- Email: [email protected]
The morphology, electrical conductivity and vapour sensing ability of inkjet-printed thin films of single-wall carbon nanotubes
CARBON
4 7 ( 2 0 0 9 ) 7 5 2 –7 5 7
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
The morphology, electrical conductivity and vapour sensing ability of inkjet-printed thin films of single-wall carbon nanotubes M.F. Mabrook*, C. Pearson, A.S. Jombert, D.A. Zeze, M.C. Petty School of Engineering and Centre for Molecular and Nanoscale Electronics, Durham University, South Road, Durham DH1 3LE, United Kingdom
A R T I C L E I N F O
A B S T R A C T
Article history:
Thin films containing single-wall carbon nanotubes (SWCNTs) have been prepared using
Received 14 July 2008
the inkjet printing (IJP) technique. Atomic force microscopy (AFM) has been used to inves-
Accepted 9 November 2008
tigate the morphology of these layers. The inkjet printed films consisted of small, ran-
Available online 18 November 2008
domly-oriented islands of nanotubes, the topography of which was dependent on the nature of the substrate surface. The in-plane electrical characteristics of the films were measured at room temperature. The current versus voltage data exhibited non-linear behaviour, which could be fitted to the theoretical model for Poole–Frenkel conductivity. Preliminary measurements are also reported on the use of the thin layers to detect alcohol vapour. 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon nanotubes are attractive materials for nanotechnology applications because of their exceptional electronic properties and mechanical strength [1]. There are two categories of nanotube: single-wall nanotubes (SWCNTs), which consist of a single rolled-up sheet of graphene, and multi-wall nanotubes (MWCNTs), which are a coaxial assembly of several graphene cylinders [2]. A typical SWCNT has dimensions of about 1.2 nm in diameter and 1–20 lm in length. The electronic structure of a SWCNT is either metallic or semiconducting, depending on its diameter and chirality. Therefore, SWCNTs offer an attractive proposition as conductive wires in micro- or nano-scale electronic devices. Many applications have now been suggested for nanotubes, including field-effect transistors, advanced composite materials and chemical sensors [3–8]. The latter devices have attracted considerable interest in the research community due to their high sensitivity. This, in part, results from the large surface area of the nanotubes. The realisation of practical devices, such as those
indicated above, has been slow for several reasons. These include the aggregation of the nanotubes during preparation and the difficulty in placing SWCNTs with high precision in micro or nano-electronic device structures. Different strategies are now available for building-up layers of carbon nanotubes on solid supports. In this work, we focus on inkjet printing of this important material and describe the properties of thin films produced by this technique. Inkjet printing (IJP) has now become a very useful technology for depositing layers of conductive organic materials [9– 13]. The method works by ejecting an ink through very fine nozzles, 10–200 lm in diameter. The advantages of IJP over other thin film techniques lie in its patterning capability, the efficient use of material, the high speed and low cost of the process, and in the fact that thin films can be printed on flexible substrates. Three important parameters influence the quality of inkjet-printed films: the printer; the ink; and the substrate. The printer controls the location of the deposit and the jetting parameters, whereas the viscosity and surface tension of the ink, together with the wettability of the droplets
* Corresponding author: Fax: +44 1913342407. E-mail address: [email protected] (M.F. Mabrook). 0008-6223/$ - see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.11.009
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on the substrate surface, influence the shape and size of the printed dots [14,15]. Inkjet printing has now been used to fabricate all-polymer transistors [16], all-polymer capacitors [17], polymer light emitting diodes [18] and chemical sensors [19,20]. Wei et al. have used a commercial inkjet printer to produce patterned thin films of MWCNTs [21]. Here, we study the use of IJP deposition to prepare thin films containing SWCNTs. We have used different substrate surfaces (polyester sheet and glass) together with atomic force microscopy (AFM) to study the morphologies and the microstructures of films deposited using this technique. The electrical characteristics and the application of these films for vapour sensing are also reported.
2.
Experimental
SWCNTs were prepared by arc discharge. All tubes were further purified using a technique reported previously [22]. Poly(ethyleneimine) (PEI) (Mw = 25,000), ethylene glycol, sodium dodecyl sulphate (SDS) were purchased from Sigma–Aldrich. Substrate sheets of 175 micron heat-stabilised polyester film coated with ITO, resistivity 170–230 X per square, were purchased from CPFilms. A commercial HP thermal printer (Deskjet 693 C) with a resolution of 600 · 600 dots per inch was used in this study. The only modification to the equipment was to replace the ink with the polyelectrolyte/SWCNT solution. The printer cartridges were emptied and thoroughly rinsed with ultra pure water to ensure that there was no ink left in the reservoir. The cartridges were then cleaned ultrasonically in water for 30 min. and dried with nitrogen gas. Arrays of six device structures, each device being 10 · 4 mm, were printed onto ITO-coated polyester film. The devices were deposited to form a bridge between two ITO electrodes separated by 2–5 lm, as depicted in Fig. 1a; Fig. 1b shows an optical micrograph of three inkjet-printed layers. In order to study the morphology of the printed films, arrays of nanotube devices were also printed on insulating polyester films (some treated with PEI to improve stability). To prepare the nanotube-based ink solution, the SWCNTs were first modified to make them suitable for inkjet printing. For this, the purified SWCNTs were mixed with a solution of SDS in ultrapure water. This compound was used to provide a satisfactory degree of separation among the bundles of nanotubes. On becoming coated with the surfactant, the relatively large clusters of nanotubes would break up into smaller aggregations. The dispersion of the nanotubes was further facilitated by being stirred and placed in an ultrasonic bath for 1–2 h. The viscosity and surface tension are both key parameters of the ink solutions. The viscosity must be sufficiently low to allow the channel to be refilled in a fraction of a second, while the surface tension must be high enough to hold the ink in the nozzle without dripping. A significant problem is to avoid clogging of the nozzles by dried ink. For the best IJP performance, the properties of the organic solution should match those of the ink used for the specific printer [19]. In this work, 10–20% ethylene glycol was added to the nanotube solution to improve its viscosity. AFM studies of the topography of these films were carried out using Digital Instruments Nanoscope E and NanoMan scanning probe microscopes. Two-point probe, in-plane DC
753
electrical measurements were performed at room temperature in a screened metal sample chamber evacuated to a pressure of 101 mbar using a mechanical rotary vane pump. Bias voltages were provided and sample currents measured using a Keithley 2400 digital sourcemeter. For vapour sensing, the thin films were mounted in a specially constructed glass chamber where a carrier gas containing known concentrations of vapour was passed over the device. Measurements of the response of the films to the vapours were made using freshly prepared nanotube films. To test the response of the sensors to the vapours, the current through the film was measured as the concentration of the vapours in the nitrogen carrier gas was increased and decreased over the range 300 ppm (parts per million) to 2000 ppm. The vapour concentration could be changed rapidly for response time measurements. After each increase in concentration of the vapours, the glass sample chamber was flushed with nitrogen. All the sensing experiments were undertaken under atmospheric pressure at room temperature (21 ± 2 C) in a normal laboratory environment. The DC current through the films was measured with a voltage applied to the devices using the digital sourcemeter. The response times reported in this paper are the times taken for the current (at a constant applied bias) to vary from 10% to 90% of the maximum current change.
3.
Results and discussion
Optical micrographs of IJP nanotubes revealed that thin films, one to three printed layers in thickness, were in the form of small and randomly-oriented islands of the SWCNT/polyelectrolyte ink, Fig. 1b. The film morphology was also influenced by the nature of the substrate. For example, the average size of printed islands on polyester substrates was about 100 lm, Fig. 2a, whereas, the PEI-coated polyester substrates resulted in smaller islands of the nanotube (average size 45 lm) with a much improved coverage of the film surface, as shown in Fig. 2b. It is evident from Fig. 2 that inkjet-printed SWCNT films contain long bundles, or ‘ropes,’ of nanotubes with lengths up to 80 lm, depending on the substrate surface. In the case of printing on untreated polyester substrates, most of the nanotubes were deposited at the centre and perimeter of the original ink droplet footprint, forming a ring-shaped pattern, Fig. 2a. In contrast, printing onto PEI-coated surfaces produced droplets which were more spread out, with the nanotube ropes forming star-shaped bundles, Fig. 2b. This adhesion of the SWCNT film with the PEI-coated substrate indicates good wettability between the two surfaces (determined to a large extent by the amount of the functional material deposited). In addition, PEI has been used as the seed layer in the layer-by-layer (LbL) deposition of SWCNT thin films to provide a positively charged substrate surface [22]. Hence, the electrostatic attraction between the substrate and the negatively charged inkjet-printed SDS/SWCNT ink solution onto the PEI-coated surfaces produced uniform SWCNT thin films. The observed SWCNT configurations are, interestingly, similar to those associated with polymer-dispersed liquid crystals, which are depicted schematically in Fig. 2 [23]. The arrangement shown on the right of Fig. 2a is obtained with the tangential anchoring of the liquid crystal molecules, whereas the radial pattern shown in Fig. 2b occurs when
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a
4 7 ( 2 0 0 9 ) 7 5 2 –7 5 7
SWCNT thin film ITO
2-5 µ m Polyester substrate
b
Fig. 1 – The arrangement of electrodes for the electrical and sensing measurements for inkjet-printed SWCNT films. (a) Schematic diagram of electrode configuration. (b) Optical micrograph of 3-printed layers of SWCNTs.
Fig. 2 – AFM image of 100 lm · 100 lm area of a single inkjet-printed film of SWCNTs on (a) a polyester substrate and (b) polyester treated with PEI. The figures on the right depict the possible configurations of a polymer dispersed liquid crystal. The directors are shown as lines, with respect to the polymer surface: top parallel alignment; bottom perpendicular alignment.
the liquid crystal molecules are anchored with their long axes perpendicular to the droplet walls. The different arrangements observed for polymer dispersed liquid crystals depend on factors such as droplet size and shape, and whether the liquid crystal molecule prefers to align parallel or perpendicular to the surrounding polymer surface. In the case of our SWCNT molecules, the nature of the underlying substrate (e.g. its surface energy) clearly plays a key role in controlling the size and shape of the printed nanotube ropes. The length and alignment of the SWCNTs will influence the electrical conductivity of the printed films. Single inkjetprinted nanotube layers were found to be electrically insulating and independent of the nature of the underlying substrate. This was due to the relatively large gap between the printed droplets (compared to the electrode separation). Nanotube-free regions of substrate are evident in Fig. 2a. AFM studies of SWCNTs on PEI-coated polyester surfaces also revealed that the connected bundles of nanotubes seen in Fig. 2b were only present on a local scale in comparison to the deposition area (4 · 10 mm). Although multiple prints on all substrates showed some coalescence of the ink droplets (e.g. Fig. 1b for three printed layers) these multilayer films were also not electrically conductive over large distances. Reliable in-plane, electrically conductive inkjet-printed SWCNT films could be achieved only if multiple printed films were made to cross two conductive electrodes separated by a very short distance, less
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than about 20 lm – smaller than the length of the nanotube bundles. Typical current versus voltage (I–V) data for a SWCNT film, printed between ITO electrodes separated by 5 lm, are shown in Fig. 3. The current clearly depends in a non-linear fashion on the applied voltage. In a previous study of LbL films containing carbon nanotubes, we have reported non-linear I–V characteristics for both in-plane and out-of-plane DC conductivity [22]. Various theoretical models were tested to fit the data, including Schottky and Poole–Frenkel conductivity, space-charge injection and quantum mechanical tunnelling. Although the electrical data could be fitted reasonably well using a simple power law (I / V2), suggesting space-charge-limited conductivity, the expected dependence of current on electrode separation was not observed. The best fit was obtained using a model based on tunnelling. Although this offered a reasonable physical explanation for the conductivity normal to the film plane, it was not clear why such a process should control the in-plane conductivity (where the nanotube spacing would be greater than normally expected for quantum mechanical tunnelling).
a
Experiment
Current (µA)
Current (µA)
Fitting
300 200
5
10
3
10
I∝ V2
10
-3
0.1
1 10 Voltage (V)
100
0 0
5
10
15
20
Voltage (V)
b
-10
Ln(I/V)
-12
-14 Experiment -16
Fitting
0
1
ð1Þ
IPF bPF / V1=2 Ln V ðkTÞd1=2
ð2Þ
Therefore, the voltage dependence at constant temperature given by [24]. Ln½IPF =V / V1=2
ð3Þ
where IPF, E are the current and electric field, respectively, T is the temperature, / is a constant related to the trap depth, A is a constant, and bPF is the Poole–Frenkel constant. 3 1=2 e bPF ¼ ð4Þ pee0
2
or Ln½I / V1=2
ð5Þ
The electrode separation d may be extracted from the slope a of ln(I/V) versus V1/2 and ln(I) versus V1/2 for the Poole–Frenkel and Schottky models, respectively. 2 b ð6Þ d¼ ðkTÞa
1
-1
100
IPF ¼ AEexpðbPF E1=2 =kT /Þ
Ln½I=T2 / V1=2
Experiment I∝ V3
10
10
In this work, we have tested the various conductivity models outlined above. Again, the data in Fig. 3 could be fitted quite well using a simple power law (I / V3, Fig. 3a, as confirmed by the log plot in the inset), clear evidence of the non-linearity of the I–V characteristics. The data in Fig. 3a were also fitted both with Poole–Frenkel and Schottky conduction models to determine the dominant mechanism. The Poole–Frenkel model is described by
where e is the electronic charge, e0 the permittivity of free space and e the permittivity of the film. The equivalent for the Schottky conduction model is described by
500 400
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3
4
5
V1/2 Fig. 3 – Current versus voltage characteristics for the three inkjet-printed layers on ITO-coated polyester substrate for the structure shown in Fig. 1. Experimental data are shown as symbols. (a) Full line corresponds to I / V3 fit. The inset shows the log–log scale for the I–V data with guidelines show I/V2 and I/V3 behaviours. (b) Full line is best Poole– Frenkel conductivity fit, log(I/V) / V1/2.
where b represents either the Poole–Frenkel (bPF) or Schottky (bS) coefficients. Note that bPF = 2bS [24]. Good fits in the higher voltage regime were obtained by using the model for Poole–Frenkel conductivity, which is normally observed under high applied electric fields [25–27]. Fig. 3b shows the experimental data in the form of a ln(I/V) versus V1/2 relationship (Eq. (3)). It is important to note that a reasonably good fit was also obtained on plotting ln(I) versus V1/2 (Eq. (5)). Our previous work (ellipsometric measurements at 632 nm) on SWCNT LBL film (characterised by material preparation similar to that of IJP) indicate a permittivity of 2.16 [24]. The pre-exponential electric field term in Eq. (1) is often neglected by workers fitting the Poole–Frenkel theory to experimental data (on the basis that the exponential term will be dominant). However, it has been shown that this can be significant under certain conditions, particularly at very high applied voltages [27]. Dielectric constants have recently been reported for SWCNT/polythiophene composites [28]. At room temperatures, the real part of the permittivity of the composite (10:1 weight ratio polymer:SWCNT) was only slightly greater than that of the conductive polymer (approximately 4.5 for the composite compared to 4 for polythiophene at 4 kHz). In our case, the permittivity of the surfactant coating of the SWCNTs used for IJP is expected to be in the range 2 to 3, consistent with the value measured.
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The value of measured permittivity was fitted in both the Poole–Frenkel and Schottky equations to determine the electrode separation d. The fitting of the data in Fig. 3a to Poole– Frenkel model led to 2.4 lm, within the 2–5 lm range of the actual electrode separation. In contrast, the Schottky model fit of the same data produced an electrode separation an order of magnitude smaller, 0.26 lm, well outside the 2–5 lm range. The slopes extracted from Eqs. (3) and (5) may vary slightly given that they are experimentally estimated. However, the values of the electrodes separation determined (using Eq. (6)) clearly suggest that: (i) Poole–Frenkel conduction mechanism is dominant; (ii) the Schottky model cannot be applied in this instance as the theoretical values are not consistent with the experimental findings. The results of vapour sensing experiments using methanol revealed that the current through the films decreased as the concentration of methanol in the carrier gas increased. Different bias voltages show a similar sensing response with higher sensitivities at lower voltages. Data are shown in Fig. 4 for methanol concentrations from 300 ppm to 2000 ppm with 3 V applied voltage. The film resistance recovered almost completely when the vapour was turned off. The response of the sensor to the vapours can be given in terms of the relative variation of the measured current, DI, where ðIvapour Io Þ 100% ð7Þ DI ¼ Io Ivapour is the current through the sensor when exposed to an atmosphere containing a known vapour concentration and Io is the current through the sensor when exposed to the carrier gas only. The calibration curve for the film response was obtained by plotting DI against the concentration of methanol in the carrier gas, and is shown as the inset to Fig. 4. Previous work has also demonstrated the electrical resistance of carbon nanotubes is influenced on exposure to gases such as CH4 and O2 [29]. It was suggested that the effect resulted from the adsorption of oxygen molecules on the surface of the SWCNTs, thereby doping them. In other studies,
4.6 4.4 300 ppm 500 ppm
4.0
4.
Conclusions
Thin films containing SWCNTs have been fabricated using the IJP technique. The morphologies of the films were studied using AFM. IJP films were electrically conductive and exhibited non-linear current versus voltage behaviour. It is suggested that the non-linear conductivity is related to the Poole–Frenkel mechanism. A preliminary study of the IJP thin films as chemical sensors revealed a decrease in the conductivity on exposure to methanol vapour. The inkjetprinted SWCNT films possessed high values of DI, but with relatively long response and recovery times (exceeding three minutes).
Acknowledgements
30
3.8
This work has partly been supported by the University Innovation Centre (UIC) in Nanomaterials, funded by the UK Government Department of Trade and Industry and One NorthEast.
25
3.6
1000 ppm
Δ I (%)
Current (µA)
4.2
exposure to methane was thought to affect the hybridisation state of the bonding within the carbon nanotubes [30]. We offer an alternative explanation for our vapour sensing results. On exposure to methanol, the vapour will diffuse into the thin SWCNT film, leading to a distribution of methanol molecules on the surface of the nanotubes and in the regions between them. From our electrical conductivity studies above, it is likely that the DC conductivity of the inkjet-printed films is influenced by the relatively insulating regions between the conductive carbon nanotubes. The incorporation of methanol in these regions will almost certainly increase their permittivity; the relative permittivity of methanol is approximately 34, compared to a value of 2–3 expected for the surfactant coating of the SWCNTs. According to Eqs. (1) and (4) above, an increase in the permittivity will result in a decrease in the current through the film. The response and recovery of the sensing devices will be limited by the rate of diffusion of the vapour molecules into and out of the film, accounting for the relatively long response and recovery times evident in Fig. 4 (180 s for the response and 210 s for the recovery for a 2000 ppm exposure to methanol). The sensitivity of the IJP films (slope of the inset curve in Fig. 4) was about 0.015% ppm1. This figure is less than values reported for inkjet-printed films of polymers exposed to methanol, e.g. 0.05% ppm1 for a film of poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate), PEDOT-PSS [19,20]. However, the response times of the SWCNT devices reported here were slightly faster, perhaps reflecting the more ‘open’ structure of the inkjet-printed films of the nanotubes.
3.4
20 15 10 5
3.2
500 1000 1500 2000 Concentration of methanol vapour (ppm)
2000 ppm
3.0 0
20
40
60
80
100
120
R E F E R E N C E S
Time (min) Fig. 4 – The variation of current through an inkjet-printed SWCNT film when exposed to step changes in the concentrations of methanol vapours. The inset shows the variation in DI as a function of the concentration of methanol vapour.
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[3] Wang XQ, Wang M, He PM, Xu YB, Li ZH. Model calculation for the field enhancement factor of carbon nanotube. J Appl Phys 2004; 96(11):6752–55. [4] Wang J, Dai J, Yarlagadda T. Carbon nanotube-conductingpolymer composite nanowires. Langmuir 2005;21(1):9–12. [5] Pirio G, Legagneux P, Pribat D, Teo KBK, Chhowalla M, Amaratunga GAJ, et al. Fabrication and electrical characteristics of carbon nanotube field emission microcathodes with an integrated gate electrode. Nanotechnology 2002;13(1):1–4. [6] Penza M, Cassano G, Aversa P, Cusano A, Cutolo A, Giordano M, et al. Carbon nanotube acoustic and optical sensors for volatile organic compound detection. Nanotechnology 2005;16(11):2536–47. [7] Suehiro J, Imakiire H, Hidaka S, Ding W, Zhou G, Imasaka K, et al. Schottky-type response of carbon nanotube NO2 gas sensor fabricated onto aluminum electrodes by dielectrophoresis. Sens Act B 2006; 114(2):943–9. [8] Fu D, Xu Y, Li L-J, Chen Y, Mhaisalkar SG, Boey FYC, et al. Electrical detection of nitric oxide using single-walled carbon nanotube network devices. Carbon 2007;45(9):1911–20. [9] Lee HH, Chou KS, Huang KC. Inkjet printing of nanosized silver colloids. Nanotechnology 2005;16(10):2436–41. [10] Mabrook MF, Pearson C, Petty MC. An inkjet-printed chemical fuse. Appl Phys Lett 2005;86:013507. [11] Liu Y, Cui T. Polymer-based rectifying diodes on a glass substrate fabricated by ink-jet printing. Macromol Rapid Commun 2005; 26(4):289–92. [12] Chen B, Cui T, Liu Y, Varahramyan K. All-polymer RC filter circuits fabricated with inkjet printing technology. Sol State Electron 2003; 47(5):841–7. [13] Calvert P. Inkjet printing for materials and devices. Chem Mater 2001; 13(10):3299–305. [14] Hebner TR, Wu CC, Marcy D, Lu MH, Sturm JC. Ink-jet printing of doped polymers for organic light emitting devices. Appl Phys Lett 1998; 72(5):519–21. [15] Percin G, Khuri-Yakub BT. Micromachined droplet ejector arrays for controlled ink-jet printing and deposition. Rev Sci Instrum 2002; 73(5):2193–6. [16] Sirringhaus H, Kawase T, Friend RH. High-resolution ink-jet printing of all-polymer transistor circuits. MRS Bulletin 2001;26(7):539–43.
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[17] Liu Y, Cui T, Varahramyan K. All-polymer capacitor fabricated with inkjet printing technique. Solid-State Electron 2003;47(9):1543–8. [18] Chang SC, Liu J, Bharathan J, Yang Y, Onohara J, Kido J. Multicolour organic light-emitting diodes processed by hybrid inkjet printing. Adv Mater 1999; 11(9):734–7. [19] Mabrook MF, Pearson C, Petty MC. Inkjet-printed polymer films for the detection of organic vapors. IEEE Sensors 2006;6(6):1435–44. [20] Mabrook MF, Pearson C, Petty MC. Inkjet-printed polypyrrole thin films for vapour sensing. Sens Act B 2006; 115(1):547– 51. [21] Wei T, Ruan J, Fan Z, Luo G, Fei W. Preparation of a carbon nanotube film by ink-jet printing. Carbon 2007;45:2692– 2716. [22] Palumbo M, Lee KU, Ahn BT, Suri A, Coleman KS, Zeze D, et al. Electrical investigations of layer-by-layer films of carbon nanotubes. J Phys D: Appl Phys 2006; 39(14):3077–85. [23] Collings PJ. Liquid crystals. Bristol: IOP Publishing Ltd.;1990. [24] Jombert AS, Coleman KS, Wood D, Petty MC, Zeze DA. Poole– Frenkel conduction in single wall carbon nanotube composite films built-up by electrostatic layer-by-layer deposition. J Appl Phys 2008;104:094603. [25] Roberts GG, Apsley N, Munn RW. Temperature-dependent electronic conduction in semiconductors. Phys Reports 1980;60(2):59–150. [26] Anderson JC. Dielectric and transport properties of insulating films. J Vac Sci Technol 1972; 9(1):1–11. [27] Hill RM. Poole–Frenkel conduction in amorphous solids. Phil Mag 1971; 23(181):59–86. [28] Eren San S, Yerli Y, Okutan M, Yılmaz F, Gunaydın O, Hames Y. Temperature dependency of electrical behaviors in single walled carbon nanotube/conducting polymer composites. Mater Sci Eng B 2007; 138(3):284–8. [29] Valentini L, Armentano I, Lozzi L, Santucci S, Kenny JM. Interaction of methane with carbon nanotube thin films: role of defects and oxygen adsorption. Mater Sci Eng. C 2004; 24(4):527–33. [30] Roy RK, Pal Chowdhury M, Pal AK. Room temperature sensor based on carbon nanotubes and nanofibres for methane detection. Vacuum 2005;77(3):223–9.
4 7 ( 2 0 0 9 ) 7 5 2 –7 5 7
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
The morphology, electrical conductivity and vapour sensing ability of inkjet-printed thin films of single-wall carbon nanotubes M.F. Mabrook*, C. Pearson, A.S. Jombert, D.A. Zeze, M.C. Petty School of Engineering and Centre for Molecular and Nanoscale Electronics, Durham University, South Road, Durham DH1 3LE, United Kingdom
A R T I C L E I N F O
A B S T R A C T
Article history:
Thin films containing single-wall carbon nanotubes (SWCNTs) have been prepared using
Received 14 July 2008
the inkjet printing (IJP) technique. Atomic force microscopy (AFM) has been used to inves-
Accepted 9 November 2008
tigate the morphology of these layers. The inkjet printed films consisted of small, ran-
Available online 18 November 2008
domly-oriented islands of nanotubes, the topography of which was dependent on the nature of the substrate surface. The in-plane electrical characteristics of the films were measured at room temperature. The current versus voltage data exhibited non-linear behaviour, which could be fitted to the theoretical model for Poole–Frenkel conductivity. Preliminary measurements are also reported on the use of the thin layers to detect alcohol vapour. 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon nanotubes are attractive materials for nanotechnology applications because of their exceptional electronic properties and mechanical strength [1]. There are two categories of nanotube: single-wall nanotubes (SWCNTs), which consist of a single rolled-up sheet of graphene, and multi-wall nanotubes (MWCNTs), which are a coaxial assembly of several graphene cylinders [2]. A typical SWCNT has dimensions of about 1.2 nm in diameter and 1–20 lm in length. The electronic structure of a SWCNT is either metallic or semiconducting, depending on its diameter and chirality. Therefore, SWCNTs offer an attractive proposition as conductive wires in micro- or nano-scale electronic devices. Many applications have now been suggested for nanotubes, including field-effect transistors, advanced composite materials and chemical sensors [3–8]. The latter devices have attracted considerable interest in the research community due to their high sensitivity. This, in part, results from the large surface area of the nanotubes. The realisation of practical devices, such as those
indicated above, has been slow for several reasons. These include the aggregation of the nanotubes during preparation and the difficulty in placing SWCNTs with high precision in micro or nano-electronic device structures. Different strategies are now available for building-up layers of carbon nanotubes on solid supports. In this work, we focus on inkjet printing of this important material and describe the properties of thin films produced by this technique. Inkjet printing (IJP) has now become a very useful technology for depositing layers of conductive organic materials [9– 13]. The method works by ejecting an ink through very fine nozzles, 10–200 lm in diameter. The advantages of IJP over other thin film techniques lie in its patterning capability, the efficient use of material, the high speed and low cost of the process, and in the fact that thin films can be printed on flexible substrates. Three important parameters influence the quality of inkjet-printed films: the printer; the ink; and the substrate. The printer controls the location of the deposit and the jetting parameters, whereas the viscosity and surface tension of the ink, together with the wettability of the droplets
* Corresponding author: Fax: +44 1913342407. E-mail address: [email protected] (M.F. Mabrook). 0008-6223/$ - see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.11.009
CARBON
4 7 (2 0 0 9) 7 5 2–75 7
on the substrate surface, influence the shape and size of the printed dots [14,15]. Inkjet printing has now been used to fabricate all-polymer transistors [16], all-polymer capacitors [17], polymer light emitting diodes [18] and chemical sensors [19,20]. Wei et al. have used a commercial inkjet printer to produce patterned thin films of MWCNTs [21]. Here, we study the use of IJP deposition to prepare thin films containing SWCNTs. We have used different substrate surfaces (polyester sheet and glass) together with atomic force microscopy (AFM) to study the morphologies and the microstructures of films deposited using this technique. The electrical characteristics and the application of these films for vapour sensing are also reported.
2.
Experimental
SWCNTs were prepared by arc discharge. All tubes were further purified using a technique reported previously [22]. Poly(ethyleneimine) (PEI) (Mw = 25,000), ethylene glycol, sodium dodecyl sulphate (SDS) were purchased from Sigma–Aldrich. Substrate sheets of 175 micron heat-stabilised polyester film coated with ITO, resistivity 170–230 X per square, were purchased from CPFilms. A commercial HP thermal printer (Deskjet 693 C) with a resolution of 600 · 600 dots per inch was used in this study. The only modification to the equipment was to replace the ink with the polyelectrolyte/SWCNT solution. The printer cartridges were emptied and thoroughly rinsed with ultra pure water to ensure that there was no ink left in the reservoir. The cartridges were then cleaned ultrasonically in water for 30 min. and dried with nitrogen gas. Arrays of six device structures, each device being 10 · 4 mm, were printed onto ITO-coated polyester film. The devices were deposited to form a bridge between two ITO electrodes separated by 2–5 lm, as depicted in Fig. 1a; Fig. 1b shows an optical micrograph of three inkjet-printed layers. In order to study the morphology of the printed films, arrays of nanotube devices were also printed on insulating polyester films (some treated with PEI to improve stability). To prepare the nanotube-based ink solution, the SWCNTs were first modified to make them suitable for inkjet printing. For this, the purified SWCNTs were mixed with a solution of SDS in ultrapure water. This compound was used to provide a satisfactory degree of separation among the bundles of nanotubes. On becoming coated with the surfactant, the relatively large clusters of nanotubes would break up into smaller aggregations. The dispersion of the nanotubes was further facilitated by being stirred and placed in an ultrasonic bath for 1–2 h. The viscosity and surface tension are both key parameters of the ink solutions. The viscosity must be sufficiently low to allow the channel to be refilled in a fraction of a second, while the surface tension must be high enough to hold the ink in the nozzle without dripping. A significant problem is to avoid clogging of the nozzles by dried ink. For the best IJP performance, the properties of the organic solution should match those of the ink used for the specific printer [19]. In this work, 10–20% ethylene glycol was added to the nanotube solution to improve its viscosity. AFM studies of the topography of these films were carried out using Digital Instruments Nanoscope E and NanoMan scanning probe microscopes. Two-point probe, in-plane DC
753
electrical measurements were performed at room temperature in a screened metal sample chamber evacuated to a pressure of 101 mbar using a mechanical rotary vane pump. Bias voltages were provided and sample currents measured using a Keithley 2400 digital sourcemeter. For vapour sensing, the thin films were mounted in a specially constructed glass chamber where a carrier gas containing known concentrations of vapour was passed over the device. Measurements of the response of the films to the vapours were made using freshly prepared nanotube films. To test the response of the sensors to the vapours, the current through the film was measured as the concentration of the vapours in the nitrogen carrier gas was increased and decreased over the range 300 ppm (parts per million) to 2000 ppm. The vapour concentration could be changed rapidly for response time measurements. After each increase in concentration of the vapours, the glass sample chamber was flushed with nitrogen. All the sensing experiments were undertaken under atmospheric pressure at room temperature (21 ± 2 C) in a normal laboratory environment. The DC current through the films was measured with a voltage applied to the devices using the digital sourcemeter. The response times reported in this paper are the times taken for the current (at a constant applied bias) to vary from 10% to 90% of the maximum current change.
3.
Results and discussion
Optical micrographs of IJP nanotubes revealed that thin films, one to three printed layers in thickness, were in the form of small and randomly-oriented islands of the SWCNT/polyelectrolyte ink, Fig. 1b. The film morphology was also influenced by the nature of the substrate. For example, the average size of printed islands on polyester substrates was about 100 lm, Fig. 2a, whereas, the PEI-coated polyester substrates resulted in smaller islands of the nanotube (average size 45 lm) with a much improved coverage of the film surface, as shown in Fig. 2b. It is evident from Fig. 2 that inkjet-printed SWCNT films contain long bundles, or ‘ropes,’ of nanotubes with lengths up to 80 lm, depending on the substrate surface. In the case of printing on untreated polyester substrates, most of the nanotubes were deposited at the centre and perimeter of the original ink droplet footprint, forming a ring-shaped pattern, Fig. 2a. In contrast, printing onto PEI-coated surfaces produced droplets which were more spread out, with the nanotube ropes forming star-shaped bundles, Fig. 2b. This adhesion of the SWCNT film with the PEI-coated substrate indicates good wettability between the two surfaces (determined to a large extent by the amount of the functional material deposited). In addition, PEI has been used as the seed layer in the layer-by-layer (LbL) deposition of SWCNT thin films to provide a positively charged substrate surface [22]. Hence, the electrostatic attraction between the substrate and the negatively charged inkjet-printed SDS/SWCNT ink solution onto the PEI-coated surfaces produced uniform SWCNT thin films. The observed SWCNT configurations are, interestingly, similar to those associated with polymer-dispersed liquid crystals, which are depicted schematically in Fig. 2 [23]. The arrangement shown on the right of Fig. 2a is obtained with the tangential anchoring of the liquid crystal molecules, whereas the radial pattern shown in Fig. 2b occurs when
754
CARBON
a
4 7 ( 2 0 0 9 ) 7 5 2 –7 5 7
SWCNT thin film ITO
2-5 µ m Polyester substrate
b
Fig. 1 – The arrangement of electrodes for the electrical and sensing measurements for inkjet-printed SWCNT films. (a) Schematic diagram of electrode configuration. (b) Optical micrograph of 3-printed layers of SWCNTs.
Fig. 2 – AFM image of 100 lm · 100 lm area of a single inkjet-printed film of SWCNTs on (a) a polyester substrate and (b) polyester treated with PEI. The figures on the right depict the possible configurations of a polymer dispersed liquid crystal. The directors are shown as lines, with respect to the polymer surface: top parallel alignment; bottom perpendicular alignment.
the liquid crystal molecules are anchored with their long axes perpendicular to the droplet walls. The different arrangements observed for polymer dispersed liquid crystals depend on factors such as droplet size and shape, and whether the liquid crystal molecule prefers to align parallel or perpendicular to the surrounding polymer surface. In the case of our SWCNT molecules, the nature of the underlying substrate (e.g. its surface energy) clearly plays a key role in controlling the size and shape of the printed nanotube ropes. The length and alignment of the SWCNTs will influence the electrical conductivity of the printed films. Single inkjetprinted nanotube layers were found to be electrically insulating and independent of the nature of the underlying substrate. This was due to the relatively large gap between the printed droplets (compared to the electrode separation). Nanotube-free regions of substrate are evident in Fig. 2a. AFM studies of SWCNTs on PEI-coated polyester surfaces also revealed that the connected bundles of nanotubes seen in Fig. 2b were only present on a local scale in comparison to the deposition area (4 · 10 mm). Although multiple prints on all substrates showed some coalescence of the ink droplets (e.g. Fig. 1b for three printed layers) these multilayer films were also not electrically conductive over large distances. Reliable in-plane, electrically conductive inkjet-printed SWCNT films could be achieved only if multiple printed films were made to cross two conductive electrodes separated by a very short distance, less
CARBON
than about 20 lm – smaller than the length of the nanotube bundles. Typical current versus voltage (I–V) data for a SWCNT film, printed between ITO electrodes separated by 5 lm, are shown in Fig. 3. The current clearly depends in a non-linear fashion on the applied voltage. In a previous study of LbL films containing carbon nanotubes, we have reported non-linear I–V characteristics for both in-plane and out-of-plane DC conductivity [22]. Various theoretical models were tested to fit the data, including Schottky and Poole–Frenkel conductivity, space-charge injection and quantum mechanical tunnelling. Although the electrical data could be fitted reasonably well using a simple power law (I / V2), suggesting space-charge-limited conductivity, the expected dependence of current on electrode separation was not observed. The best fit was obtained using a model based on tunnelling. Although this offered a reasonable physical explanation for the conductivity normal to the film plane, it was not clear why such a process should control the in-plane conductivity (where the nanotube spacing would be greater than normally expected for quantum mechanical tunnelling).
a
Experiment
Current (µA)
Current (µA)
Fitting
300 200
5
10
3
10
I∝ V2
10
-3
0.1
1 10 Voltage (V)
100
0 0
5
10
15
20
Voltage (V)
b
-10
Ln(I/V)
-12
-14 Experiment -16
Fitting
0
1
ð1Þ
IPF bPF / V1=2 Ln V ðkTÞd1=2
ð2Þ
Therefore, the voltage dependence at constant temperature given by [24]. Ln½IPF =V / V1=2
ð3Þ
where IPF, E are the current and electric field, respectively, T is the temperature, / is a constant related to the trap depth, A is a constant, and bPF is the Poole–Frenkel constant. 3 1=2 e bPF ¼ ð4Þ pee0
2
or Ln½I / V1=2
ð5Þ
The electrode separation d may be extracted from the slope a of ln(I/V) versus V1/2 and ln(I) versus V1/2 for the Poole–Frenkel and Schottky models, respectively. 2 b ð6Þ d¼ ðkTÞa
1
-1
100
IPF ¼ AEexpðbPF E1=2 =kT /Þ
Ln½I=T2 / V1=2
Experiment I∝ V3
10
10
In this work, we have tested the various conductivity models outlined above. Again, the data in Fig. 3 could be fitted quite well using a simple power law (I / V3, Fig. 3a, as confirmed by the log plot in the inset), clear evidence of the non-linearity of the I–V characteristics. The data in Fig. 3a were also fitted both with Poole–Frenkel and Schottky conduction models to determine the dominant mechanism. The Poole–Frenkel model is described by
where e is the electronic charge, e0 the permittivity of free space and e the permittivity of the film. The equivalent for the Schottky conduction model is described by
500 400
755
4 7 (2 0 0 9) 7 5 2–75 7
3
4
5
V1/2 Fig. 3 – Current versus voltage characteristics for the three inkjet-printed layers on ITO-coated polyester substrate for the structure shown in Fig. 1. Experimental data are shown as symbols. (a) Full line corresponds to I / V3 fit. The inset shows the log–log scale for the I–V data with guidelines show I/V2 and I/V3 behaviours. (b) Full line is best Poole– Frenkel conductivity fit, log(I/V) / V1/2.
where b represents either the Poole–Frenkel (bPF) or Schottky (bS) coefficients. Note that bPF = 2bS [24]. Good fits in the higher voltage regime were obtained by using the model for Poole–Frenkel conductivity, which is normally observed under high applied electric fields [25–27]. Fig. 3b shows the experimental data in the form of a ln(I/V) versus V1/2 relationship (Eq. (3)). It is important to note that a reasonably good fit was also obtained on plotting ln(I) versus V1/2 (Eq. (5)). Our previous work (ellipsometric measurements at 632 nm) on SWCNT LBL film (characterised by material preparation similar to that of IJP) indicate a permittivity of 2.16 [24]. The pre-exponential electric field term in Eq. (1) is often neglected by workers fitting the Poole–Frenkel theory to experimental data (on the basis that the exponential term will be dominant). However, it has been shown that this can be significant under certain conditions, particularly at very high applied voltages [27]. Dielectric constants have recently been reported for SWCNT/polythiophene composites [28]. At room temperatures, the real part of the permittivity of the composite (10:1 weight ratio polymer:SWCNT) was only slightly greater than that of the conductive polymer (approximately 4.5 for the composite compared to 4 for polythiophene at 4 kHz). In our case, the permittivity of the surfactant coating of the SWCNTs used for IJP is expected to be in the range 2 to 3, consistent with the value measured.
756
CARBON
4 7 ( 2 0 0 9 ) 7 5 2 –7 5 7
The value of measured permittivity was fitted in both the Poole–Frenkel and Schottky equations to determine the electrode separation d. The fitting of the data in Fig. 3a to Poole– Frenkel model led to 2.4 lm, within the 2–5 lm range of the actual electrode separation. In contrast, the Schottky model fit of the same data produced an electrode separation an order of magnitude smaller, 0.26 lm, well outside the 2–5 lm range. The slopes extracted from Eqs. (3) and (5) may vary slightly given that they are experimentally estimated. However, the values of the electrodes separation determined (using Eq. (6)) clearly suggest that: (i) Poole–Frenkel conduction mechanism is dominant; (ii) the Schottky model cannot be applied in this instance as the theoretical values are not consistent with the experimental findings. The results of vapour sensing experiments using methanol revealed that the current through the films decreased as the concentration of methanol in the carrier gas increased. Different bias voltages show a similar sensing response with higher sensitivities at lower voltages. Data are shown in Fig. 4 for methanol concentrations from 300 ppm to 2000 ppm with 3 V applied voltage. The film resistance recovered almost completely when the vapour was turned off. The response of the sensor to the vapours can be given in terms of the relative variation of the measured current, DI, where ðIvapour Io Þ 100% ð7Þ DI ¼ Io Ivapour is the current through the sensor when exposed to an atmosphere containing a known vapour concentration and Io is the current through the sensor when exposed to the carrier gas only. The calibration curve for the film response was obtained by plotting DI against the concentration of methanol in the carrier gas, and is shown as the inset to Fig. 4. Previous work has also demonstrated the electrical resistance of carbon nanotubes is influenced on exposure to gases such as CH4 and O2 [29]. It was suggested that the effect resulted from the adsorption of oxygen molecules on the surface of the SWCNTs, thereby doping them. In other studies,
4.6 4.4 300 ppm 500 ppm
4.0
4.
Conclusions
Thin films containing SWCNTs have been fabricated using the IJP technique. The morphologies of the films were studied using AFM. IJP films were electrically conductive and exhibited non-linear current versus voltage behaviour. It is suggested that the non-linear conductivity is related to the Poole–Frenkel mechanism. A preliminary study of the IJP thin films as chemical sensors revealed a decrease in the conductivity on exposure to methanol vapour. The inkjetprinted SWCNT films possessed high values of DI, but with relatively long response and recovery times (exceeding three minutes).
Acknowledgements
30
3.8
This work has partly been supported by the University Innovation Centre (UIC) in Nanomaterials, funded by the UK Government Department of Trade and Industry and One NorthEast.
25
3.6
1000 ppm
Δ I (%)
Current (µA)
4.2
exposure to methane was thought to affect the hybridisation state of the bonding within the carbon nanotubes [30]. We offer an alternative explanation for our vapour sensing results. On exposure to methanol, the vapour will diffuse into the thin SWCNT film, leading to a distribution of methanol molecules on the surface of the nanotubes and in the regions between them. From our electrical conductivity studies above, it is likely that the DC conductivity of the inkjet-printed films is influenced by the relatively insulating regions between the conductive carbon nanotubes. The incorporation of methanol in these regions will almost certainly increase their permittivity; the relative permittivity of methanol is approximately 34, compared to a value of 2–3 expected for the surfactant coating of the SWCNTs. According to Eqs. (1) and (4) above, an increase in the permittivity will result in a decrease in the current through the film. The response and recovery of the sensing devices will be limited by the rate of diffusion of the vapour molecules into and out of the film, accounting for the relatively long response and recovery times evident in Fig. 4 (180 s for the response and 210 s for the recovery for a 2000 ppm exposure to methanol). The sensitivity of the IJP films (slope of the inset curve in Fig. 4) was about 0.015% ppm1. This figure is less than values reported for inkjet-printed films of polymers exposed to methanol, e.g. 0.05% ppm1 for a film of poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate), PEDOT-PSS [19,20]. However, the response times of the SWCNT devices reported here were slightly faster, perhaps reflecting the more ‘open’ structure of the inkjet-printed films of the nanotubes.
3.4
20 15 10 5
3.2
500 1000 1500 2000 Concentration of methanol vapour (ppm)
2000 ppm
3.0 0
20
40
60
80
100
120
R E F E R E N C E S
Time (min) Fig. 4 – The variation of current through an inkjet-printed SWCNT film when exposed to step changes in the concentrations of methanol vapours. The inset shows the variation in DI as a function of the concentration of methanol vapour.
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