Reversible electron-induced conductance in polymer nanostructuresA. R. Laracuente, M. Yang, W. K. Lee, L. Senapati, J. W. Baldwin, P. E. Sheehan, W. P. King, S. C. Erwin, andL. J. Whitman Citation: Journal of Applied Physics 107, 103723 (2010); doi: 10.1063/1.3428963 View online: http://dx.doi.org/10.1063/1.3428963 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/107/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Nano-electron beam induced current and hole charge dynamics through uncapped Ge nanocrystals Appl. Phys. Lett. 100, 163117 (2012); 10.1063/1.4705299 Simulation of electron beam lithography of nanostructures J. Vac. Sci. Technol. B 28, C6C48 (2010); 10.1116/1.3497019 Local electron beam induced reduction and crystallization of amorphous titania films Appl. Phys. Lett. 89, 021902 (2006); 10.1063/1.2219398 Electron beam induced conductivity in polymethyl methacrylate, polyimide, and SiO 2 thin films J. Vac. Sci. Technol. B 22, 2907 (2004); 10.1116/1.1826062 Electron-beam-induced conduction in a ruthenium carbonyl nanoparticle polymer Appl. Phys. Lett. 76, 1773 (2000); 10.1063/1.126163

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Reversible electron-induced conductance in polymer nanostructuresA. R. Laracuente,1,a� M. Yang,1 W. K. Lee,1 L. Senapati,1 J. W. Baldwin,1 P. E. Sheehan,1

W. P. King,2 S. C. Erwin,1 and L. J. Whitman1

1Naval Research Laboratory, Washington, DC 20375-5342, USA2Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana,Illinois 61801, USA

�Received 30 September 2009; accepted 15 April 2010; published online 27 May 2010�

We report a mechanism for controlling conductance in polymer nanostructures. Poly�3-dodecylthiophene-2,5-diyl� �PDDT� nanostructures were directly written between gold electrodesusing thermal dip pen nanolithography and then characterized in UHV. We find that the conductivityof a PDDT nanostructure can be increased by more than five orders of magnitude �from �10−4 to10 S cm−1� by exposure to energetic electrons, and then repeatedly returned to a semi-insulatingstate by subsequent exposure to hydrogen. Based on systematic measurements complemented bycalculations of electronic structure and electron transport in PDDT, we conclude that theconductance modulation is caused by H desorption and reabsorption. The phenomenon has potentialapplications in hydrogen sensing and molecular electronics. © 2010 American Institute of Physics.�doi:10.1063/1.3428963�

I. INTRODUCTION

Polymers are attractive materials for a wide variety ofelectronic and optoelectronic applications,1–5 in part becausethe structure and properties can be varied by customsynthesis.6–10 Although conductive polymers �which are in-trinsic semiconductors� have begun to find commercial suc-cess, their poor conductivity prevents their use in applica-tions that require fast switching or high conductivity. Manypolymers can be chemically doped to increase theirconductivity;11–16 however, common doping methods are un-reliable, are plagued by impurities, and are incompatiblewith conventional processes used to fabricate microelec-tronic devices. An alternate, unusual way to temporarily in-crease polymer conductivity is via radiation exposure, in-cluding electron beams, ultraviolet light, or x-rays.14,17,18

Unfortunately, after the exposure the polymer conductivitytypically decays back to near its original value.11 The ob-served increase in conductivity is commonly explained interms of trapped carriers19–21 but the mechanism for the sub-sequent conductivity decay is poorly understood.22–25

Herein we present a study of electron-induced conduc-tivity in poly�3-dodecylthiophene-2,5-diyl� �PDDT� nano-structures directly written between electrodes using thermaldip pen nanolithography �tDPN�.26,27 Creating a structure onthe same size scale as the electron beam enabled us to studyelectron-induced transport as a function of the precise loca-tion and dose of energetic electrons. We find that the conduc-tivity of a PDDT nanostructure increases dramatically by ir-radiation with energetic electrons. Subsequent gas exposurehas varied effects on the conductivity that depends specifi-cally on the gas, and we conclude that the induced conduc-tivity and its reversal is caused by H desorption and reab-sorption.

II. EXPERIMENTAL METHODS

PDDT was directly deposited between gold electrodesthat had been patterned by e-beam lithography onto a 100nm thick silicon oxide film on a silicon substrate. Before use,the gold electrodes were sonicated in acetone and thenplaced in an oxygen plasma chamber to remove any residualorganic contamination. Commercially-obtained, regioregular��99%�, head-to-tail coupled PDDT was used. tDPN uses acustom atomic-force-microscopy �AFM� cantilever with anintegrated resistive heater28 to control the fluidity of mol-ecules �the “ink”� previously deposited on the cantilevertip.26 Because the PDDT ink is solid at room temperature, nodeposition occurs when the unheated tip is in contact withthe surface. When the AFM tip is heated close to or abovethe PDDT glass-transition temperature, PDDT flows fromthe tip to the substrate surface. By adjusting the tip heatingpower and the writing speed, the deposition rate can be con-trolled and deposition turned on and off.27 To inhibit oxida-tion of the PDDT by water or oxygen during heating, depo-sition was performed inside a dry-nitrogen-filled glove box�humidity�0%�. The PDDT-coated tip was first degassed byheating the tip, and then brought into contact with the sub-strate. The tip was first used for imaging the surface in orderto locate the center of the electrode gap, and then a PDDTline was deposited across the gap.

After confirming the morphology of each tDPN nano-structure via AFM, each sample was transferred to UHV��1�10−10 Torr� for chemical and electrical analysis in aunique facility that includes a high-resolution scanning elec-tron microscope �SEM� and a scanning Auger nanoprobe in-tegrated with a four-tip nanoprobe. Auger electron spectra�AES� from spots of diameter �50 nm can be obtained fromthe secondary electrons generated by the SEM electron beamand collected with a hemispherical energy analyzer. The mul-titip nanoprobe enables multiple conductivity measurementsto be made across structures �1 �m long in situ, without

a�Author to whom correspondence should be addressed. Electronic mail:arnaldo.laracuente@nrl.navy.mil.

JOURNAL OF APPLIED PHYSICS 107, 103723 �2010�

0021-8979/2010/107�10�/103723/6/$30.00 © 2010 American Institute of Physics107, 103723-1

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the need for fan-out and wiring to external connections.AFM and SEM images of a typical polymer nanostructurespanning electrodes are shown in Fig. 1. �Note that this AFMimage was obtained using a clean, commercial tip in tappingmode, not the original tDPN tip.�

III. THEORETICAL METHODS

Conductivity in PDDT was studied theoretically using anidealized model system, depicted in Fig. 2, consisting of asingle polymer chain of nine PDDT molecules, bonded togold electrodes via a sulfur atom. The ground-state physicaland electronic structure of the chain was determined usingdensity-functional theory in the generalized-gradient ap-proximation with Becke’s three-parameter hybrid functional

B3LYP, as implemented in Gaussian 03. The electronic struc-ture was calculated using all-electron 6-311+G� basis func-tions for carbon, hydrogen, and sulfur. �For gold, we usedLANL2DZ basis sets, i.e., Los Alamos effective core poten-tial with double zeta.� The convergence criterion for self-consistency of the total energy was 10−8 Hartrees.

Electronic transport calculations were carried out in thelow-bias regime, using the multichannel Landauer–Buttikerformalism based on the Green’s function. Since spin-orbitand hyperfine interactions in this molecular system are weak,the effects of incoherent and spin-flip scattering were ne-glected. Hence, the total current is simply the sum of spin-up�↑↓� and spin-down �↓� currents,

I�↑/↓� =e

h�

�R

�L

TLR�↑/↓��E��fL�E� − fR�E��dE , �1�

where the limits of integration are the electrochemical poten-tials of the Au contacts, �1 and �2, under the applied biasvoltage V. For strongly coupled molecular system we ex-pected an equally distributed potential drop between the leftand right contacts. Thus the electrochemical potentials usedfor the calculations were �1=Ef −eV /2 and �2=Ef +eV /2,where Ef is the Fermi energy of bulk Au. The transmissionfunction T�↑ /↓� represents the sum of transmission prob-abilities for electrons transmitted through the molecule, andwas calculated from the Green’s function using the Fisher–Lee formalism

TLR�↑/↓� = Tr��LG�↑/↓��RG�↑ /↓�+

� , �2�

where the Green’s functions are

G�↑/↓� = �E � I − H�↑/↓� − �L − �R�−1, �3�

and E is the injection energy of the electrons. Au atoms areexplicitly included in our model system, as a single layer of25 gold atoms, to obtain the coupling matrices needed tocalculate the self-energy functions, �. We have used the localdensity of states of the 6s band of bulk Au �0.035 eV perelectron spin� to approximate the Green’s function of the Aucontact.

IV. RESULTS AND DISCUSSION

An array of PDDT nanostructures deposited by tDPNbetween gold electrodes is depicted in Fig. 1�a�. The struc-ture shown in Fig. 1�b� was approximately 370 nm wide and30 nm thick between the electrodes. One challenge in char-acterizing nanostructures of this scale is confirming their ma-terial composition, especially for soft materials not easilyanalyzed by energy dispersive x-ray analysis. Using our Au-

(b)

(c)

1 µm

40 µm

(a)Nanostructure

Pt Probe

AuElectrode

PDDT

Nanostructu

re

FIG. 1. �a� Array of PDDT nanostructures �highlighted with arrows� depos-ited by tDPN between gold electrodes. �b� Tapping-mode AFM image of aPDDT nanostructure deposited by tDPN between two gold electrodes�spaced 930 nm apart�. �c� SEM image of the same nanostructure; the PDDTappears as a dark feature between the electrodes.

C H12 25

S

9

SS

AuElectrode

AuElectrode

FIG. 2. Model of a PDDT molecule sandwiched between two metal goldcontacts used for calculations of electron transport. The single polymerchain of nine PDDT molecules bonded to gold electrodes via sulfur atoms.

103723-2 Laracuente et al. J. Appl. Phys. 107, 103723 �2010�

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ger nanoprobe, however, we can determine the local chemi-cal composition of each device component. Typical Augerspectra of this PDDT nanostructure and the surrounding sur-faces are shown in Fig. 3, with the Si, C, O, S, and Au peaklocations indicated. Spectra were collected on the silicon ox-ide substrate, the gold contact pads �the triangular regions�,and on the PDDT nanostructure. The major contaminants, Con the substrate and S on the pads, are attributed to air ex-posure and handling prior to loading the sample into theUHV chamber. Note that on the PDDT nanostructure, theratio of the constituent S and C AES peaks is consistent withnondissociative polymer deposition.

Electrical measurements across the PDDT nanostruc-tures were made using the multitip nanoprobe with Pt probetips in contact with the gold electrode pads. Current versusvoltage �IV� measurements were recorded by sweeping thecurrent and measuring the voltage drop across the nanostruc-ture. The nanostructure resistance was determined from theslope of the IV curves near 0 V.29 As deposited, this PDDTnanostructure had a resistance �10 G� �limit of our mea-surement�. As a reference, the contact resistance between thePt/Ir probes and the Au was �10 �, as measured by twoprobes in contact with the same Au electrode.

Surprisingly, the nanostructure conductance can be re-versibly increased by more than five orders of magnitude viacontrolled exposure to the 5 keV SEM electron beam. Asshown in Fig. 4, the shape of the IV curve depends on thetotal electron exposure. Short exposures produce low-conductivity states ���1 S cm−1� that result in non-OhmicIV curves; long exposures produce high-conductivity statesand Ohmic IV curves. Because the metal-polymer contactwas not exposed, we attribute the non-Ohmic to Ohmic tran-

sition solely to an increase in carrier concentration and adecrease in the depletion width within the PDDT. Assuminga PDDT carrier mobility of 0.1–1.0 cm2 V−1 s−1 �an upperlimit based on reported values6,30,31�, we estimate the carrierconcentration at the non-Ohmic to Ohmic transition to be inthe order of 1018–1019 cm−3. Note that such a high concen-tration in a polymer is very unusual, suggesting that the mo-bility may actually be much larger than 1 cm2 V−1 s−1 andclose to the intrachain mobility of ladder-type polymers.32

To elucidate the effect of electron irradiation on the con-ductance, the nanostructure was exposed to the 5 keV elec-tron beam in 15 s increments, and an IV measurement wasmade immediately after each exposure. Two types of expo-sures were performed with the same cumulative electrondose per 15 s exposure. Spot exposures were made using astationary spot with a primary-electron-beam flux of 8�1021 cm−2 s−1. Raster exposures were made by rastering abeam flux of 1�1020 cm−2 s−1 across the whole PDDTnanostructure. The resulting changes in conductivity versusexposure time are shown in Fig. 5. Repeated spot exposureson one location in the middle of the structure produced asudden increase in the conductivity during the first exposure,followed by a slow increase �5�10−5 S cm−1 s−1� in theconductivity during subsequent exposures. If, instead, thebeam was moved to a new location after each spot exposure,the conductivity increased seven times faster than for thefixed-spot case. Exposing the whole PDDT nanostructure byrastering the electron beam induced the maximum rate ofconductivity increase �8�10−3 S cm−1 s−1�. After repeatedraster exposures, the conductivity saturated at �10 S cm−1.

The increase observed in conductivity could resultfrom either carrier generation or from film damage �whichwe expect would be irreversible�. To determine if theelectron beam permanently damages the polymer in thenanostructure—e.g., by decomposition—we studied the re-versibility of the highly conductive state. On the time scale

Energy (eV)6000 400200

dN/dE(a.u.)

OO

S

C O

SiOx

PDDT

Au electrode

Si

Au

FIG. 3. Auger electron nanoprobe spectra collected on the silicon oxidesubstrate, the gold electrode, and on the PDDT nanostructure. The locationsof the Si, C, O, Au, and S Auger peaks are indicated.

Voltage (V)100-10

-10

10

Current(nA)

(a)(b)

(c)

FIG. 4. IV measurements from the PDDT nanostructure after �a� a 45 s spotelectron beam exposure; �b� a 15 s raster e-beam exposure; and �c� H2

exposure.

103723-3 Laracuente et al. J. Appl. Phys. 107, 103723 �2010�

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of days, a highly conductive nanostructure left in the UHVchamber will gradually return to a resistive state. However, ifthis film is re-exposed to the beam, it will return to the highlyconductive state. Such a cycle has been repeated more than15 times without noticeable change in either resistance state,leading us to conclude that the electron exposure does notirreversibly alter the polymer structure.

We propose that the mechanism for the electron beam-induced conductance is the creation of charge carriers byelectron-induced H desorption. Because charge accumulationis mostly short lived and cannot persist for long times, wepropose that dehydrogenation is the major phenomenon con-trolling the increase in current in the PDDT nanostructure. Adehydrogenation mechanism is supported by the effects ofspecific gas exposures on PDDT nanostructures that wereinitially in a conductive state, as follows. We exposed such ananostructure to 2�10−5 Torr of Ar, H2, or O2 in a second,interconnected UHV chamber and performed IV measure-ments after each gas exposure �Table I�. Because the conduc-tivity naturally decays over time, a control experiment �theentry labeled “none”� was also performed to monitor the

change in resistance simply associated with the time in UHVduring the sample transfers. Argon exposure had no measur-able effect on the conductivity, and the oxygen exposurecaused a small increase in the resistance. In contrast, H2

exposure dramatically increased the resistance by several or-ders of magnitude.

It is well known that electrons can remove hydrogenfrom surfaces.33,34 On diamond surfaces, the electron-induced H-desorption cross-section �which depends on sur-face coverage and electron energy� is between 4�10−18 and5�10−19 cm2.35,36 A cross-section of similar magnitude isobserved on H-terminated Si surfaces;37 therefore, a cross-section of �10−18 cm2 would be a reasonable assumptionfor PDDT. The H density in a PDDT lamellar crystal is�7�1022 cm−3, which suggests our electron flux could cre-ate radicals in the PDDT at a rate �1023 cm−3 s−1. Even thissimple estimate would account for far more radicals than thenumber of carriers needed to create the additional conductiv-ity we observe.

Our model of the reversible change in conductance isbased on electron-induced H desorption and resorption. Webelieve the electron beam removes a significant number of Hatoms from the polymer, leaving behind ions and radicals.Given the facile H diffusion within the polymer, radicalscreated by H desorption would be expected to recombine toform double bonds. In contrast, rearrangement of the ionscould create new carriers on both the alkyl groups and thepolymer chains.38 Charge rearrangement on the alkyl groupswould promote charge transport across adjacent PDDTchains, thereby increasing the conductance. The quenchingof the conductivity by H2 exposure suggests that these radi-cals are passivated by hydrogen. Hydrogen passivationwould readily account for the conductance decrease observedover days in UHV, where there is always a low backgroundpressure of H2 �and potentially H radicals, when hot fila-ments are present�.

The generation of carriers by electron-induced H desorp-tion is also consistent with the conductivity of the PDDTnanostructure as a function of electron-beam exposure �Fig.5�. When the electron beam is fixed during the exposure,most of the H atoms on the exposed region desorb, and theconductivity suddenly increases. The conductivity thenslowly increases, presumably due to carriers diffusing awayfrom the exposed region and H diffusing back into the re-gion. Therefore, moving the beam to an unexposed regionproduces a constant increase in conductivity. By rastering thebeam over the whole nanostructure, the removal of H and theconductivity increase are maximized, until most of the H isremoved and the conductivity reaches its maximum.

It is interesting to point out that spin-casted PDDT thin-films of a similar thickness �20–40 nm thick� deposited oversimilar gold electrodes did not exhibit any conductivity in-crease even after an hour of electron irradiation. However,electron exposures longer than 10 h resulted in creating apermanent conductive path between the electrodes which areconsistent with a permanent damage of the polymer film.

We attribute the difference in behavior to a different mo-lecular structure within the spin cast films than in nanostruc-tures those deposited via tDPN.39 During tDPN deposition

Time (sec)

Conductivity(S/cm)

0 200100 300 400 5000.001

0.01

1

0.1

10

Fixed Spot

Varied Spot

Raster

FIG. 5. The conductivity of the PDDT nanostructure as a function of elec-tron beam exposure time on a fixed spot on the nanostructure, varied spotlocations, and a raster exposure �covering the whole nanostructure�.

TABLE I. The change in resistance of a conductive PDDT nanostructurecaused by various gas exposures. The nanostructure was first made conduc-tive by electron-beam exposure in the SEM, then transferred to an intercon-nected UHV chamber and exposed to 2�10−5 Torr of the gas indicated.The time interval between the electrical measurements recorded before andafter the exposure is indicated. The resistance change is expressed as theratio of the final to the initial resistance.

GasMeasurement interval

�min�Exposure

�s� Resistance change

None 65 ¯ 0.5Argon 46 170 0.5Oxygen 45 730 1.4Hydrogen 41 760 126

103723-4 Laracuente et al. J. Appl. Phys. 107, 103723 �2010�

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the PDDT is heated close to or above its glass-transitiontemperature and then sheared between the heated tip andsubstrate. This leads to a high degree of molecular order withthe PDDT stacked layer-by-layer and aligned along the pathof the tip.27 In the present case, the polymer would bealigned with the electrodes. Second, tDPN deposits thePDDT from the polymer melt which creates films that aremore uniform and denser than those created usingsolvents.40,41

Our proposal that electron beam-induced conductance inthe PDDT is caused by H desorption is strongly supported byour theoretical calculations. The energy difference betweenremoving H from the side chains and the backbone is about0.017 24 eV/atom. Therefore, we computed electron trans-port across three related PDDT-chain structures where H wasremoved from the side chains. The first was the fully hydro-genated structure depicted in Fig. 2, with nine complete−C12H25 side chains, representing the “as-deposited” PDDTstructure. A second structure was formed by removing, fromeach −C12H25 side chain, one of the two H atoms bonded toeach C atom, leaving nine dehydrogenated −C12H13 sidechains. A third, intermediate, structure was formed by re-moving one H atom from each side chain, leaving −C12H24

side chains. The calculated IV characteristics for these threestructures are shown in Fig. 6�a�. In agreement with our hy-pothesis, the lowest conductivity was observed for the as-deposited structure with −C12H25 chains, while the highestconductivity was observed for the dehydrogenated −C12H13

chains. Even the intermediate structure, with −C12H24 chains,shows a substantial increase in conductivity, demonstratingthat the removal of a single H atom can strongly affect con-ductivity.

Of course, the actual PDDT nanostructure obtained ex-perimentally has a much more complex internal structurethan the single PDDT-chain shown in Fig. 2. In particular,the single PDDT-chain does not allow for the possibility oftransport across different chains, which could arise eitherfrom “interlinking” by forming chemical bonds betweenchains, or simply from the hopping of electrons acrossnearby chains. We briefly investigated this latter possibilityby performing transport calculations for a double PDDT-chain consisting of two single PDDT-chains separated by 0.4nm, with both chains bonded to the gold contacts. As withour calculations for single chains, we considered two vari-ants: a fully hydrogenated double chain �with −C12H25 sidechains�, and a dehydrogenated double chain �with −C12H13

side chains�. The calculated IV characteristics of these twostructures are shown in Fig. 6�b�. The conductivity of thefully hydrogenated double chain is similar to that of the cor-responding single chain, but the conductivity of the dehydro-genated double chain is strongly enhanced, by a factor ofabout 4 in the low-bias regime. Thus, the impact of dehydro-genation on conductivity for double PDDT-chains is evengreater than for single chains. Moreover, it is expected thatthe greater order and density of the tDPN structure as com-pared to the spin cast polymer would enhance the effect ofthese interchain electron interactions. This could explain whythe enhancement is observed in the tDPN structures and notin the spin-cast films.

The impressive sensitivity of the PDDT nanostructure tohydrogen under UHV conditions suggests possible applica-tions in hydrogen sensing �of increasing importance to sup-port a future hydrogen energy infrastructure�. The most sen-sitive detectors can currently detect parts per million H2.42,43

The observed four orders of magnitude conductivity changein our PDDT nanostructure after exposure to the backgroundH2 in UHV ��10−11 Torr� shows it has the potential to de-tect much less than parts per billion �under vacuum condi-tions�. To explore the potential for sensing applications inambient conditions, a nanostructure was prepared in a con-ductive state �as usual, in UHV�, and its resistance was thenmonitored during ambient exposure to ambient air �nomi-nally 0.5 ppm H2� and to a 145 ppm H2-in-air mixture. Fig-ure 7 shows the time response of the nanostructure resistanceunder the two different hydrogen concentrations. While theinitial resistance change is very similar in both exposures,suggesting a surface adsorption effect, the long term resis-

Voltage (V)2-2 0

10

-10

Current(nA)

Voltage (V)2-2 0

10

-10

Current(nA)

(b)

(a)

C H12 13

C H12 25

C H12 25

C H12 24

C H12 13

FIG. 6. �a� Calculated IV curves of PDDT molecules fully hydrogenated�−C12H25 side chain�, dehydrogenated �−C12H13 side chain�, and with onehydrogen removed �−C12H24� from a side chain. �b� Calculated IV charac-teristic of a two-layer PDDT nanostructure with fully hydrogenated −C12H25

side chains and dehydrogenated −C12H13 side chains.

103723-5 Laracuente et al. J. Appl. Phys. 107, 103723 �2010�

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tance change depended on H2 concentration, indicating abulk diffusion effect. Finally, if a similar radical creation/quenching cycle could be controlled in a device architecture,this phenomenon might also have applications in polymer-based switching in molecular electronics.

V. CONCLUSIONS

In summary, PDDT nanostructures were directly writtenbetween gold electrodes using thermal dip-pen nanolithogra-phy and then characterized on the nanometer scale in UHV.We find that the conductivity of a PDDT nanostructure canbe increased by more than five orders of magnitude �from�10−4 to 10 S cm−1� by exposure to a 5 keV electron beam,and then repeatedly returned to the original semi-insulatingstate by subsequent exposure to hydrogen. First-principleselectronic structure calculations and calculations of electrontransport using the multichannel Landauer–Buttiker formal-ism based on the Green’s function that indicate the conduc-tivity of PDDT is strongly enhanced by dehydrogenation. Weconclude that exposure to energetic electrons induces desorp-tion of H from the polymer, creating neutral and chargedradicals whose rearrangement create conductive channelsalong and across polymer molecules.

ACKNOWLEDGMENTS

This work was supported by the Office of Naval Re-search and the National Science Foundation �W.P.K.�.

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Time (min)

Resistance(MΩ) Air

20 60 100 140

20

60

100

120

80

40

H in Air2

FIG. 7. PDDT nanostructure resistance measured as a function of timeexposed to ambient air �nominally 0.5 ppm H2� and a 145 ppm H2-in-airmixture.

103723-6 Laracuente et al. J. Appl. Phys. 107, 103723 �2010�

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