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16496   DOI: 10.1021/la101385e   Langmuir 2010, 26(21),  16496–16502Published on Web 07/02/2010

pubs.acs.org/Langmuir

©2010 American Chemical Society

Low-Temperature Large-Scale Synthesis and Electrical Testingof Ultralong Copper Nanowires†

Melinda Mohl, Peter Pusztai, Akos Kukovecz, and Zoltan Konya*

Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. ter 1.,6720 Szeged, Hungary

Jarmo Kukkola and Krisztian Kordas

Microelectronics and Materials Physics Laboratories, University of Oulu, PL 4500 FIN-90014 Oulu, Finland 

Robert Vajtai and Pulickel M. Ajayan

Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas 77251

Received April 7, 2010. Revised Manuscript Received May 21, 2010

Copper nanowires (NWs) with uniform diameters and lengths ranging from several hundreds of nanometers toseveral micrometers have been prepared with high yield by a simple hydrothermal procedure. The X-ray diffraction(XRD) and energy dispersive X-ray spectroscopy (EDS) analysis data indicate that the copper nanowires are free of any contamination, while the electron diffraction (ED) analysis has revealed the nanowires to be single crystals. Thenanowire growth mechanism has also been discussed. Hexadecylamine is the surface stabilizing agent in our method,while glucose facilitates formation of single-crystalline seeds on which the copper nanowires grow. The electricalproperties of the as-synthesized copper NWs have also been investigated.

Introduction

Copper is probably the most frequently used metallic nano-wire. Lately, one-dimensional (1D) nanocopper received out-standing attention because of its high electrical conductance.This property makes it especially sought after by the microelec-

tronics industry.

1

Being used in a broad range of applicationsstarting from electric circuits to electric power leads, 1D nano-copper offers truly unique electrical and thermal properties.1

Typical fields of intensive use include devices constructed fromnanosize electronic, optoelectronic, and magnetic building ele-ments. Somorjai and co-workers have pioneered research on theheterogeneous catalytic application potential of copper nano-structures of controlled morphology and oxidation state.2,3 Theirmost important results are related to uncovering the details of surface chemistry on copper4 and to the bridging the gap betweenmodel catalysts and real heterogeneous catalytic systems.5

Today, it is generally agreed that the original intrinsic proper-ties of metal nanostructures are mostly determined by their size,shape, composition, structure, and crystallinity.6 Intensive re-

search has been ongoing for nanostructures of well-defined shapeandstructure, since size and shape influencethe physical and chemi-cal properties significantly. With a few exceptions (e.g., vacuum

vapor deposition,7 etc.), the controlled nanowire synthesisis gene-rally performed by growing crystals in the presence of physicaltemplates or capping agents selectively adhering to crystallinefacets such as surfactants and/or organic polymers. Applicationsrequire the nanowires to have uniform size distribution and to befree of defects. Therefore, a facile synthesis approach capable of 

producing large quantities of nanowires with tunable aspect ratiois in great demand.

There are a number of methods for preparing copper nano-structures: chemical vapor deposition,8 electrochemical deposi-tion,9,10 template or membrane processes,11-13 reverse micellarsystems,14,15 andso on.Probablythe most widelyused methodforthe preparation of Cu nanocrystals is the template-directed elec-trochemical method.16,17 It is easy to perform, and the morpho-logy and crystallinity of nanowires (NWs) are relatively easilyadjustable.18 However,the method consists of several steps,and itis generally only applicable for producing very small amounts of nanowires.

† Part of the Molecular Surface Chemistry and Its Applications specialissue.

*To whom correspondence should be addressed. E-mail: konya@chem.u-szeged.hu. Telephone: þ36-62-544-620. Fax: þ36-62-544-619.

(1) Azarian, A.; Irajizad, A.; Dolati, A.  Condens. Matter 2007, 19, 446007.

(2) Jernigan, G. G.; Somorjai, G. A. J. Catal.  1994, 147 , 567 – 577.

(3) Zhang, Y. W.; Huang, W. Y.; Habas, S. E.; Kuhn, J. N.; Grass, M. E.;Yamada, Y.; Yang, P.; Somorjai, G. A. J. Phys. Chem. C  2008, 112, 12092 – 12095.

(4) Fu, S. S.; Somorjai, G. A. J. Phys. Chem. 1992, 96, 4542 – 4549.

(5) Somorjai, G. A.; York, R. L.; Butcher, D.; Park, J. Y.  Phys. Chem. Chem.Phys. 2007, 9, 3500 – 3513.

(6) Somorjai, G. A.; Tao, F.; Park, J. Y. Top. Catal. 2008, 47 , 1 – 14.

(7) Liu, Z.; Bando, Y.  Adv. Mater. 2003, 15, 303 – 305.

(8) Choi, H.; Park, S.  J. Am. Chem. Soc.  2004, 126, 6248 – 

6249.(9) Toimil Molares, M. E.; H€ohberger, E. M.; Schaeflein, Ch.; Blick, R.;

Neumann, H. R.; Trautmann, C. Appl. Phys. Lett.  2003, 82, 2139 – 2141.

(10) Toimil Molares, M. E.; Buschmann, V.; Dobrev, D.; Neumann, R.; Scholz,R.; Schuchert, I. U.; Vetter, J.  Adv. Mater. 2001, 13, 62 – 65.

(11) Gerein, N. J.; Haber, J. A. J. Phys. Chem. B  2005, 109, 17372 – 17385.

(12) Liu, Z.; Yang, Y.; Liang, J.; Hu, Z.; Li, S.; Peng,S.; Qian, Y. J. Phys. Chem.B 2003, 107 , 12658 – 12661.

(13) Yen, M.; Chiu,C.; Hsia,C.; Chen,F.; Kai, J.;Lee,C.; Chiu, H.Adv.Mater.2003, 15, 235 – 237.

(14) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B  2000, 104, 5865 – 5868.

(15) Pileni, M. Nat. Mater. 2003, 2, 145 – 150.

(16) Dobrev, D.; Vetter, J.; Angert, N.; Neumann, R. Appl.Phys.A: Mater. Sci.Process. 1999, 69, 233 – 237.

(17) Gao, T.; Meng, G. W.; Zhang, J.; Wang, Y. W.; Liang, C. H.; Fan, J. C.;Zhang, L. D. Appl. Phys. A: Mater. Sci. Process. 2001, 73, 251 – 254.

(18) Tian, M.; Wang, J.; Kurtz, J.; Mallouk, T. E.; Chan, M. H. W.  Nano Lett.2003, 3, 919 – 923.

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Mohl et al. Article

In 1996, Setlur et al. described an intriguing novel method of producing large quantities of carbon nanotubes and encapsulatedcopper nanowires in a hydrogen arc.19 At the same time a newmethod has been presented by Filankembo and Pileni whereincopperNWs areproducedin a micelle template system((AOT)2-

isooctane-water system) andthe shape of coppernanocrystal canbe adjusted/controlled by the addition of different salts while thesame template remains unchanged.14

Presumably, hydrothermal methods seem to be one of the

simplest ways for the production of metal nanowires in a largescale. Liu et al., who were the first among those who applied theCu(II)-glycerol complex in the presence of phosphite (HPO3

2-)and SDBS, successfully prepared some 10 μm long copper nano-wires at 393 K by a hydrothermal method. In the course of thesynthesis, SDBS acts as a capping agent.12 A few years later,Zheng et al. prepared porous copper nanowires with rectangularcross sections in a one-step hydrothermal synthesis using ascorbicacid as reducing agent and pvp as capping agent at relatively lowtemperature.20,21 While Wang et al.22 prepared ultralong sub-micrometer nanowiresby applyingascorbicacid, Shi et al.23 usingoctadecylamine similarly succeeded in obtaining copper nano-wires with a large aspect ratio. In both cases, the added reducingagent acted as the capping agent as well. At the same time Deng

et al.24 elaborated a simple hydrothermal method to produce inone step copper core/carbon sheath nanocables. These copper@carbon nanocables are formed in the presence of cetyltrimethyl-ammonium bromide (CTAB) by a hydrothermal reduction/carbonization step. In the hydrothermal process, CTAB plays thekey role as a structure-directing agent.

Contrary to several publications, which detail possible elec-trical applications, available for the production of copper nano-wires by a hydrothermal way, the determination of electricalproperties of coppernanostructures is rather far from complete inthe relevant literature. So far, determination of current-voltage(I -V ) characteristics of copper nanowires prepared by electro-deposition9,25 have been performed.

In this paper, we describe an effective protocol for preparing

single-crystalline ultralong copper nanowires in a large scale. Inthe new hydrothermal method, glucose reduces copper in thepresence of hexadecylamine (HDA). In order to explore the roleof glucose and HDA in the synthesis, different amounts wereadded. Surprisingly enough, varying the amount of glucose had adramatic effect on the shape of the copper nanowires. Further-more, the effects of reaction time on the products were studiedas well. In order to exploit the electrical properties of coppernanowires, we aimed to synthesize copper nanowires with uni-form diameter and single-crystalline nature. Current-voltage,resistance-gate voltage, and resistance-temperature characteri-stics in different gas environments have been studied.

Experimental Section

Chemicals.   Reagent-grade chemicals copper(II) chloride(CuCl2 3 2H2O Reanal) puriss,   D-glucose (C6H12O6 3 H2O, Re-anal), hexadecylamine (C16H35N, Sigma-Aldrich, Tech 90%,),

n-hexane (C6H14, Sigma-Aldrich, 96.5%), and ethanol abs.(C2H6O, Molar Chemical, 99,99%) were used as without furtherpurification.

1. Synthesis Process.   In the course of a typical prepara-tion procedure, copper chloride (0.17 g, 12.5 mM) and glucose(0.391 g,∼2 mmol) were dissolvedin 80 mL of distilledwaterin anErlenmeyer flask. Thenhexadecylamine(1.44 g) was slowly addedto the water solution followed by vigorous mixing for 5 h bymagnetic stirrer until a light blue emulsion was obtained. Theemulsion was placed in a Teflon-lined stainless steel autoclave of 

200mL capacity.The autoclavewas rotatedat 393K for 2 h underautogenous pressure and then allowed to cool to room tempera-ture. The resulting reddish brown (foxy color) solution wascentrifuged (2000 rpm) and then washed with deionized water,n-hexane, and ethanol sequentially. The process was repeatedseveral times to remove the excess surfactant, and a reddish fluffysolid was obtained. The product was keptundern-hexane to avoidthe oxidation of the copper NWs.

2. Characterization. The morphology and the progress of thesynthesis were assessed by transmission electron microscopy(TEM; Philips CM10). Samples were sonicated in ethanol beforebeing dropped on a copper mounted holey carbon film and dried.The statistical characterization of the diameter size distributionof each sample was performed based on the TEM images. Themorphology and elemental composition of the prepared copper

nanowires were characterized by scanning electron microscopy(SEM; Hitachi S-4700) and energy dispersive X-ray spectroscopy(EDS), respectively. Samples were dropped on the surface of apiece of silicon wafer which was attached to the aluminum sampleholder. The powder X-ray diffraction (XRD) patterns wereobtained from powder samples mounted on glass slides in aRigaku Miniflex II XRD instrument operating with Cu KR

radiation ( λ = 1.5406 A ̊).3. Electrical Measurements.   For the electrical measure-

ments, copper nanowires were dispersed in ethanol, sonicated,and drop-casted on a Si/SiO2 chip equipped with Pt/Ti electrodes(15   μm spacing). All the current-temperature and current-voltage measurements were performed in a Linkam THMS600heating and freezing stage applying 6 K/min heating and coolingrates. A Keithley 2636A sourcemeter was used in all resistance-

temperature, current-voltage, and resistance-gate voltage mea-surements. The samples were investigated in the 150-625 Ktemperature range under different gas atmospheres (N2, syntheticair, H2/Ar, at 1 bar pressure). The change of electric propertiesduring a transition of metallic copper to Cu(I)2O and Cu(II)Osemiconductor was also assessed.

Results and Discussion

The synthesis resulted in a reddish brown suspension contain-ing both nanowires and nanoparticles. After centrifugation, areddish brown solid precipitate was obtained. For the separationof nanowires from nanoparticles, a rotation speed of 2000 rpm 26

was critical during centrifugation. The yield by weight after dryingwas about 50%, indicating a significant material loss that pre-

sumably happened in the washing phase.1. Phase, Morphology, and Structure Characterization

of the Products. A characteristic TEM image of a single nano-wire (Figure 1) clearly reveals the nanowire morphology. Electrondiffraction analysis gave evidence of the single crystalline natureof the as-synthesized nanomaterial. The morphology and dimen-sion of theas-preparedproducts were also examinedby SEM. It isshown a SEM image (Figure 1, left) that the final product iscomposed of a large quantity of Cu nanowires having a uniformdiameter of 64  (  8 nm and length of a few micrometers, thus

(19) Setlur,A. A.;Lauerhaas,J. M.;Dai,J. Y.;Chang,R. P. H.Appl.Phys. Lett.1996,  69, 345.

(20) Zhang, X.; Zhang, D.; Ni, X.; Zheng, H. Solid State Commun.  2006, 139,

412.

(21) Zhang, X.; Zhang, D.; Ni, X.; Zheng, H. Chem. Lett. 2006, 35, 1142.

(22) Wang, W.; Li, G.; Zhang, Z.  J. Cryst. Growth  2007, 299, 158 – 164.

(23) Shi, Y.; Li, H.; Chen, L.; Huang, X.   Sci. Technol. Adv. Mater.  2005,  6,

761 – 765.

(24) Deng, B.; Xu, A.; Chen, G.; Song, R.; Chen, L. J. Phys. Chem. B 2006, 110,

11711.

(25) Cao, H.;Wang, L.;Qiu,Y.; Zhang,L. Nanotechnology2006, 17 , 1736 – 1739.

(26) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y.  Chem. Mater. 2002,

14, 4736 – 4745.

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Article Mohl et al.

featuring an aspect ratio above 50. Consequently, the methodmentioned above is applicable for the production of uniform CuNWs in bulk quantities. As shown in the images, the Cu NWs

produced in our lab are mostly straight; nevertheless, some of them do have angular and bending sections as shown in the TEMimage (Figure 1, right). We tentatively assume that the angula-rities are brought about by the simultaneous growth of differentfacets of the seed nanoparticles. Even though the ED patternsgave evidence for the copper nanowires to be single crystalline innature (inset in Figure 1), the straight sections of NWs showangularities andother forms as shown in Figure 1. Thus, based onTEM images, it is assumed that straight and angular NWs arecomposed of micrometer long single crystals or assemblies of such single crystals, respectively. More details will be given below.The chemical composition of nanowires has been investigated byEDS. A typical EDS spectrum shows presence of Al (1.48 keV), Si(1.74 keV), and Cu (8.07 and 8.92 keV) lines. The presence of theCu KR   line in the EDS spectrum (Figure 1) confirms theformation of Cu nanowires, whereas the Si and Al lines originatefrom the silicon substrate and aluminum sample holder, respec-tively.

Figure 2 depicts the XRD pattern of copper nanowire filmsdrop-casted on a glass substrate. Copper nanowires were identi-fied on the basis of the three clearly distinguishable diffractionpeaks at 2θ = 43.3, 50.4, and 74.2 corresponding to the {111},{200}, and {220} crystal planes of face-centered cubic (fcc)copper, respectively. The lattice constant of this cubic phasewas found to be  a0 = 3.619 A ˚ which agrees well with the litera-ture12 value of a0 = 3.615 A ˚ . In the X-ray diffractograms, neitherany peak proving presence of contaminating species nor that of 

CuO appeared. Peaks related to potential contaminating speciesor CuO were absent from the XRD profile, indicating that puremetallic copper products were obtained. However, the color of 

copper nanowires turned from foxy to drab during prolongedstorage in air. This phenomenon could be avoided by storing theCu NWs under ethanol. XRD analysis of the discolored productsverified the presence of oxidized nanowires as shown in Figure 2,left: a minor peak appeared at 36.5 which can be attributed to the{111} crystal plane of Cu2O ascribed to the oxidized surface layerof thenanowires. It is likely that the oxygendissolved in thewaterused in the HDA removal step initiates the formation of a thinCu2O layer on the surface of the nanocrystal. In summary, theXRD peak positions were in good agreement with the valuesreported in the literature and confirmed the crystalline nature of copper nanowires.

2. Growth Mechanism of the Cu Nanowires. To investi-gate the influence of the amount of the surfactant (HDA) and thereducing agent (glucose) on the morphology of the final product,copper nanowires with different molar ratios were prepared byvarying the amount of HDA or glucose while keeping theamounts of water and CuCl2*2H2O fixed.

a. Amount of Glucose. At the molar ratio of glucose/copper1:1, mostly nanowires were formed. Copper particles of othermorphologies were practically absent from this product (Figure 3,sample B). Upon increasing the ratio of glucose by a factor of 1.5-2, more and more nanoparticles of diverse shapes and sizesappeared in the SEM images (Figure 3, sample C and D). At theratio of glucos/copper 4:1, the product consisted almost exclu-sively of 50-300 nm nanoparticles and hardly any nanowireswere formed. Supposedly, at high glucose contents, only a few

Figure 1.   Copper NW products of a 24 h synthesis investigated by SEM (left) and TEM (right).

Figure 2.   Left panel: Powder XRD patterns of copper nanowires after synthesis (bottom curve) and copper nanowires exposed to air (topcurve). Right panel: d  spacing versus 1/M  based on XRD measurements.

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seeds are active in wire growth. Decreasing the amount of glucoseto 1/10 yielded exclusively nanowires; however, a blue suspensionwasformed with a little amount of reddish-brown precipitate. It isassumed that in this case only a small amount of copper wasreduced during synthesis. The SEM image (Figure 3, sample A)clearly reveals the resulting crystal defects of these wires.

b. Influence of the Ratio of HDA to CuCl 2  3 H 2O.   Themorphology and the aspect ratio of the nanowires were foundto be a complex function of the molar ratio of HDA to copperchloride as evidenced by SEM. (Figure SI-1 in the SupportingInformation depicts copper nanowires prepared in the presenceof various amounts of HDA while keeping all other synthesisparameters constant. Samples A, B, C, and D correspond to50%, 75%, 125%, and 150% of the original HDA amount, res-pectively.) It is assumed that increasing the amount of HDAmight cause too high surfactant coverage on the initial NW seedswhereas an insufficient HDA amount is unable to effectivelypassivate the required faces.26 Both cases result in a ratherisotropic growth.

c. Time-Dependent Analysis.  Since the growth mechanismof copper nanowires is not yet fully understood, hydrothermalsyntheses of various durations were performed in order to gaininsight into the growth kinetics of copper nanowires. The pro-ducts were investigated by TEM, SEM, and XRD. Productsobtained after 1-72 h hydrothermaltreatment at 393K exhibitedalmost identical morphologies, indicating that synthesis time isnot a critical factor under these experimental conditions.

d. Suggested Growth Mechanism.  The key parameters de-termining the final shapes of nanocrystals are known to be thecrystallographic structure of the seeds in the nucleation processand in the following stages of growth.27 The color of the productCu NW suspensions was bluish green after 1-2 h synthesis time

yet turned into reddish brown with increasing synthesis time(Supporting Information Figure SI-2). TEM images of samplessynthesized for 1, 3, and 6 h are shown in Figure 4. In the case of 1 h synthesis, the samples were mostly composed of 2.1 ( 0.7 nmcopper nanoparticles and hardly any nanowires appeared. At thisearly stage, the HDA is observable as planes in the TEM images.Whereas in the case of 2 and 3 h samples, some copper nanowiresshow up and the structure ofHDA appears tobe coiled23,28 ratherthan planar. SEM images (Supporting Information Figure SI-3)confirmed the coiled structure of HDA wrapped around thecopper nanowires. The outer diameter of the whole structurewas 214  ( 39 nm, and the wrapped nanowires measured 31  (

7 nm in diameter. Consequently, the growth of copper nanowiresis supposed to be initiated by the formation of the coppernanoparticles, that is, single-crystalline seeds appearing at theearly stage of growth. HDA plays an essential role in thenucleation process. The image of a sample containing either noHDA (B) or no glucose (C) is shown in Supporting InformationFigure SI-2. The color of the mixture indicates that no nanowiresareformed in theabsence of glucose. We suggest that glucose, as areducing agent, plays a significant role in the synthesis. It isresponsible for bringing about the seeds, that is, the nanoparticlesfrom which the nanowires grow at later stages. In the absence of HDA, a light yellow suspension is formed that yields a whiteprecipitate after 1 day. No copper nanowires were identified inthis sample. In the complex-surfactant-assisted hydrothermalmethod presented by Liu et al.,12 the role of HDA is like thatof SDBS. First of all, it hinders the agglomeration of coppernanoparticles at the early synthesis stages. Moreover, the singlecrystalline nature of Cu NWs is also attributed to the preferentialadsorption of HDA on selected crystallographic facets of copper

Figure 3.   SEMimages of copper nanowires synthesized with the following glucose/copper salt ratios: (A) 0.2:1, (B) 1:1, (C) 3:1, and (D) 4:1.

(27) Wiley, B.; Herricks, T.; Sun, Y.; Xia, Y. Nano Lett. 2004, 4, 1733 – 1739.

(28) Li, Y.; Li, X.; Deng, Z.; Zhou, B.; F an, S.; Wang, J.; Sun, X Angew. Chem.,Int. Ed. 2002, 41, 333 – 335.

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Article Mohl et al.

nanoparticles. Even though ED results indicated that the nano-wires are single crystals, they may contain angular and bendingsections as well. This apparent contradiction can be resolved byrealizing that early in the synthesis (3 h, Figure 4D) the reactionmixture contains wire composites of micrometer size that areformed by the adhesion of several seeds, that is, nanoparticles.

In some synthesis products, “spotted” wires could be observed(Supporting Information Figure SI-4). Further investigation isneededto give a precise andreliable account of thegrowth mecha-nism of such nanowires. Two formation mechanisms appear tobe plausible. Either the nanowire and nanoparticle growth arecompetitive processes leading to complex NW surfaces due toproximity induced aggregation and recrystallization reactions, orthe “spotted” NW is the result of direct growth complicated bylocally varying facet growth speeds due to concentration fluctua-tions along the wire axis.

3. Electrical Measurements.   a. Oxidation/ReductionStudy of the Copper NWs.  The nanowires were exposed todifferent temperature treatments under altering gas (N2, air, H2/Ar) environment. Even at a relatively low temperature of 550 K,clusterswere formedon thesurface of thenanowires. Supposedly,they are brought about by HDA residues or other contaminants

melting and crystallizing during temperature cycles. Samplesheated in air atmosphere were oxidized as shown by the fieldemission SEM (FESEM) and EDS measurements. In the courseof temperature cycles, the resistance increased and decreasedsignificantly in synchrony with the first few oxidation-reductionstages.

The FESEM images of selected samples A-D are shown inSupporting Information Figure SI-5. Sample A is the originalCu NW before temperature treatment, sample B reduced in anatmosphere of 5% H2 in argon, and sample C oxidized in air andsubjected to the following heat cycles with a rate of 6 K/min:300 K   f 550 K   f 350 K   f 550 K   f 300 K. Sample D wasexposed to several heat cycles up to 625 K in air and in 1000 ppmH2 in argon. While minor cluster formation is observable even onsampleA, onsamples B (treatedat 550 K inH2)andC(550K,air)the cluster formation is much more intensive. By far, the biggestclusters were found in the case of sampleD, which was repeatedlyheated to 625 K in both hydrogen and air atmospheres.

The Cu/O atomic ratios calculated from EDS measurementsareshown in Table1. It is clearly seen that sampleA containsonlya small amount of oxygen, supposedly due to a thin surface Cu2Olayer. The low oxygen contents of sample B may also be related to

Figure 4.   TEM images of samples at different stages of hydrothermal treatment (A) 1 h, (B) 2 h, (C and D) 3 h, (E) 6 h, and (F) 24 h.

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a surface oxide layer. On the other hand, in sample C, very highoxygen content is detected, most probably caused by the oxida-tion of copper in the air atmosphere. Let us note here that onsample C the size of the clusters is bigger and their surface isrougher than that of sample B. This correlates well with oxideformation on the surface that expands and merges nanowirestogether. Sample D was exposed to several heat cycles in bothoxidative and reductive atmosphere. This sample has clearly thebiggest clustersof allwhich is explainedby thehigher temperatureof 625 K and the longer duration of the heat treatments makingcluster formation and growth possible. The atomic ratio wasfound to be Cu/O = 6:5 for this sample. The oxygen content wasvery high in spite of the reductive atmosphere (1000 ppm H2)

applied in the last treatment. Consequently, the intensive clusterformation appears to prevent nanowire reduction.

In conclusion, it is clear that Cu nanowires form clusters evenat 550 K. Two possible explanations can be suggested for theformation of clusters: (i) There might be some contamination lefton the surface of the nanowires which gets melted or fused duringthe heating treatments. In view of the circumstances of the pre-paration of the nanowires, HDA residues on the surface or glu-cose might bring about this phenomenon. This seems to be themost likely explanation for cluster formation. (ii) In principle, thewell-known phenomenon of melting point lowering in nano-structures could also be responsible for cluster formation. How-ever, bulk copper melts at 1358 K, a temperature significantlyhigher than the one occurring in our experiments (625 K).Therefore, this argument does not seem to be relevant enough.

b. Results of the Electrical Measurements. The  I -V  char-acteristics shown in Figure 5 are linear and independent of thegate voltage as is expected for a metallic conductor. The sampleswere very sensitive for oxidation in air. In Figure 6, the sampleseems to be oxidized at 360 K, which iscausedby a low amount of laboratory air mixed into the inert atmosphere of the measure-ment chamber. Seemingly, the sample reaches an oxidized state atabout 360 K as theresistance suddenly increasesby a great extent.It wasnot possible to changeit back to itsoriginalstatevia furthertemperature changes. Figure 7 shows exponential temperaturedependence of the resistance for the oxidized nanowires measuredin nitrogen atmosphere. From theArrhenius fit(inset in Figure 7)

of conductance, two energy barriers could be distinguished(0.16 and 0.29 eV for the low and the high temperature regimes,respectively) which might be related to Schottky contacts eitherat theinterfaces of semiconducting metal oxide nanowires andCunanowires or at the Pt electrodes; however, we cannot exclude thepossibility that the exponential temperature dependence of carrierconcentration, typical for semiconducting materials, also plays arole. Earlier, Singh et al. have observed the I -V characteristics of Cu-Se nanowire heterostructures to exhibit a resonant tunnelingdiode (RTD) behavior.29

In the next step, the reduction of nanowires to metallic copperwas attempted, while measuring the change of resistance whilethe temperature was varied (Figure 8a) in 1000 ppm hydrogenatmosphere. Reduction of the copper oxide to metallic copperhappens around 570 K. The resistance of the reduced nanowirenetwork (Figure 8b) was found to be quite low, exhibiting linearI -V  characteristics with no gate tunable transport, and showeda linear temperature dependence with a positive temperaturecoefficient (Figure 8c), both typical for metals.

Upon oxidation of the reduced wires in synthetic air atmo-sphere, the resistance considerably increases around 450 K(Figure 9a). Note that the oxidation temperature is higher thanthat in Figure6 and theresistanceresult here is significantly lowerthan that afterthe first oxidation. Figure9b presents theresults of attempted reductions of the sample (curves 1-3) in 1000 ppmhydrogen. It appears that the sample cannot be reduced backto the metallic copper state with these heat and gas treatments.The irreversibility in the process is most probably caused bythe formation of nanowire clusters shown in FESEM images

Table 1. Cu/O Atomic Ratios Calculated from EDS Measurements

Cu O

sample A 8.4 1sample B 9.7 1sample C 3.3 1sample D 1.2 1

Figure 5.   I -V  curves for different gate voltages in room atmo-sphere of the freshly prepared samples before any temperature orgas treatments. Inset shows source-drain resistance versus gatevoltage at 50 mV SD bias.

Figure 6.   Resistance-temperature sweep in nitrogen atmosphere.Temperature wasswept 300Kf500Kf150Kf500Kf150Kf 300 K. Note, because of the low currents through this sample,resistance values could not be acquired reliably below 230 K.

Figure 7.   Resistance-temperature curves measured in nitrogenatmosphere. Temperature was swept 300 K f 150 Kf 500 Kf150 Kf 500 Kf 300 K. Inset: Arrhenius plot for conductance inthe temperature range 150-500 K.

(29) Singh, R.; Kumar, R.; Chakarvarti, S. K.  Phys. E  2008, 40, 591 – 593.

8/9/2019 Low-Temperature Large-Scale Synthesis and Electrical Testing of Ultralong Copper Nanowires

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16502   DOI:  10.1021/la101385e   Langmuir 2010, 26(21), 16496–16502

Article Mohl et al.

(Supporting Information Figure SI-5). As a result of cluster for-mation, the copper cannot be exposed to the gas in its environ-ment and thereby it cannotbe reduced properly.It is possible thatHDA residues cover the surface of the nanowires hinderingfurther redox cycles. The measured poor sample conductivitywith nonlinear I -V  behavior justifies that the reduction attemptfailed and the network is composed of some oxide phases of thenanowires (Figure 9c).

Conclusions

Copper nanowires with a large aspect ratio have been success-

fully obtained by a simple hydrothermal method. In the process,glucose acts as a reducing agent and hexadecylamine functions asa template molecule. Our results indicate that both glucose andHDA play a significant role in the reaction and have a stronginfluence on copper nanowire formation. The electrical propertiesof copper nanowires have also been investigated and were foundto be similar to those reported forelectrodeposited Cu nanowires;however, due to clustering and multiple contacts between thewires in our films,the overall conduction process is more complexthan that for individual nanowires.30 The sample could not be

reduced and reoxidized again after the first oxidation-reductioncycle. In subsequent cycles, the agglomeration of nanopar-ticles was observed and effective redox steps could not beperformed. This phenomenon hinders considerably the poten-tial applicability of copper nanowires as gas sensors. On theother hand, once the wires are protected from the surroundingenvironment, they may find applications in electrical intercon-nects as well as in thermal composites due to their excellentthermal properties inherited from the bulk copper phase. Furtherresearch is necessary to find a more effective purification of nanowires.

Acknowledgment.  This work was supported by the Academyof Finland (Projects 1120853 and 1128626) and TEKES (Projects52478,52433, and52467).The financialsupport of theHungarianScientific Research Fund (OTKA) through Projects NNF-78920and 73676 is acknowledged.

Supporting Information Available:  Additional SEM, FES-EM, and TEM images; visual comparison of selected reac-

tion mixtures involved in the hydrothermal synthesis of copper nanowires; plot of HDA coverage on copper nano-

wires. This material is available free of chargevia the Internetat http://pubs.acs.org.

Figure 8.   (a) Change of the resistance of the nanowire network when the nanowires are reduced in 1000 ppm H2 in argon. (b) Source-draincurrent-voltagecurves for different gate voltages in laboratory air atmosphere after reduction. (c) Temperaturedependenceof resistance forthe reduced nanowires network measured in N2 atmosphere.

Figure 9.   (a) Change of resistance duringoxidation of the nanowire network in synthetic air. Temperature was swept in the sequence: 300 Kf 625 Kf 350 Kf 625 Kf 300 K. (b) Resistance plot as a function of temperature while attempting to reduce the oxidized nanowires in

1000 ppm H2. (c) Current-

voltage curve in laboratory air atmosphere; measurements were carried out after the ones had been finished inpanels (a) and (b).

(30) Toimil Molares, M. E.; H€ohberger, E. M.; Schaeflein, Ch.; Blick, R. H.;Neumann, R.; Trautmann, C. Appl. Phys. Lett.  2003, 82, 2139.