Abstract— We demonstrate three types ofspray-deposited Carbon Nanotube (CNT) netsubstrates: humidity sensors, dew-point setemperature indicators. The presencDodecylsulphate (SDS) significantly increasesthe film resistance of CNT networks to chhumidity. We observe up to a 3% change in the 30-75% range of relative humidity, wrelationship. When these SDS-impregnatedcooled to the dew-point of air, with the tempemonitored, the associated increase in sheet used to establish the dew-point temperatudoped CNT networks as time-temperaexploiting the de-doping of the CNT nettemperature. We observe an increase in film networks at temperatures higher than 50 °Cresistance increase follows the Arrhenius lawresistance increase ranges from ~30%at 50 °C°C.

I. INTRODUCTION INGLE wall Carbon Nanotubes (CNTs)over large areas to form entangled con

[1,2]. Such networks, especially sparse osurface area and can extensively interactenvironment.

Humidity sensors find application in mweather stations, research or analytichumidity sensitive goods storage and anhumidity control is required. However, esensors mainly consist of large physical based on the smallest elements rely on comprocesses . Simpler indicators instead rely but with no quantitative data [3,4].

Manuscript received June 20, 2012. V. Scardaci is with Hewlett-Packard Inkjet Supplie

Technology Campus, Leixlip, Co. Kildare, Ireland (phemail: vittorio.scardaci@hp.com), and with CentrAdvanced Nanostructures and Nanodevices (CRANDublin, Dublin 2, Ireland.

R. Coull is with Hewlett-Packard Inkjet SuppliesTechnology Campus, Leixlip, Co. Kildare, Ireland @hp.com), and with Centre for Research on AdvanceNanodevices (CRANN), Trinity College Dublin, Dubli

J. N. Coleman is with School of Physics, Trinity Co2, Ireland (email: colemaj@tcd.ie), and with CenAdvanced Nanostructures and Nanodevices (CRANDublin, Dublin 2, Ireland.

L. Byrne is with Hewlett-Packard Inkjet SuppliesTechnology Campus, Leixlip, Co. Kildare, lorraine.byrne@hp.com).

G. Scott is with Hewlett-Packard Inkjet SuppliesTechnology Campus, Leixlip, Co. Kildare, graeme.scott@hp.com)

Carbon NVittorio Scardaci, Richard C

S

f sensors based on tworks on flexible ensors and time-ce of Sodium s the sensitivity of hanges of relative

film resistance in with a non-linear d CNT films are erature of the film resistance can be re. We use acid-ature indicators, tworks at higher resistance of such

C. The rate of the . The extent of the

C to >300% at 100

) can be deposited nductive networks ones, have large t with the outer

many fields, e.g. cal laboratories, nywhere a strict existing humidity

elements. Those mplex fabrication on color change,

es Ireland, Liffey Park one: +353 1 6158936, re for Research on NN), Trinity College

s Ireland, Liffey Park (e-mail: richard.coull

ed Nanostructures and in 2, Ireland. ollege Dublin, Dublin

ntre for Research on NN), Trinity College

s Ireland, Liffey Park Ireland (e-mail:

s Ireland, Liffey Park Ireland (e-mail:

Dew point detection is a morehumidity sensing, but is an importaindustries ranging from pharmaceutiCommercially available sensors areso a cheap and easy-to-use dew-poinapplications in new fields [4].

Time-temperature indicators (Twhere goods or items degrade at temgiven threshold, at a rate dependitself. Current solutions involve a e.g. diffusion or a chemical reactivisual inspection by an individual [5

In this work, we demonstrate a hpoint detector and a TTI, all based oinvolve a cheap and simple wprocess, and at the same time prtransduction mechanism based on re

II. EXPERIMEN

CNTs were purchased from Hanwused as received. These were disperconcentration, by surfactant-assistsurfactant used was sodium dodecyat 1% concentration. Larger aggrecentrifugation and the top-half supermg/ml concentration after the inmeasured by UV-Vis Spectrophotomspray-coated onto polyethylene tereto form a continuous uniform CNT film size was 10cm x 10cm, which wsize desired for the specific applicafilm was used for humidity sensing

Fig. 1: Washed (dashed line) and unresistance variation when RH is increasdecreased back to 30%.

Nanotube Network Based Sensors Coull, Jonathan N. Coleman, Lorraine Byrne and Gra

e niche application than ant process parameter in icals to paper production. complex and expensive, nt sensor may boost their

TTIs) find application mperatures higher than a

ding on the temperature visual change given by

ion, and thus rely on a 5]. humidity sensors, a dew-on CNT networks, which et-chemistry fabrication rovide a straightforward esistance changes.

NTAL wha Nanotechnology and rsed in water, at 1 mg/ml ted ultrasonication. The ylbenzenesulphate (SDS) egates were removed by rnatant was diluted to 0.1 nitial concentration was metry. This solution was ephtalate (PET) at 80 °C networked film [2]. Our

was then cut down to the ation. The resulting CNT

and dew point detection

nwashed (solid line) film sed from 30% to 75% and

aeme Scott

Fig. 2: time change of unwashed CNT film res(line + solid circles)) due to ambient changes in Rmeasured with a Sensirion commercial sensor.

investigation. To measure the dew-point SDS-CNT on PET sample was mounted onchip and the temperature of the CNT sampmeasured with a thermocouple, while the was simultaneously recorded. For TTI isurfactant was removed by rinsing the filmthen treated in concentrated nitric acid frinsed with water and rested for 24 hCommercial sensors used as references SHT75 for relative humidity experimentsmeter for dew-point. Resistance mearecorded using a Keithley source meter.probe technique was used for time-temperat

III. RESULTS AND DISCUSSIO

Fig. 1 shows the electrical resistance onetwork when this is exposed to ~30% r(RH) measured in the laboratory, ansubsequently exposed to ~75% RH, asaturated sodium chloride solution. In thobserve a ~3% resistance increase after a 15comparison, a CNT film, where SDS was also tested in similar conditions. In this caresistance increase of only 0.7%. The preseincreases the sensitivity by a 4x factor. WSDS molecules realign in the presence ocausing the network to swell, increasing theCNTs at junctions and hence the film resista

Fig. 2 shows the resistance variation ofilm, monitored over a few days, plottedhumidity measured by a commercial sensorSDS-CNT film resistance well tracks the hmeasured by the commercial sensor.

Fig. 3 shows the electrical resistance of aon PET attached to a Peltier chip, as it is cothe dew-point temperature. We can obscooling, the film resistance increases slowdew point it increases very rapidly, to rapidly when cooling is stopped. A no

sistance variation RH and RH (line)

temperature the n a Peltier cooler ple’s surface was film’s resistance

investigation, the m in water. This is for 10 min, then h before testing.

were Sensirion s and Testo 610 asurements were The four point ture sensing.

ON of an SDS-CNT relative humidity nd when it is achieved with a his case we can 50s exposure. For washed out, was

ase we observe a ence of SDS thus

We speculate that of water vapour, e spacing between ance [6]. of the SDS-CNT

d in parallel with r. In this case, the humidity trend as

an SDS-CNT film ooled down below serve that, upon wly, but near the decrease equally

ominal dew-point

Fig. 3: SDS-CNT film resistance (solid as the temperature (line + circles) is decr(dashed line)

temperature is then manually derivthe two straight lines extrapolatedcurve shown in fig. 2. The derived fit when plotted against the vacommercial meter, but with an offsebecause of the arbitrary choice of nodelay in cooling penetrating to the Pfilm.

For the TTI application, Fig. 4 shRs, change, measured at room tempdifferent CNT films treated at differfrom 30 to 100 °C for a fixed time oobserve that Rs decreases slightly aat 40 °C, while increases linearly fro60% for 20 min exposure at 100 °C.Fig. 5 shows how the electrical resiCNT film changes with time whtemperatures in the 50-100 °C ranrates of resistance increase, as wel

Fig. 4: change in Rs of a number ofmeasured at room temperature after treatment (30-100 °C) for 10 and 20 minrespectively)

line) monitored with time

reased below the dew point

ed by the intersection of d from the characteristic

dew-point gives a linear alue measured with a et of ±2-4 °C, most likely ominal dew-point and the PET mounted SDS-CNT

hows the sheet resistance, perature, of a number of rent temperatures ranging of 10 and 20 min. We can at 30 °C, it is unchanged om 50 °C upwards, up to istance of an acid treated en exposed to different ge. We observe that the ll as the final resistance

f different CNT networks treatment a temperature

n (solid squares and circles

Fig. 5: Acid treated normalized resistance variatiotime at different temperatures. Inset: Arrhenius pconstant against inverse temperature

variation, are higher at higher temperaturewell fitted by a double exponential decay, order rate constant can be extracted. Wcomponent does not change significantly wthe short one can be plotted in an Arrheniu5 inset), showing an activation energy of thus infer that, in this case, the sensindesorption of the species that were absorbnetwork during the acid treatment. Tintegrated within an appropriate electronicused as an automatic alarm system in tenvironments that degrade or are in any sort50 °C. While this obviously does not includit may include for example the chemicalcertain reactions must be temperature contro

IV. CONCLUSION We demonstrated how SDS-impregnated

CNT networks can be used as sensing matsensors, dew-point detectors and indicators. Our material is fabricated by a qscalable wet chemistry process which makcheap alternative to commercially available

ACKNOWLEDGMENT The authors acknowledge David

developing the labview interface to run Rexperiments, and Pat Reilly for building the

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