion (dependent on beam energy), Fig. 1(c). The location of

the electron-beam exposure was controlled by the RAITH ELPHY

4.0 lithography software. Single, continuous point dwelling

spots were used to create arrays of silver deposits on the mem-

brane. The microscope settings during lithography were:

magnification¼ 1000�, beam settling time¼ 5 ms, working

distance¼ 15 mm, beam current¼ 13 pA for 20 and 2 kV

acceleration voltages and 5 pA for 5 kV acceleration voltage,

minimum dwell time¼ 40 ms for 13 pA and 104 ms for 5 pA

beam current, thus keeping the minimum charge deposited at

0.52 pC for all beam energies. After lithography, the capsule

was opened, rinsed with water, and dried. The metal grid-

mounted membrane was removed from the capsule, and the

deposits were imaged by 5 kV scanning electron microscopy

(SEM) and tapping-mode atomic force microscopy (AFM)

(Park Scientific XE-HDD).

III. MODELING

Monte Carlo simulation of electron trajectories through

the membrane and into the solution was done using the pop-

ular CASINO software by Gauvin and Drouin.10 A composition

of H100C220N20O51 and density of 1.31275 were set to model

the polyimide membrane, and AgNO1003H2000 composition

and 0.54003 density were set to model the silver nitrite solu-

tion. The electron beam was simulated with 6 nm beam

diameter and with perpendicular incidence to the membrane.a)Electronic mail: ocola@anl.gov

06FF08-1 J. Vac. Sci. Technol. B 30(6), Nov/Dec 2012 2166-2746/2012/30(6)/06FF08/7/$30.00 VC 2012 American Vacuum Society 06FF08-1

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IV. RESULTS AND DISSCUSION

Deposits of aggregated silver particles were successfully

grown at all three investigated electron energies. Our test pat-

tern was designed as a set of arrays of single point dwellings,

as seen in Fig. 2(a), where every array contains 25 silver

deposits grown with the same electron dose. Electron dosage,

pattern layout, and other lithography parameters were kept the

same for all electron energies investigated. We have found the

deposits in an array to be of uniform size, and thus, we could

clearly correlate the deposit size with the electron dose. Figure

2(b) depicts the lateral size increase with increasing electron

dose for 2 and 5 keV energies as recorded by SEM imaging.

At 20 keV beam energy, the deposits appear to grow mostly

vertically, and we observed little lateral size increase. In fact,

the vertical growth with increasing dose was confirmed by

AFM imaging for all three energies. However, comparing the

three energies at the same electron dose shows that the height

of the deposits for 5 and 20 keV are similar while it is less for

2 keV, as seen from the AFM data in Figs. 3 and 4(b). Based

on two separately exposed sets and numerous SEM- and

AFM-based size measurements in each, the average deposit

diameters and the average deposit heights are plotted in

Figs. 4(a) and 4(b), respectively. Assuming that the deposit

volume can be approximated to that of a sphere cap Eq. (1),

then the calculated volume growth is highest at 5 kV, smaller

at 2 kV, and even smaller at 20 kV, as seen in Fig. 4(c). This

shows that there is an optimum acceleration voltage for effi-

cient deposition.

V ¼ ph

6

3

4d2 þ h2

� �; (1)

V¼ volume, h¼ deposit height, d¼ deposit diameter.

Simulated electron trajectories are plotted in Fig. 5 for all

three electron-energies. In case of 2 keV, the electrons are

shown to be stopped inside the 150 nm thick membrane, and

they do not make it through into the silver solution. Contrary

to this result, we did observe silver deposition from the solu-

tion. However, we also observed “dimpling” of the membrane

FIG. 1. (Color online) Photos (a) and (b) and schematic (c) of liquid cell

used in this experiment.

FIG. 2. (a) Example of the pattern used for growth characterization: arrays of

5 by 5 point dwellings with 500 nm spacing were repeated at a range of

doses. This SEM image shows 5 kV deposits and the applicable dose is

marked next to every array. Scale bar is 10 lm. In (b), individual deposits

from the arrays are shown for three different doses and three different elec-

tron energies.

06FF08-2 Ocola et al.: Growth characterization of electron-beam-induced silver deposition 06FF08-2

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at the place of the arrays, as it is visible in the AFM image in

Fig. 3. Evidently, the membrane thins down during exposure

as it absorbs the energy of the electrons. We did not get a

good measure to how much thinning has happened at certain

electron doses, but it is apparently sufficient to allow some

electrons through. A more localized thinning and “dimpling”

do happen at higher beam energies (20 kV) as well, as is

shown in Fig. 6. The exposure to high-energy electrons indu-

ces radiation damage in the polyimide as scissioning of mo-

lecular chains prior to crosslinking.11 The smallest and more

volatile molecular fractions accumulate as dissolved gas in

the polymer matrix, producing swelling prior to their diffusion

out of the polymer. The metal deposit follows the swelled

shape. In time the membrane relaxes by out-diffusion of the

volatile fragments and in that location “dimples” appear.12,13

After polyimide relaxation, some Ag deposits detach but pre-

serve the swollen shape of the polyimide, therefore, appearing

as concave shells.

The simulated 5 keV electron trajectories in Fig. 5 explain

the observed silver accumulation in between silver deposits

in the arrays, as electrons scatter over a micrometer diameter

circle at the solution side of the membrane. Both the AFM

image in Fig. 3 and the SEM images in Fig. 2(b) show this

excessive silver growth. The 20 keV electrons are maintain-

ing a rather confined beam and relatively high energy as they

FIG. 3. (Color online) 3D plots of AFM images of deposits grown with 1.56 pC

electron dose. While at 20 kV the background of the deposits is nearly flat, at

5 kV silver accumulation in between the individual deposits is observed, and at

2 kV a dimple inside the array is observed as a result of the thinning of the poly-

imide membrane.

FIG. 4. (Color online) Plots showing (a) average deposit diameter extracted from

SEM images and (b) average deposit height extracted from AFM images. For all

three energies there were two sets of arrays considered and the “error bars” are

thus representatives of the two values from different experiments. Using deposit

volume estimation by the sphere cap equation, (c) demonstrates that the most

efficient incident electron energy for deposition in this study is 5 keV.

06FF08-3 Ocola et al.: Growth characterization of electron-beam-induced silver deposition 06FF08-3

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travel across the membrane (also shown in Fig. 5), which is

the reason why the silver deposits grow more confined as

well even if somewhat enlarged by the localized polymer

swelling.

In order to test the quality of silver grown on extended

area, we investigated closer spaced arrays of point dwellings.

Figure 7 displays some examples with different amount of sil-

ver coverage. Figures 7(b) and 7(c) are examples of control-

ling deposit thickness on an area by changing the spacing of

dwell points. The thickness can also be controlled by the

dwell time (i.e., electron dose). A target thickness is best

achieved by selecting a combination of appropriate spacing

and dwell time, very similar to the method used in gas-phase

EBID. Increasing the deposit thickness, however, results in

more profound changes in deposit quality than in gas-phase

EBID of other metals. We observed significant coarsening of

the growing silver grains, and measured over 100 nm particles

in areas receiving the highest set electron dose of 50 pC per

dwell point and 500 nm spacing. Figure 7(f) shows details of

such a deposit. Besides increasing in size, the particles also

show more structure most likely due to multiple twinning.14

Optical absorbance of the silver deposits was measured

with an inverted optical microscope connected to a spectro-

graph (Princeton Instruments Acton SP2300). The absorb-

ance curves show a narrower and higher intensity peak at

around 420 nm and a broader, lower intensity peak at around

660 nm, which is typical for silver nanoparticle samples.15 In

Fig. 8, two measurements on two different samples are dis-

played as an example. Both samples were deposited on an

extended area of 50 by 50 lm, and exposed as a regular array

of point dwellings with 300 nm spacing at 20 kV acceleration

voltage. The sample with higher dose (18 pC versus 9 pC)

and thus with more silver and larger deposits show stronger

absorbance and peaks slightly shifted to larger wavelengths.

Optical activity of the deposits was also confirmed by the

successful collection of surface enhanced Raman signal

from Rhodamine 6 G molecules, as shown in Fig. 9(a).

Further indication that enhancement has been achieved

when a Raman image map is run within a short range around

FIG. 5. (Color online) 2 lm by 2 lm area plots of simulated electron

trajectories for (a) 2, (b) 5, and (c) 20 keV electron energies. These

results were obtained using the “CASINO” software (by Gauvin and Drouin).

The trajectories mark incoming, low energy and backscattered-electron

paths.

FIG. 6. (Color online) SEM image of silver deposits at 20 kV on polyimide.

The turned-over silver structures have a concave shape, with the concavity

originally facing the polyimide, while the polyimide locations left behind

show dimples.

06FF08-4 Ocola et al.: Growth characterization of electron-beam-induced silver deposition 06FF08-4

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1515 cm�1 wavenumber. Figure 9(b) shows an optical

micrograph of a set of nanodeposit arrays coated with Rho-

damine 6 G molecules. Figure 9(c) has a Raman image map

superimposed over the previous optical micrograph. Areas

coated in Rhodamine that are away from the silver nanode-

posits have no enhancement, therefore are dark, while the

areas over the silver nanodeposits are bright, Fig. 9(c). This

data indicate EBID in liquid can achieve high purity and

appropriate sample quality for important applications that

aim to take advantage of the optical properties of silver at

the nanoscale.

Composition of the silver deposited was obtained using

EDS (Thermo Scientific Ultra Dry, 129 eV resolution, 180 V

Operating bias) that was installed on the same FEI Nova 600

NanoLab microscope a few months later. The experimental

setup consisted of the grid supported free of a back substrate

to eliminate background signal, 20 kV acceleration voltage,

FIG. 7. (Color online) SEM images showing examples of silver deposits in attempts to cover extended area. The patterns are defined using single-point dwell-

ings of 5 keV, 2.5 pC dose in (a)–(c) and 5 keV, 50 pC dose in (d)–(f). While (a) is a stand-alone single-point dwelling, (b) is a 5 by 5 array with 200 nm spac-

ing, and (c) is a 5 by 5 array with 100 nm spacing. (d) is again a single-point dwelling, (e) is a 5 by 5 array with 500 nm spacing, and (f) is an enlarged detail

showing faceted crystals at the order of 100 nm in size as a result of large volume accumulation.

FIG. 8. (Color online) Optical absorbance measurements on extended-area

deposits with 300 nm spacing display silver peaks at 420 nm and 663 nm for

9.0 pC dose deposits and at 425 nm and 681 nm for 18 pC dose deposits.

FIG. 9. (Color online) Surface enhanced Raman signal collected from Rho-

damine 6 G molecules drop-coated on silver deposits. (a) Rhodamine spectra

with imaging bandwidth indicated within dashed rectangle, (b) optical

image of silver particle arrays coated with Rhodamine, (c) exact same region

with superimposed Raman image map obtained within bandwidth indicated

in part (a).

06FF08-5 Ocola et al.: Growth characterization of electron-beam-induced silver deposition 06FF08-5

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2.4 nA current and 4 mm working distance, 10 K� magnifi-

cation. The raw compositional data of a blank membrane

and five others with silver nanodeposit arrays are shown

in Table I, and Fig. 10, with different conditions of spacing

(in microns) and exposure dose (in pC) deposited at 20 kV.

The data show the composition of the supporting mem-

brane along with the silver deposits. Imide, the polyimide

monomer, is known to typically have two oxygen atoms per

nitrogen atom.16 The ratio is close to 2 for all sets of data.

Therefore, it can be inferred that the compositional data

from C, N, and O are only from the polyimide membrane

and that the sulphur and silver data are only from the nano-

deposit arrays. Once the polyimide contribution is removed

from the data, one can observe clear trends of particle

growth (silver and sulphur content) as a function of spacing

between the dots (0.3–0.7 lm) and as a function of exposure

dose (4.5–18 pC), Fig. 11(a). The highest density of dots

will correspond to 0.3 mm spacing and 18 pC dose. The rela-

tive content of sulphur appears to be lowest when the nano-

deposits are largest, suggesting that the sulphur may be

confined to the surface, Fig. 11(b). Future work will be con-

ducted to verify this assumption.

The presence of sulphur is known to be from the decom-

position of H2S and SO2 from air.17 In normal laboratory air

(measured concentrations of H2S and SO2 less than 0.2 parts

per billion), a 0.1 nm-thick tarnish film can be expected to

form in 1 h, 1.5–3.0 nm in 1 week.17 Therefore, the sulphur

found in the samples is not surprising and also suggests that

the initial purity of the nanoparticles was close to 100%

silver.

V. SUMMARY AND CONCLUSIONS

We have fabricated various patterns of aggregated silver

particles including isolated deposits of about 100 nm in di-

ameter and above, and extended-area coatings. Our investi-

gations unveiled that by adjusting electron energy, the

deposition process can be optimized either for maximum

deposit-volume per electron-dose ratio or for achieving min-

imal feature sizes. The deposits were used for Raman ampli-

fication, and the composition measured with EDS suggests

pure silver was deposited. The only impurity found was sul-

phur, which is known to come from exposure to air. The

results obtained make suggest that the liquid-phase silver

EBID process is interesting for applications as sensors and

plasmonic structures.

ACKNOWLEDGMENTS

The authors would like to acknowledge the help of Dr.

Galyna Krylova, who provided us with the silver solution

and Dr. Aiqing Chen for Raman data acquisition. They thank

the discussions with the group of Professor Todd Hastings at

the University of Kentucky, who have pioneered liquid-

phase EBID. Use of the Center for Nanoscale Materials was

supported by the U. S. Department of Energy, Office of Sci-

ence, Office of Basic Energy Sciences, under Contract No.

DE-AC02-06CH11357.

1K. Landheer, S. G. Rosenberg, L. Bernau, P. Swiderek, I. Utke, C. W.

Hagen, and D. H. Fairbrother, J. Phys. Chem. C 115, 17452 (2011).2P. Roy, R. Lynch, and P. Schmuki, Electrochem. Commun. 11, 1567

(2009).

TABLE I. Raw compositional EDS data of a blank membrane and five others

with arrays of nanodeposited silver under different conditions of spacing (in

microns) and exposure dose (in pC) deposited at 20 kV.

Sample C N O S Ag

0.7 lm 18 pC 35.7 19.7 41.9 0.6 2.1

0.5 lm 18 pC 33.7 20.2 40.1 0.8 4.9

0.3 lm 18 pC 31.3 18.3 37.6 1.7 9.8

0.3 lm 9 pC 33.9 20.2 37.8 1.2 5.8

0.3 lm 4.5 pC 35.5 19.8 41.5 0.6 2.3

Blank 35.9 18.5 44.9 0.0 0.0

FIG. 10. (Color online) EDS spectra of a silver nanodeposit array.

FIG. 11. (Color online) (a) Compositional data from EDS measurements of a

select set of nanodeposit arrays. (b) Relative composition of silver nanodeposits.

06FF08-6 Ocola et al.: Growth characterization of electron-beam-induced silver deposition 06FF08-6

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3E. U. Donev and J. T. Hastings, Nano Lett. 9, 2715 (2009).4E. U. Donev and J. T. Hastings, Nanotechnology 20, 505302 (2009).5N. Kolmakova and A. Kolmakov, J. Phys. Chem. C 114, 17233 (2010).6G. Schardein, E. U. Donev, and J. T. Hastings, Nanotechnology 22,

015301 (2011).7E. U. Donev, G. Schardein, J. C. Wright, and J. T. Hastings, Nanoscale 3,

2709 (2011).8E. U. Donev, N. Nehru, G. Schardein, J. Wright, A. Chamberlain,

C. Samantaray, and J. T. Hastings, Microsc. Microanal. 17 (Suppl. 2), 438

(2011).9J. J. Park, A. Joshi-Imre, L. E. Ocola, and R. Divan, MRS Proc. 1371,

imrc-1371-s1-p083 (2012).

10D. Drouin, A. R. Couture, D. Joly, X. Tastet, V. Aimez, and R. Gauvin,

Scanning 29, 92 (2007).11E. R. Long, Jr., and S. A. T. Long, NASA Technical Paper 2663, April (1987).12Y. Vladimirsky, O. Vladimirsky, K. J. Morris, G. M. Calderon, V. Saile,

and J. M. Klopf, Microelectron. Eng. 30, 543 (1996).13N. Moldovan, Proc. SPIE 3875, 155 (1999).14B. Wiley, Y. Sun, and Y. Xia, Acc. Chem. Res. 40, 1067 (2007).15S. S. Martinos, Phys. Rev. B 39, 1363 (1989).16T. Takekoshi, Polyimides: Fundamentals and Applications, edited by M. K.

Ghosh, and K. L. Mittal (Marcel Dekker, New York, 1996), pp. 7–48.17H. E. Bennett, R. L. Peck, D. K. Burge, and J. M. Bennett, J. Appl. Phys.

40, 3351 (1969).

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