Two-terminal nanoelectromechanical bistable switches based on molybdenum–sulfur–iodine

molecular wire bundles

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Nanotechnology 21 (2010) 125706 (6pp) doi:10.1088/0957-4484/21/12/125706

Two-terminal nanoelectromechanicalbistable switches based onmolybdenum–sulfur–iodine molecularwire bundlesJ Andzane1, J Prikulis1, D Dvorsek2, D Mihailovic2 and D Erts1

1 Institute of Chemical Physics, University of Latvia, Riga, LV-1586, Latvia2 Josef Stafan Institute, Ljubljana, Slovenia

E-mail: donats.erts@lu.lv

Received 9 December 2009, in final form 10 February 2010Published 5 March 2010Online at stacks.iop.org/Nano/21/125706

AbstractWe demonstrate the application of Mo6S3I6 molecular wire bundles for electrically controllabletwo-terminal on–off switches. We investigate how changes in the contact electrode material andgeometry influence the device characteristics, hysteretic switching behavior and device stability.We also determine the device operating parameters, particularly the Young’s moduli(40–270 GPa), operating current densities (3.2 × 105–7 × 106 A m−2) and force constants.Although qualitatively, the properties of Mo6S3I6 nanowires in nanoelectromechanical (NEM)switches are similar to those of carbon nanotubes (CNTs), their lower friction coefficient,higher mechanical stability and higher operation voltages give specific advantages in terms ofsmaller differences in on–off operating potentials, higher switching speeds and lower energyconsumption than CNTs, which are critical for applications in NEM devices.

1. Introduction

Nanoelectromechanical systems (NEMS) have recentlyattracted much attention due to their unique propertiesand possible applications that differ greatly from those ofmicroelectromechanical systems [1]. In NEMS the criticaldimensions are in nanometers and therefore the active mass isextremely small. Moreover, the internal operating frequenciesof NEMS may achieve giga- and tera-Hertz [2, 3], andNEMS power consumption and heat capacity is expected tobe extremely low. One of the most important advantagesof NEMS is the high integration level that can be achieved,and the possibility of integration into CMOS process flow [4].NEMS in general are attractive for applications as frequencycontrollers [5], radio frequency mobile transceivers [3], nan-otweezers [6], nanorelays and switches [4, 7–17]. ResonatorNEMS have been reported to yield mass sensitivity in zeptorange [5] and can be used also for displacement [18, 19], spindetection [20], quantum measurements [21] and for the fab-rication of nanoelectromechanical transistors [22]. Switches,relays and tweezers can be used as gates for micro andmillimeter waves [23], memory and logic elements [8, 13, 24]

etc. During the past few years, carbon nanotubes (CNTs)have emerged as the most promising components for suchdevices [7, 9–11, 13–15, 24], but it has been shown that othermaterials such as TiN [16], various semiconductors [8, 12, 17],C60 [25] and DNA molecules [26] can also be used as switchingelements.

The discovery of nanowires based on transition metalchalcogenide-halides and subsequent research on the molybde-num–sulfur–iodine system has led to the synthesis ofa new class of nanowire materials with the formulaMo6S9−x Ix , (MoSIx), where S and I can appear in differentproportions [27, 28]. Excellent functional properties ofMoSIx wires, combined with ease of synthesis and intrinsicabsence of impurities are expected to result in widespreadapplications in diverse areas, such as high conductivitycomposites, low friction composites and lubricant additives,field emission devices and nanoscale electrodes. MoSIxnanowires are synthesized in a straightforward one-stepsynthesis process and can be easily and controllably dispersedwithout use of surfactants. At the same time the MoSIxmaterial very effectively satisfies many of the requirementsin various applications defined by more than 10 years of

0957-4484/10/125706+06$30.00 © 2010 IOP Publishing Ltd Printed in the UK1

Nanotechnology 21 (2010) 125706 J Andzane et al

Figure 1. TEM image sequence illustrating a Mo6S3I6 nanowire based on/off device: (a) simplified diagram with forces acting on thenanowire, (b) initial position of the nanowire and counter electrode at zero voltage applied between the electrodes; (c) the nanowirejumps-to-contact at voltage 21 V; (d) the nanowire in contact with the counter electrode (voltage sweep up to 35 and down to 1 V); (e) thenanowire after jump-off contact at 1.2 V; (f) I (V ) characteristic of the on/off cycle. The arrows show the direction in which the hystereticloop is followed during the increase and decrease of the driving voltage.

CNT research [29]: their functional properties such asmechanical strength, electrical conductivity and field emissioncharacteristics are indeed very similar to CNTs. Ultralowfriction coefficients of MoSIx nanowires [30] and relativelyhigh resistivity may serve as the basis for their applicationas active elements in NEM switches. Experiments andcalculations revealed the potential of Mo6S6 nanowires to beused as a nanocable to flexibly transfer information betweenelectromechanical switches, which can also be constructedfrom the same nanowires [31].

In this work we present the operation of a two-terminalon–off NEM device based on Mo6S3I6 nanowire bundles.By observing the hysteresis in I –V curves we show thatdevice stability is influenced by the contact geometry. Inpotential applications, the stability becomes important whendesigning the devices that work at high speeds and lowdrive voltages, where system noise becomes significant. Thehysteresis characteristics are also directly related to the energyconsumption of such devices.

2. Results and discussion

A schematic of our nanowire based two-terminal NEMdevice and the forces acting between the nanowire and theopposite electrode in a two-terminal NEM device are shownin figure 1(a).

In the first step we demonstrate a stable on–off operationof an electrically controlled mechanical switch with a singlenanowire bundle. All changes in contacts and in thenanowire during electrical measurements are visualized insidethe transmission electron microscope (TEM). A voltage sweepfrom 0 to maximum voltage (35–45 V) and back to 0 at a rateof 1 V s−1 was applied between the nanowire and the goldelectrode. A 1 G� resistor was connected in series to thenanowire in order to limit the current through the contact and

nanowire to avoid the risk of Joule heating and the meltingof the contact or nanowire. A TEM image sequence of theoperation of the Mo6S3I6 nanowire based on–off device isshown in figures 1(b)–(e).

In this particular case the length l and radius R of thenanowire are 15 μm and 100 nm respectively. The initialdistance between the end of the nanowire and the oppositeelectrode is 3 μm. At such a distance, the gradient of the vander Waals force (around 7.4×10−10 N m−1) is much lower thanthe force constant of the nanowire of 4.8 N m−1 and switchingon of the device can be realized only by applying an electricalfield. The van der Waals force between the sphere and the planesurface is calculated as F = AR/6s2 [32], where A is theHamaker constant (typically 4 × 10−19 J for metals), R is theradius of the nanowire and s is the distance between the end ofthe nanowire and the electrode surface. The force gradient iscalculated as k = dF/ds = AR/2s3.

The elastic force of a cylindrical nanowire (F) isproportional to the spring constant (k) and the deflection ofthe nanowire end from the equilibrium position x : F =−kx . The spring constant of the beam depends on the beamk = 12E I/L3 [33], where E is the Young’s modulus andI = π R4/4 is the cross section moment of inertia.

When the voltage between the nanowire and goldelectrode increases, the nanowire begins to move toward theopposite electrode. When the attractive electrostatic nanowire-electrode force exceeds the elastic force of the nanowire—in this case at 21 V—jump-to-contact occurs (figures 1(c)and (d)). Since the jump is faster than the image acquisitiontime (tenths of milliseconds), the positions of the nanowireboth before and after the jump-to-contact can be seen infigure 1. Simultaneously a jump in current from the systemnoise floor up to 5.5 nA is observed (figure 1(f)). As the appliedvoltage increases to 35 V, the current through the contact andthe nanowire increases almost linearly (figure 1(f)). When the

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Figure 2. TEM images of (a) the Mo6S3I6 nanowire bundle and the counter electrode in initial positions (at zero voltage); (b) the samenanowire bundle in contact with the counter electrode; (c) a higher magnification image of the contact (contact area of 100 nm2); (d) a highermagnification image of the contact (contact area of 45 nm2); (e) I (V ) curve; the jump-to-contact and jump-off-contact events occurred atvoltages 42 V and 5 V accordingly for the contact shown in (c); (f) I (V ) curve; the jump-to-contact and jump-off-contact events occurred atvoltages 42 V and 27.5 V accordingly for the contact shown in (d).

voltage is subsequently decreased back to 0 V, a hysteresisin the I (V ) characteristic is observed because the sum ofadhesive and electrostatic forces is higher than the elastic forceexerted in the nanowire [10, 12, 17]. Jump-off the contact isobserved when the voltage decreases to 1.2 V. At this point theattractive force acting between the nanowire and the electrodebecomes weaker than the elastic force of the bent nanowire andjump-off-contact occurs (figure 1(e)).

In the next step we address the electrical behavior ofthe system in an on–off type of device realized between theAu electrode and the Mo6S3I6 nanowire without the currentlimiting resistor, with different contact geometries and counterelectrode materials. The factors influencing operation of thedevice are: van der Waals, electrostatic, adhesion and elasticforces, which also depend on the contact geometry (area) andthe contact materials. Three requirements must be fulfilled forthe device to operate in the on regime. (1) Before switchingthe device on, it has to be ensured that the gradient of thevan der Waals force is lower than the force constant of thenanowire. This can be achieved by changing the distancebetween the nanowire and the second electrode. (2) The devicewill be turned on only if the sum of electrostatic and van derWaals forces is large enough to force the nanowire to jumpinto contact. To achieve an off regime, the elastic force ofthe nanowire has to exceed the adhesion force in the contactwhen the electrostatic field is reduced. (3) The device will beturned off if the elastic force exceeds the total attractive force(adhesion and electrostatic) at the contact.

A comparison between adhesion and elastic forces wasdone for the device shown in figure 2. As has been shown

previously [34, 35], for nanocontacts the adhesion forces andthe contact area can be described using Maugis theory [36].In our case a simplified fitting version given by Carpicket al [37] was used for the adhesion force Fa calculation:

Fa = F�

a(λ)/(π√

γ1γ2 R), where F�

a(λ) is force expressed indimensionless units, λ is a dimensionless parameter and γ1

and γ2 are the surface energies of two interacting surfaces.R is the effective radius of the two interacting bodies

with radii R1 and R2: 1/R = 1/R1 + 1/R2. F�

a(λ)

was calculated using the empirical equation given in [37]:

F�

a(λ) = (4.04λ1.4 − 1)/(4(4.04λ1.4 + 1)) − 7/4, where λ

can be expressed as λ = (64Rγ1γ2/(9K 2z30))

1/3 and z0 is atypical atomic dimension. The reduced Young’s modulus isgiven by: K = 4/3((1 − ν2

1 )/E1 + (1 − ν22 )/E2)

−1, where ν1

and ν2 are the Poisson’s ratios and E1 and E2 are the Young’smoduli for two contacting spheres. For force calculationswe used the Young’s modulus of the molybdenum–sulfur–iodine molecular wire bundles determined as a function of thebundle diameter [38]. In calculations the values of surfaceenergy of molybdenum sulfide of 0.26 J m−2 [39], Poisson’sratio ν = 0.28 and z0 = 0.28 nm were used. For goldγ = 1.37 J m−2 [36], E = 117 GPa, ν = 0.42 [40] andz0 = 0.28 nm.

Electrostatic force was calculated using the electrostaticforce formula for the sphere plane interaction [41] F =πεε0 RV 2/s where R is the tip radius, V —voltage, s—distance between the end of the nanowire and the electrode andε = 1 was used for the average dielectric constant of the gap.

In the on position, the calculated adhesion force for acontact area of 100 nm2 is around 190 nN, while the elastic

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Figure 3. TEM images of (a) contact between the Mo6S3I6 nanowire bundle and the Mo6S3I6 electrode; (b) contact between the Mo6S3I6

nanowire bundle and the gold electrode.

force of the bent nanowire with diameter of 150 nm and lengthof 3 μm is around 525 nN. The electrostatic force at thejump-off-contact voltage does not exceed 20 nN and can beneglected. Thus, at the off event the retracting elastic force is2.5 times larger than the sum of the attractive adhesion andelectrostatic forces. The reason for such a force differencemay be that the adhesion forces between gold and the Mo6S3I6

nanowire cannot be calculated using classical contact theories,since chemical interaction between gold and sulfur atoms mayhave to be taken into account [42].

The width of hysteresis loops and the operation of theNEMS can be significantly modified by variations in thecontact area. For example, by changing the contact pointon the gold electrode, different contact radii are obtained, asshown under higher magnification in figures 2(c) and (d). Aswe can see from the corresponding I (V ) characteristics infigures 2(e) and (f), the jump-off events are shifted in thisparticular case from 5 to 27.5 V, which we attribute to areduction of the contact area from ∼100 nm2 (figure 2(c)) to∼45 nm2 (figure 2(d)).

Figure 3 shows on–off events between two Mo6S3I6

nanowire bundles with different diameters.In this device the nanowire with lower force constant

jumps to the contact with a stiffer nanowire at 60 V andjumps off at 0 V. In figure 3, Mo6S3I6–Au and Mo6S3I6–Mo6S3I6 contact switches are shown for comparison. Theelastic forces pulling the nanowires apart are found to becomparable for the Mo6S3I6 nanowire bundle in contactwith the Mo6S3I6 and with Au electrodes (7.5 × 10−7 and9 × 10−7 N respectively), but the contact area between twoMo6S3I6 nanowires was approximately nine times larger thanthe nanowire contact to the gold electrode (900 and 100 nm2,respectively). Consequently the adhesion per unit area isan order of magnitude lower for the MoSIx–MoSIx type ofcontact, which implies that smaller operation distances can beachieved with MoSIx–MoSIx NEM switches.

To further evaluate the usability of Mo6S3I6 materialin functional devices, we now investigate the critical

Figure 4. TEM images of Mo6S3I6 nanowire bundle (length 3.3 μm,diameter 50 nm): (a) at excitation frequency 1.6 MHz; (b) atresonance frequency 1.885 MHz. The Young’s modulus of thenanowire bundle is 41.3 GPa.

device parameters: Young’s modulus, resistivity and failureparameters of the nanowire bundles. For force calculationswe tested the applicability of the published Young’s modulusof the nanowire bundles determined as a function of bundlediameter [38]. We tested these data by selective measurementof the resonance frequency of various diameters and lengthsof Mo6S3I6 nanowires. An example of such a measurement isshown in figure 4.

The Young’s moduli of the molecular wire bundles used inour switch applications varied from 40 to 270 GPa and were inagreement with data published in [38].

The resistivities of nanowire bundles in the NEMSconfiguration varied from 1.8 � m (figure 2) to 5.2 � m(figure 1) and the current densities during two-contactcharacterization varied accordingly from 3.2 × 105 to 7 ×106 A m−2. The high intrinsic resistivities of the Mo6S3I6

nanowire devices proves to be an advantage for the applicationof nanowires in bistable NEM devices: in carbon nanotubebased switches the surface of one of the electrodes needs to becoated with an insulator layer [9, 16] or a resistor is added inseries [10] in order to decrease the current through the contact.

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Figure 5. Failure of a Mo6S3I6 nanowire bundle: TEM image of thenanowire before (a) and after breaking at 30 V (b); (c) I (V )characteristic before breaking.

(This figure is in colour only in the electronic version)

In contrast, Mo6S3I6 nanowires can be used in NEM deviceswithout additional insulating coating or external resistor.

Mo6S3I6 nanowire bundles are mechanically morestable than carbon nanotubes in two-terminal switches; themechanical stability is comparable with Ge nanowire [17]based devices. Carbon nanotubes operating at similarconditions were shortened whenever jumping into contactwith the electrode occurred [10], whereas Mo6S3I6 nanowirebundles can be used repeatedly under even harsher conditions(larger jump to the contact distances, higher voltages).

The Mo6S3I6 nanowire bundles withstand high voltagesbut show failure at voltages higher than 25 V. In somecases, nanowires exhibiting high resistivity are breakingat voltages closer to 100 V. Failure currents of Mo6S3I6

nanowires are comparable with failure currents of CNTs andare in the range 3–12 μA (current densities are greater than106 A m−2). Figure 5 shows a typical nanowire bundledisrupted near its middle after excessive current has beenapplied. Similarly to previous observations on CNTs [43],this suggests that these nanowires were resistively heated andthat the temperature becomes locally high enough to vaporizethe nanowire material. As in the case of CNTs, the failuremechanism of Mo6S3I6 appears to be Joule heating and notdefects, which is very important from the applications point ofview.

3. Conclusions

The properties of single Mo6S3I6 nanowire based two-terminalNEMS devices were experimentally investigated by in situ

TEM testing. It was shown that a single MoSI nanowireexhibits inherent bistability and can be effectively used inapplications such as NEM switches and memory elements.While properties are similar to those reported for CNTs,the devices exhibit low friction coefficients at contact andhigher operation voltages. The low adhesion leads tosmaller differences in driving potentials for on–off switching,which leads to smaller energy consumption in high speed,high density devices. From the investigation of the failureparameters we deduce that the MoSI-based NEMS exhibitvery robust operating behavior and their high current failureis related to Joule heating of the bulk nanowire rather thandefects or contact failure. We conclude by commenting onthe feasibility of integration of such switching devices intoCMOS technology. Recent rapid advances in dielectrophoreticpositioning allows arrays of nanowires to be made on top ofexisting architectures on silicon wafers. Such manufacturingtechniques have already been demonstrated, for example,for manufacturing individual field emission tips [44], in thefabrication of nanowire resonator arrays [45] and in thefabrication of NEM switches [12]. The addition of flow-orientation and microfluidic cell technology have alreadydemonstrated on-chip devices. The MoSI wire diameters canbe quite effectively selected by centrifugation [46], particularlyfor single molecular wires. Thus, before this kind of devicecan be incorporated into larger scale electronic processorsor sensors, control over contact reproducibility needs to beinvestigated. The use of controlled gap geometry using singlemolecular wires or thin wires with selected diameters offers thepossibility of studying the contact properties in a systematicfashion.

4. Experimental section

Mo6S3I6 nanowires were produced following the methoddescribed previously [27, 28]. A TEM holder equipped witha scanning probe piezo actuator was used to manipulate thenanowires. The measurement setup is shown in figure 1(a) andis described in detail elsewhere [47, 48]. The nanowires wereglued to the gold tip with conductive epoxy; a second gold tipwas used as a counter electrode. The lengths of the nanowireswere between 3 and 15 μm, and the radii were around 100 nm.The gold tips were fabricated by electrochemical etching at 5 VDC of a gold wire (99.999% pure Au) with diameter 0.25 mmin a hydrochloric acid water solution (a 1:1 mixture of 37.5%HCl and distilled water). The apex radius of conical gold tipsvaried from 20 to 200 nm. Etched tips were washed in distilledwater. To remove any contamination from the gold surface, thetips were etched inside the etching coating system (Gatan) withAr ions at an energy of 6.5 keV at a current of 210 μA for 10 s.

Acknowledgments

This work was supported by the ERAF, Ministry of Educationand Science of Latvia, Council of Science of Latvia and the EUSTREP project DESYGN-IT (No. NMP4-CT-2004-505626).

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