We use a highly localised resistive heating technique to grow vertically aligned multiwalled nanotube

films and aligned single-walled nanotubes on substrates with an average temperature of less than 100°C. The

temperature at the catalyst can easily be as high as 1000 °C but an extremely high temperature gradient

ensures that the surrounding chip is held at much lower temperatures, even as close as 1μm away from the

local heater. We demonstrate the influence of temperature on the height of multi-walled nanotube films,

illustrate the feasibility of sequential growth of single-walled nanotubes by switching between local heaters

and also show that nanotubes can be grown over temperature sensitive materials such as resist polymer

Campbell, E.E.B.

Dittmer, S.

Mudgal, S.

Nerushev, O.A.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

Fizika Nizkikh Temperatur, 2008, v. 34, No. 10, p. 1058–1062Local heating method for growth of aligned carbonnanotubes at low ambient temperatureS. Dittmer1, S. Mudgal1,2, O.A. Nerushev3, and E.E.B. Campbell1,31Department of Physics, Gteborg University, SE-41296 Gteborg, Sweden2Indian Institute of Technology Madras, I.I.T. Post Office, Chennai 600-036, India3School of Chemistry, Edinburgh University, Edinburgh EH9 3JJ, ScotlandE-mail: Eleanor.campbell@ed.ac.ukReceived January 29, 2008We use a highly localised resistive heating technique to grow vertically aligned multiwalled nanotubefilms and aligned single-walled nanotubes on substrates with an average temperature of less than 100oC. Thetemperature at the catalyst can easily be as high as 1000oC but an extremely high temperature gradientensures that the surrounding chip is held at much lower temperatures, even as close as 1µm away from thelocal heater. We demonstrate the influence of temperature on the height of multi-walled nanotube films,illustrate the feasibility of sequential growth of single-walled nanotubes by switching between local heatersand also show that nanotubes can be grown over temperature sensitive materials such as resist polymer.PACS: 61.46.Fg Nanotubes.Keywords: carbon nanotubes, growth.1. IntroductionChemical vapour deposition methods are generallyregarded as the methods of choice for directly growingcarbon nanotubes with control over density, placement,length and morphology (single-walled nanotubes versusdouble- or multi-walled nanotubes) [1–4].One of the most interesting potential applications ofcarbon nanotubes is within the electronics industry wheretheir electrical, mechanical and thermal properties makethem very promising systems to combine with silicon tech-nology. However, a major problem concerns the tem-perature required to grow good quality carbon nanotubes.Typically, temperatures in the range 600–900 oC are re-quired, depending on the details of the growth conditionsand catalyst preparation. Although nanotubes can be grownat lower temperatures [5], their structural integrity dete-riorates rapidly as the growth temperature is decreased andtheir mechanical and electrical properties suffer as aconsequence. In order to grow good quality carbon nano-tubes on CMOS chips (aiming for temperatures below 450oC) or on other temperature-sensitive substrates such aspolymers, it is necessary to find ways to overcome thissevere limitation.Englander et al. [6] showed that it was possible togrow multiwalled nanotubes (10–50 nm thick) where thecatalyst was deposited on suspended polysiliconmicrobridges that were resistively heated. This methodhas also been followed up more recently by Hierold andco-workers [7], however, it requires complex fabricationtechnology and complicates the integration of the grownnanotubes. We have recently reported a related but simp-ler technique [8] involving the direct growth of nanotubeson metal electrodes fabricated on a silicon chip. We wereable to change between multi-walled and single-wallednanotube growth by changing the precursor gas and alsoindicated the very high temperature gradients that mustexist on the local heater. In this paper we extend theprevious results to model the temperature distribution inthe local heater. We also consider the influence of tem-perature on the growth of vertically aligned multi-wallednanotube films and discuss the changes in the heaterproperties as growth develops. In addition, we illustratethat aligned single-walled nanotubes can be grown se-quentially between different pairs of electrodes and that itis possible to grow horizontally aligned single-wallednanotube arrays over temperature sensitive substrates.© S. Dittmer, S. Mudgal, O. A. Nerushev, and E.E.B. Campbell, 20082. Experimental setup2.1. Growth chamber and substrate preparationThe small vacuum chamber used to carry out thegrowth studies is shown in Fig. 1. A patterned chip isplaced in the centre of the chamber and electrical contactsare made via movable probe needles. The small chambervolume allows very rapid pump-out, making it possible toquickly change precursor gases and have accurate controlover growth times.The chamber is pumped by a small turbo pump and isevacuated to high vacuum before the gases are intro-duced. A similar chamber has been designed to fit underthe microscope of a micro-Raman spectrometer allowingoptical emission and Raman spectroscopy to be carriedout during growth. The results from this second chamberwill be the subject of a future publication.The basic substrate used for the growth studies pre-sented here is an n-type doped Si wafer with a 400 nmthick, thermally grown oxide layer. The electrode patternsare made with a combination of electron beam and pho-to-lithography using a lift-off process. Molybdenum,with a thickness of 100 nm, is typically used as the elect-rode material. In our earlier report, tungsten was the cho-sen electrode material [8]. Typical examples of thegrowth electrodes are shown in Fig. 1,c,d. The lowerlayout is designed to be used for in situ four-probe measu-rements during growth. The upper layout was designedfor studying the influence of the application of an alig-ning electric field during growth.The catalyst consists of a 1nm thick layer of Fe depo-sited on a 5nm thick layer of aluminium oxide. The gasmixture used for multiwalled nanotube growth was 6sccm C2H2, 300 sccm H2 and 500 sccm Ar at atmosphericpressure. For single-walled nanotubes we used 6 sccmC2H4 in place of C2H2. The results presented here werefor 5 minutes growth time. However, especially in thecase of acetylene, the nanotube growth ceases before theend of the stipulated growth time.2.2. Temperature modellingThe temperature across the small metal heater wasmodelled using COMSOL. The heater was modelled as 4micrometer long, 100 nm thick and 400 nm wide. Figure 2shows the results of the temperature modelling when app-lying a current of 14 mA across the small metal bridge. Itis apparent that the temperature falls very rapidly at theedges of the growth bridge. It is relatively easy to reachtemperatures as high as 1000 oC at the centre of thegrowth region, rapidly dropping to below 100 oC at theedges.We find in the modelling that the silicon oxide layer iscritically important to thermally insulate the heater fromthe pure silicon. Without this boundary layer the heat isdissipated into the wafer resulting in a larger area beingLocal heating method for growth of aligned carbon nanotubes at low ambient temperatureFizika Nizkikh Temperatur, 2008, v. 34, No. 10 1059abcdFig. 1. Small vacuum chamber used to grow nanotubes withthe local heating method. The substrate can be electricallycontacted to supply a heating current to the electrodes andalso to monitor the voltage drop across the small metallicbridge heater during growth (a). The scheme for 4-probemeasurements is shown in (b) and an example of a substratedesign for such measurements is shown in (d). (c) is anexample of a substrate design to study the influence of analigning electric field during growth.1000800600400200–10 –5 0 5 10Fig. 2. The results of temperature modelling of a typicalelectrode geometry using COMSOL. In this example, a currentof 14 mA was passed through the metal bridge and a tem-perature as high as 1000oC was produced at the centre. Theresults illustrate the extremely high temperature gradients atthe edge of the metal heater.heated but without the heater reaching temperaturessuitable for nanotube growth.The resistivity used in the model was initiallydetermined from in situ I–V measurements on the actualelectrodes to be 220   cm. However this turned out tobe too high when used in the simulations leading to anon-converging system.To reach reasonable temperature values, the resistivityin the model was reduced to 90   cm. We believe thatone of the main causes for the model to overestimate thetemperature when using the measured resistivity valueswas that we are not able to correctly account for heatbeing transferred by cooling to the surrounding gases.Neither did we include any additional changes to thermalconductivity or other parameters due to physical or che-mical changes in the heater resulting from the interactionwith hydrocarbons during growth. Evidence for suchchanges will be presented below and studied in detail infuture publications.Earl ier resul ts , based on where mult i -wal lednanotubes grew on the electrode and underlying sub-strate, provided an estimate of the temperature gradient atthe edge of the heater to be on the order of 100 oC/µm [8].This is confirmed by the results of the modelling. We havealso recently confirmed the high temperatures and tem-perature gradients by in situ measurement of black-bodyradiation from the heater during growth. The optimuminitial catalyst growth temperature (defined here as thetemperature for which the maximum length is found) formultiwalled nanotubes is found to be 700–750 oC, in verygood agreement with studies of the optimum growthtemperature for similar catalyst and precursor materials innormal thermal CVD experiments [9,10].3. Growth results3.1. Arrays of vertically aligned multi-walled nanotubesAcetylene is a suitable precursor gas for the growth ofarrays of vertically aligned multiwalled nanotube films[9]. Figure 3 shows an array grown on top of a Mo heaterfor the standard growth conditions and a current of 20 mApassing through the heater corresponding to a tem-perature on the order of 600–700 oC. A well-aligned arrayis produced with the length of the nanotubes decreasingtowards the cooler zones at the heater edges.The temperature is very critically dependent on boththe current through the heater and the interaction with thegas environment. In particular, when using ethylene gas,changes induced in the heater material can lead tosubstantially higher temperatures in the presence of thegas than without hydrocarbon gas and also to expansionof the electrode material. An early example where wehave used the 4-probe substrate (Fig. 1,d) to monitor thevoltage drop across the heater as the current was inc-reased showed a clear change in the transport properties1060 Fizika Nizkikh Temperatur, 2008, v. 34, No. 10S. Dittmer, S. Mudgal, O. A. Nerushev, and E.E.B. CampbellFig. 3. A example of an array of vertically-alignedmulti-walled nanotubes grown with the local heatingtechnique. In this example, a current of 20 mA was used, cor-responding to a maximum temperature at the centre of theheater of 600-700oC.3.02.52.01.51.00.502 4 6 8 10 12 16Voltage,VabCurrent, mA14Fig. 4. Results of a 4-probe measurement showing that theproperties of the Mo heater change as the current is increased inthe presence of the growth gas mixture. This can lead tochanges of the electrode temperature during growth for a cons-tant heating current. Up triangles: first run of increasing thetemperature. Down-triangles: decreasing the temperature (a).Picture of an electrode that was accidentally overheated duringgrowth (b).of the molybdenum as the current was increased. Theresults of the measurement are shown in Fig. 4,a. Theresults of an experiment where the electrode wasaccidentally overheated can be seen in Fig. 4,b where onecan clearly see the electrode expansion and lift-off fromthe underlying substrate.3.2. Horizontally aligned single-walled nanotubesWe showed earlier that by changing the precursor hy-drocarbon gas under otherwise identical conditions it waspossible to change from growth of multi-walled nano-tubes to predominantly single-walled nanotubes [8]. Thesingle-walled nanotube growth from ethylene occursmore slowly and with less efficiency than the multi-walled growth from acetylene and one can achieve greatercontrol of the growth direction by applying an electricfield during growth. An example of an array of single-walled nanotubes grown in this way is shown in Fig. 5.The Raman spectra show that the nanotubes are of goodquality, as indicated by the high G/D band ratio. A typicalexample of a Raman spectrum taken with a 568 nm laserafter 15 seconds of growth is shown in Fig. 5,b.One advantage of the local heating growth method isthat it can be used to grow nanotubes over temperaturesensitive substrates. In Fig. 6,a, we show an examplewhere single-walled nanotubes have been grown, in thepresence of an aligning electric field (0.5 V/µm), across atrench filled with an inorganic polymer resist (FOx-12e-beam resist). The nanotubes can clearly be grown overthis material which can be removed after growth to pro-duce an array of suspended single-walled nanotubes. Thisopens up many new opportunities for device fabricationand integration of nanotubes on a variety of substrates.A second advantage is the possibility to sequentiallygrow nanotubes from different electrodes. In Fig. 6,b wehave first grown the nanotubes from the bottom righthand electrode towards the upper left hand corner. Theheating current was then switched off and the lower lefthand electrode was then heated to grow the second layerof nanotubes from the lower left hand corner to the upperright. Although the alignment is not perfect in this firstattempt, the results indicate that this technique has goodpotential for sequentially growing nanotubes to build uprather complex structures or, in combination with a po-lymer support as shown in Fig. 6,a, to produce a regularsuspended nanotube mesh that could have interestingfilter properties.Local heating method for growth of aligned carbon nanotubes at low ambient temperatureFizika Nizkikh Temperatur, 2008, v. 34, No. 10 1061abGD1.1 nm1.9 nm 1.0 nmRaman hift cms –1100 200 300 1000 1400 1800Fig. 5. Array of single-walled nanotubes grown usingethylene as precursor gas. An aligning electric field was ap-plied during growth (a). Typical Raman spectrum (568 nmlaser excitation) of suspended single-walled nanotubes grownwith this technique. The nanotube diameters corresponding tothe detected radial breathing mode peaks are indicated on thefigure (b).abFig. 6. Single-walled nanotubes grown across a temperaturesensitive substrate (a). Illustration of sequential nanotubegrowth. Nanotubes were first grown from the lower righthand corner to the upper left, the heating current was thenswitched to the lower left electrode and nanotubes weregrown from it to the upper right (b).4. ConclusionWe have shown that a relatively simple local heatingmethod can be used to grow either multi-walled orsingle-walled nanotubes on molybdenum electrodes.With this method the average temperature of the substratecan be less than 100 oC, making it possible to grow highquality nanotubes close to temperature sensitive ma-terials. It is also possible to sequentially grow nanotubesfrom different electrodes.AcknowledgementsFinancial support from the Stiftelsen fr StrategiskForskning (SSF), the EU (STREP projects CANEL andNANORF), Vetenskapsrådet (VR) and the WallenbergFoundation is gratefully acknowledged.1. N.R. Franklin and H. Dai, Adv. Mater. 12, 890 (2002).2. Y. Homma, Y. Kobayashi, T. Ogino, and T, Yamashita,Appl. Phys. Lett. 81, 2261 (2002).3. A. Ural, Y. Li, and H. Dai, Appl. Phys. Lett. 81, 3463(2002).4. E. Joselevich and C.M. Lieber, Nano Lett. 2, 1137 (2002).5. M. Cantoro, S. Hofmann, S. Pisana, V. Scardaci, A. Par-vez, C. Ducati, A.C. Ferrari, A.M. Blackburn, K.-Y. Wang,and J. Robertson, Nano Lett. 6, 1107 (2006).6. O. Englander, D. Christensen, and L. Lin, Appl. Phys.Lett. 82, 4797 (2003).7. C. Hierold, A. Jungen, C. Stampfer, and T. Helbling, Sen-sors and Actuators A136, 51 (2007).8. S. Dittmer, O.A. Nerushev, and E.E.B. Campbell, Appl.Phys. A84, 243 (2006).9. A.A. Puretzky, D.B. Geohegan, S. Jesse, I.N. Ivanov, andG. Eres, Appl. Phys. A81, 223 (2005).10. G.-H. Jeong, N. Olofsson, and E.E.B. Campbell, submitted.1062 Fizika Nizkikh Temperatur, 2008, v. 34, No. 10S. Dittmer, S. Mudgal, O. A. Nerushev, and E.E.B. Campbell

Local heating method for growth of aligned carbon nanotubes at low ambient temperature

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Local heating method for growth of aligned carbon nanotubes at low ambient temperature

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