IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 19 (2009) 055002 (9pp) doi:10.1088/0960-1317/19/5/055002

Electrical characterization of suspendedgold nanowire bridges with functionalizedself-assembled monolayers using atop-down fabrication methodZhiwei Zou1, Junhai Kai and Chong H Ahn

MicroSystems and BioMEMS Laboratory, Department of Electrical and Computer Engineering,University of Cincinnati, Cincinnati, OH 45221, USA

E-mail: zouz@ececs.uc.edu

Received 3 December 2008, in final form 11 March 2009Published 15 April 2009Online at stacks.iop.org/JMM/19/055002

AbstractIn this paper, a very simple top-down fabrication method, which is compatible with standardsilicon (Si) fabrication processes, is proposed to fabricate new suspended gold nanowirebridges with flexible designs. The electrical characteristics of the nanowire bridges whichinclude the V–I curve, thermoresistive and impedance spectra change before and afternanowire bridges release and resistivity change with different design parameters are measured.The suspended nanowire bridge structures show the reduction of interference from thesubstrate and a large design flexibility to fit varying application desires. Furthermore, thenanowire bridge has shown a high potential for biomolecular detection by the mechanical,electrical or optical sensing mechanism through the formation of functionalizedself-assembled monolayers (SAMs) on the bridge structure.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

One-dimensional nanostructures such as carbon nanotubes,semiconductor, metal and polymer nanowires exhibit uniqueelectrical, optical and mechanical properties that can beexploited for biochemical sensing [1–9], nanomechanicalactuation [10–14] and nanoelectronics [15–23]. Highlysensitive sensors can be achieved by utilizing a one-dimensional nanostructure due to their extremely high surface-to-volume ratio [1]. Most existing nanowire structuresfor biosensors involve a direct contact with the substrate,while suspended nanotubes and nanowires are more suitablefor exploring the interplay of electronic behavior withmechanical, thermal and chemical properties without theinteraction with the substrate, which can lead to possibleapplications in sensors, transducers and memory elements.The suspended structure is also critical for nanoscale actuatingapplications. As a result, efforts have been made to manipulate

1 Author to whom any correspondence should be addressed.

carbon nanotubes or nanowires over gaps between metalmicroelectrodes as suspended bridge structures [24–35]. Forexample, a carbon nanotube has been synthesized across theinter-electrodes gap as a bridge by pre-patterning growth of thecatalysts island on top of the electrode [24]. Another exampleshows that synthesized gold nanowires (diameter rangingfrom ∼50 to 250 nm) modified with the biomolecular biotinhave been manipulated by dielectrophoresis to span acrossa gap between avidin functionalized gold microelectrodes.The change of electrical response associated with individualnanowire bridges was then characterized [29, 31].

However, the chemically synthesized nanowires ornanotubes have their own limitations to build functionalsuspended bridge structures, such as complex integration,requiring transfer and manipulation of an individualnanostructure, with subsequent ohmic contact formation. Thenanowires have to be connected to electrodes that can interfacewith other circuitry. The mismatch of the nanowires and theconventional electrodes has hindered this adoption. Also, a

0960-1317/09/055002+09$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK

J. Micromech. Microeng. 19 (2009) 055002 Z Zou et al

Nano-micro interface

Au nanowire bridge

Figure 1. Schematic view of the suspended gold nanowire bridgewith SAMs.

low-resistance connection is critical, so that small changes inconductance of the nanowires can be precisely measured andcontact resistance does not reduce the accessible sensitivity[32].

The standard top-down semiconductor process hasadvantages to overcome these limitations for certainapplications. Different methods have been applied basedon standard top-down Si fabrication to make Si nanowiresfor biochemical sensors [4–7] and nanoelectromechanicalresonators [11–13]. The bridging Si nanowire structure isamenable to integrate the nanowire sensing elements withrelated electronics formed by conventional IC technology [32].However, in order to fabricate Si structures with dimensionsdown to the nanoscale, specific silicon-on-insulator (SOI)wafers with a very thin top Si layer (<100 nm) have been usedin these works with carefully controlled complicated thermaldiffusion, thermal oxidation and RIE dry etching process.

By using a modified microfabrication technique,suspended metal nanowire bridge (e.g. gold) structures canbe realized in a more controllable and flexible way. As anexample, gold nanowires with a height of 160 nm, widthof 350 nm and length of 5 μm have been fabricated usingthe top-down method and mechanically characterized (e.g.hardness and elastic modulus) by nanoindentation techniquesusing a nanoindenter [35]. In this work, by using a verysimple top-down fabrication method, we have realized agold nanowire bridge that has a freedom of interaction formechanical, electrical and biochemical characterizations asshown in figure 1. This method is completely compatiblewith the standard Si fabrication process and is more easilyimplemented or integrated into NEMS sensing and actuatingsystems compared with the method of synthesizing gold andSi nanowires. In addition, the top-down fabrication methodprovides a high flexibility to realize various types of suspendednanowire bridge structures in response to changing demands.Furthermore, it is desirable to form self-assembled monolayers(SAMs) on nanostructures to make potential nano-bio hybriddevices. Ti nanopatterns (e.g. dots and lines) fabricated bythe top-down method has been used, for instance, to directthe macromolecular assembly by controlling the deposition of

(a)

(b)

(c)

Figure 2. Fabrication flow of the suspended gold nanowire bridge.

polyelectrolyte layers [36]. In this work, gold was selectedas the structural material due to its excellent physical andchemical stability as well as mechanical strength. Gold isalso well known as one of the most suitable materials forbiomolecular immobilization through the formation of SAMson its surface. Therefore, it is an excellent material for nano-bio hybrid devices [29].

2. Methods

Figure 2 shows the fabrication procedure, which involvesnanofabrication, nano-micro interface fabrication andsuspended nanowire releasing. First, nanofabrication wasperformed using e-beam lithography, e-beam evaporation andlift-off to create nanowire patterns on the Si/SiO2 substrate.Next, microfabrication techniques including photolithography,e-beam evaporation and lift-off were used to build nano-microinterconnects or interfaces which provide electric connections.Then, the nanowire bridge was released by SiO2 isotropicetching in the third step. Finally, the nanowire bridge wasequipped with SAMs using standard methods [37].

2.1. Nanofabrication

The fabrication process started from the nanofabrication ofgold nanowires. A thermal oxidation process was performed

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at 1100 ◦C to grow a 1 μm thick SiO2 layer on normal〈1 0 0〉 p-type Si wafers (substrate resistivity: 1–10 � μm).A 300 nm thick positive electron sensitive resist(polymethylmethacrylate, PMMA) layer was spin-coated onthe wafer at 2500 rpm and baked at 180 ◦C for 2 min.E-beam lithography was performed by using a Raith-150system. After exposure (200 μA cm−2), nano patterns weredeveloped in a solution of 1:3 mixed methylisobutylketone(MIBK) and isopropanol (IPA) for 30 s. The whole processingtime of e-beam lithography was less than 1 h while most timewas spent on patterning the relatively large area of contactpads. A gold metal layer (∼50 nm) and a Ti adhesive layer(∼5 nm) were evaporated on the patterned substrate using ane-beam evaporator. The sample was then dipped into acetonefor lift-off. Nanowires with different widths (down to 50 nm),lengths (up to 50 μm) and shapes (e.g. straight and serpentinewires) were made in this step. Contact pads for nano-microinterfaces were also fabricated.

2.2. Nano-micro interface fabrication

Photolithography was used for nano-micro interface patterningand integration of nanowire with larger electric connectionpads. Positive photoresist (Shipley) 1818 was spin-coatedon the sample surface at 4000 rpm and baked at 90 ◦C for30 min, followed by immersion in chlorobenzene for 45 s.The sample was then exposed to UV light (10 mJ cm−2) for10 s and developed in the Microposite R© 351 developer for1 min. Another gold layer (∼200 nm) and Ti adhesive layer(∼5 nm) were deposited using the e-beam evaporator. In thisstep, the second gold layer covered the existing contact padsmade during nanofabrication. The sample was then dippedinto acetone for lift-off.

2.3. Nanowire bridge structure release

In this step, because of the isotropic etching properties of SiO2

wet etching, the nanowire bridge was released by undercutetching of SiO2 under the nanowire. Shipley 1818 photoresistwas patterned again by photolithography, only leaving the areaaround the nanowire open for etching. Buffered oxide etchant(BOE) with the 0.1 μm min−1 etching rate was used to etchthe SiO2 under the nanowire for 15 min at room temperature.Subsequently, Si etchant (tetramethylammonium hydroxide,TMAH) was used at 70 ◦C to further increase the depth of thetrench if necessary.

2.4. SAM formation

Gold is well known as one of the most suitable materials tobe equipped with SAMs of the alkanethiols group [35]. Toguide the further development as a nano-bio hybrid device,it is useful to consider the surface functionalization of theproposed suspended gold nanowire bridge. A method to forma SAM layer on the gold nano-bio hybrid device was utilizedand reported in our group before [38]. The chip was cleanedusing the piranha solution and oxygen plasma first to removecontaminations on the gold surface. The sample was rinsed byDI water thoroughly before SAM preparation. After cleaning,

the sample was preconditioned by being immersed in a 10 mMethanolic solution of mixing 11-mercaptoundecanoic acid(MUA) and 3-mercaptopropionic acid (MPA) at a volume ratio1:10 for 16 h at room temperature. After preconditioning, thesample surface was completely rinsed by ethanol to removeany unattached species. The sample was then activated inan aqueous solution of 75 mM 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDAC) and 15 mM n-hydroxysuccinimide(NHS) for 10 min at room temperature and washed by a PBSbuffer solution and dried by nitrogen.

3. Results and discussion

3.1. Fabrication results

Suspended gold nanowire bridges have been successfullyfabricated with a very good yield and used for the furthercharacterization. Structures with thickness down to 50 nmand width down to 75 nm demonstrate no obvious structurefailure caused by the residual stress after release. Severalmethods such as SEM and AFM have been employed todemonstrate the fabrication result of the suspended goldnanowire bridge. Figure 3 illustrates the entire chip with thenano-micro interface and nanowires. A gold nanowire bridgewith 100 nm width and 10 μm length before releasing is clearlyshown by the AFM in figure 4. A SEM image (figure 5)shows a serpentine gold nanowire bridge with a width of250 nm and length of 5 μm before releasing. The SEM imagesof figure 6 show released suspended nanowire bridges withdifferent designs: (a) straight nanowire: 100 nm width, 10 μmlength; (b) straight nanowire with a central platform:200 nm width, 20 μm length for the wire and 2 μm × 2 μmfor the central platform; (c) serpentine nanowire with a centralplatform: 200 nm width, 20 μm length for the wire, and 2 μm ×1 μm for the central platform; and (d) straight nanowire array:200 nm width, 10 μm length for each bridge for a total of tenbridges with 10 μm spacing.

One of the most important aspects of this work isthe flexibility in design. Since the microfabrication andnanofabrication process steps are performed independently,one can design any nanostructure desired in thenanofabrication process and still use the same nano-microinterface scheme. This enables the fabrication and testingof many different nanowire bridge structures in parallel, thusreducing cycle times during prototype design [39].

3.2. Electrical characterizations

The electrical characteristics of the gold nanowire bridge havebeen determined. Using electrical measurement, it is possibleto characterize the electrical properties with different designsto guide further development and to use electrical responseto characterize the physical and chemical properties of thenano-bio hybrid system for future use as sensors [29].

In order to reduce the coupled parasitic resistance andcapacitance, the microprobe station was used to perform themeasurement. In addition, the total resistance of the nano-micro interface and contact pads is less than 10% of theresistance of the nanowire due to the much larger width

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500 μm

20 μm

Nanowire

Figure 3. Microscopic images of the entire chip and nano-micro structures.

(a) (b)

μM

μM

Figure 4. AFM images: (a) 2D and (b) 3D AFM images of the gold nanowire bridge before releasing.

and thickness. Therefore, the measured resistance can beconsidered as the resistance of the nanowire. Figure 7shows the voltage–current (V–I) characteristics of the nanowirebridge at room temperature using the Agilent 6611C dcsystem power supply and 34401A digital multimeter. TheV–I characteristics of the nanowire bridge have expected linearrelation; thus, ohmic contact was achieved.

The suspended bridge structures benefit from theelimination of the mechanical, electrical and thermalinteraction with the substrate. Resistance changes ofnanowires were measured as substrate temperatures werevaried. After nanowire bridge release, the heat transfer fromthe heated substrate to the nanowire is reduced as shown infigure 8(a). As the substrate temperature increases, both theresistance of the isolated nanowire bridge and the slope of theresistance changes become lower than those for the unreleasednanowire, which indicates that both the actual temperatureand temperature change of the suspended nanowire are lower

than those of the substrate due to the thermal isolation afterrelease.

In addition to the thermal effect on resistance change,impedance spectra were also used to investigate the effectof coupled parasitic impedance elements from the substrate(figure 8(b)) using a Agilent 4284A precision LCR meter(bandwidth of 20 Hz to 1 MHz). A constant voltage sourceof 20 mV was typically used. The impedance measurementwas carried out using the LCR meter in its parallelcapacitance mode based on the simplified model coupling ofa parallel capacitance (C) with the nanowire resistance (R) byconsidering the relative impedance magnitude of the reactance.The parasitic capacitance starts to have an effect above 100 kHzand can be introduced as seen in the inset of figure 8(b). Thus,this simplified electrical model can be presented as

|Z| =∣∣∣∣R · j(1/2πf C)

R + j(1/2πf C)

∣∣∣∣ = R√

(2πf C · R)2 + 1. (1)

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Figure 5. SEM pictures of the serpentine gold nanowire bridge before releasing.

(d)

20 μm

10 μm

100 nm 200 nm

20 μm

10 μm

200 nm

(a) (b)

(c) (d)

Figure 6. SEM pictures of suspended nanowire bridges with different designs after releasing.

It has also been found that the effect of the coupledcapacitance is reduced after the nanowire bridge is released(figure 8(b)). Parallel capacitance can be derived usingfitting curves from the electrical model and equation (1).Resistance and capacitance were found to be 63.5 � and1 × 10−10 F, respectively. This result suggests that theparallel capacitance is not only simply attributed to thecapacitance of the SiO2 underneath the estimated nanowirecontact area, but also probably due to a large contactcapacitance from the contact pads [28]. Nevertheless, thissimplified model predicts the change of impedance spectraafter the nanowire release, which introduces the decrease inthe parallel capacitance caused by etching the SiO2 layer.

Future study will include a more detailed electrical modelsuch as the nano-transmission wire model [15] in orderto gain a full understanding of the nanowire’s electricalresponse. At this stage, these preliminary results suggestfuture improvements in the electrical characteristics of thesuspended gold nanowire bridge structure by modulatingthermal and electrical interference from the substrate.

The relationship between the different design parametersand the nanowire bridge properties is necessary. The effect ofdesign parameters such as nanowire length, width and numberof nanowires in an array of electrical properties (i.e. resistance)is shown in figure 9.

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-60 -40 -20 0 20 40 60

-0.8

-0.4

0.0

0.4

0.8

Cu

rre

nt

(mA

)

Voltage (mV)

R2=0.9998

Figure 7. Plot of the V–I characteristics of a suspended goldnanowire bridge. Calculated resistance is 63.7 � and thedimensions of the gold nanowire bridge are 30 μm (L), 200 nm (W)and 100 nm (H).

10 100 1k 10k 100k 1M61

62

63

64

65 Before release

Fitting curve

After release

Imp

ed

an

ce

(Ω

)

Frequency (Hz)

Si

AuR

C SiO2

20 30 40 50 60 70

64

66

68

70

72

0.0017 K-1, R

2=0.9886

Before release

After release

Resis

tance (

Ω)

Substrate temperature (οC)

0.002 K-1, R

2=0.9918

(b)

(a)

Figure 8. Plot of the electrical characteristics change of nanowirebridges before and after release: (a) nanowire resistance changeversus substrate temperature change and (b) impedance spectrachange at different frequency ranges. (Inset) A simplified model toexplain the impedance change. Fitting curves from this modelbefore and after the nanowire release. R is 63.55 � and C is 1 ×10−10 F for the fitting curve. The dimensions of the gold nanowirebridge are 30 μm (L), 200 nm (W) and 100 nm (H).

0 10 20 30 40 500

20

40

60

80

100

Resistance

Resistivity

Length (μm)

Re

sis

tance (

Ω)

0

2

4

6

8

Resis

tivity

(μΩ-c

m)

50 100 150 200 250 300 3500

20

40

60

Resistance

Resistivity

W idth (nm)

Resis

tance (

Ω)

0

2

4

6

8

Resis

tivity

( μΩ-c

m)

0 2 4 6 8 10

50

100

150

200

250

300

Resis

tance (

Ω)

Number of nanowires in the array

(c)

(b)

(a)

Figure 9. Plots of the resistance change versus (a) nanowire lengthchange, and the width is kept at 300 nm and the thickness is 100 nm;(b) nanowire width change, and the length is kept at 5 μm and thethickness is 100 nm; and (c) the different number of nanowires inthe array. The dimensions of the gold nanowire bridge are 150 μm(L), 300 nm (W) and 100 nm (H). (Serpentine nanowire bridges havebeen used in this testing and the effective length is three times of thegap width, i.e. 50 μm.)

Figure 9(a) shows how resistance and resistivity changewhen only the length of the nanowire bridge varies. Theresistivity (∼6 μ� cm−1) of the suspended gold nanowireswith different lengths is consistently several times larger thanthat of the bulk gold (∼2.4 μ� cm−1). Several theories couldbe applied to explain this phenomenon. First, the resistivityof the nanowire increases when one or more dimensions ofthe nanowire become comparable with the electronic meanfree path (i.e. gold: 40 nm). This effect is attributed tothe scattering at the nanowire surface boundaries and grainboundaries [40, 41]. Although the boundary scattering effectsare more obvious when at least one of the dimensions is less

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75o 50o

Clean SAM SAM+IgG0

20

40

60

80

67

50

Co

nta

ct a

ngle

(d

eg

)

75

67o

Figure 10. Contact angle changes after the SAM formation and protein (mouse anti-rabbit IgG) binding on the gold surface.

than 50 nm, a recent report shows that it is still possible to havea larger grain boundary scattering even with grains that are100 nm in diameter. A theory named ‘surface contaminationeffects’ has also been introduced in this paper [41]. It is knownthat a thin layer of contaminant organic molecules will formspontaneously on clean gold nanowire surfaces when exposedto ambient conditions. An insulating effect occurs from thesurface contamination where electron flow is forced to tunnelthrough the wire instead of propagating on the surface. Thismay cause a larger grain boundary scattering and may alsolead to increased electron scattering at the surface. Thiseffect clearly becomes significant as the surface-to-volumeratio increases [42, 43].

As for the nanowire’s width change (figure 9(b)), whenthe width of the gold nanowire is reduced to around 100 nm,which is close to the gold mean free path and grain size, theresistivity increases as the width is further decreased. Thiscan also be explained by the boundary scattering effect andthe surface contamination effect, which play a more importantrole when the surface-to-volume ratio of the nanowire structurebecomes very large at the nanoscale [42, 43]. These resultsmatch the existing literature [40–42, 44] indicating that thisdesign can be pursued further.

We also looked at the resistance for arrays of nanowirebridges in figure 9(c). All the nanowires in the array areidentical in design, and the resistance of the nanowire bridgearray is normalized to a single wire. It is shown that theresistance change is not linearly related to the number ofnanowires in the array, which is different from the bulkgold wires, so adding more parallel nanowire bridges cannotbe simply considered as increasing the cross-section area.For the array with more parallel nanowire bridges, moresurfaces are exposed. Thus, the surface scattering and surfacecontamination effects become larger in this case.

3.3. Characterization of SAMs

Another advantage of this suspended gold nanowire bridgeis that it is easily equipped with SAMs of the thiols groupon its surface. Thiol groups are very good at covalentlyimmobilizing DNA or proteins of interest for biomolecularanalysis application. Therefore, nanowire bridges can be usedto sense the presence of biomolecular.

Contact angle measurement and AFM have beenutilized for the recognition of SAM formation and proteinimmobilization. For the contact angle measurement, a dropof DI wafer was applied to the gold surface and the contactangle measurement was performed within 1 min of contact.Figure 10 shows the contact angle change before and afterSAM formation and protein (mouse anti-rabbit IgG) bindingusing a FTA125 contact angle analyzer (First Ten Angstroms,VA, USA). These data correlate well with the previous work[37] and the contact angle change corresponds to the surfaceproperty change before and after the immobilizations. AFMimages (Nanosurf R© easyScan 2 AFM) taken in a tapping modehave also been used for the characterization of biomolecularimmobilization at the solid surface [37]. In this research,figure 11 illustrates the change of gold surface morphologyafter protein binding through SAMs in the 1 μm × 1 μmAFM scanning area. The protein (mouse anti-rabbit IgG)immobilization on the gold surface can be clearly seen infigure 11(b) in comparison with the clean gold surface infigure 11(a). Covalent binding of the protein layer to thealkanethiols chain could provide a very stable surface forconstruction of biosensors. The functionalization of thesurface of the gold nanowire bridge presents potential ofthe nano-bio hybrid system and nanoscaled bio-electronicsdevice which could convert biological binding informationinto electrical signals [29].

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(a)

(b)

Figure 11. AFM images: (a) before and (b) after SAM formationand protein binding on the gold surface.

After successful fabrication and electrical characterizationof the suspended gold nanowire bridge structure, furtherresearch is ongoing to optimize the design of the structureand to achieve the biomolecular sensing through the electricalproperty changes such as frequency response, and resistanceand conductance changes after the biomolecular binding.

4. Conclusions

In conclusion, suspended gold nanowire bridges have beenfabricated and electrically characterized in this work. Thegold nanowire bridge has been realized by a very simple top-down nanofabrication method and shows unique properties.Specifically, this method is fully compatible with standard Sifabrication processes. In addition, this method ensures thefreedom of mechanical, thermal and electrical interaction andthe flexibility in the design to allow different nanowire bridgestructures. Furthermore, the gold nanowire bridge that wasequipped with functionalized SAMs has shown the naturalproperties desirable for chemical and biological applications.

Acknowledgments

This work has been partly supported by National ScienceFoundation (NSF) grant no 0216374 for the Raith-150e-beam lithography system in the University of Cincinnati.The authors gratefully thank Mr Ron Flenniken, Mr RobertJones and Mr Jeff Simkins of the Institute for NanoscaleScience and Technology at the University of Cincinnati fortheir technical support.

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