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Fabrication of planar organic nanotransistors using low temperature thermal nanoimprint lithography for chemical sensor applications

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Please note that terms and conditions apply. IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 21 (2010) 075301 (7pp) doi:10.1088/0957-4484/21/7/075301 Fabrication of planar organic nanotransistors using low temperature thermal nanoimprint lithography for chemical sensor applications

J Kettle1,5, S Whitelegg1,AMSong1,DCWedge2, L Kotacka3, V Kolarik3,MBMadec4,SGYeates4 and M L Turner4

1 Microelectronics and Nanostructures Group, School of Electrical and Electronics Engineering, University of Manchester, Sackville Street, Manchester M60 1QD, UK 2 Manchester Interdisciplinary Biocentre, University of Manchester, 131 Princess Street, Manchester M1 7ND, UK 3 Optaglio, s.r.o., Rez 199, 250 68, Czech Republic 4 Organic Materials Innovation Centre (OMIC), School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK

Received 13 November 2009 Published 18 January 2010 Online at stacks.iop.org/Nano/21/075301

Abstract A new fabrication process for the patterning of organic semiconductors at the nanoscale has been developed using low temperature thermal nanoimprint lithography and the details of this process are discussed. Novel planar nanotransistors have been fabricated and characterized from poly(3-hexylthiophene) (P3HT) and we demonstrate the feasibility of using such devices as highly sensitive chemical sensors. (Some figures in this article are in colour only in the electronic version)

1. Introduction being fabricated via NIL technology [8]. In this technique, the source and drain electrodes are patterned using NIL followed A key advantage of organic over inorganic semiconductors is by the chemical vapor deposition (CVD) of pentacene on top the ability to use low cost, large area printing technologies of the nanogap electrodes for the active layer. Similarly, such as roll-to-roll, with the potential of incorporating Zhang et al have demonstrated a novel UV-transfer embossing nanoimprint lithography (NIL) into the process chain. Organic technique, whereby electrode arrays were patterned, with an field effect transistors (OFETs) based on low cost solution- estimated 5 µm gap, onto a substrate, followed by spin-casting processable materials have yielded impressive improvements of P3HT [9]. Nanofeatures using micro-contact printing of in performance, with mobilities of up to 2.5 cm2 V−1 s−1 being poly(dimethylsiloxane) stamps has been demonstrated on large demonstrated [1, 2]. With conventional printing techniques, area, flexible substrates, though the yield and reproducibility the minimum feature size is currently limited to around 20 µm, of this technique remain an issue [10]. However, to the best which is suitable for most device geometries used in 3D-OFET of our knowledge no ‘direct’ patterning of the active organic fabrication but only allow quite low performance, particularly semiconductor layer for OFETs by NIL has been accomplished to date. Nanoimprint lithography is an important emerging in terms of operational speed [3, 4]. Recently, conventional technology and is not restricted to small areas, and roll-to- photolithography techniques have been modified to enable roll processing [11] and large area imprinting are achievable. patterning of organic semiconductors down to feature sizes Structures with sub-10 nm resolution have been reported in the of 1 µm[5Ð7], with OFETs having submicron geometries literature [12]. 5 Present address: Organic Materials Innovation Centre (OMIC), School of Recent work has developed a novel 2D device based on the Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, design of a self-switching diode (SSD) fabricated from atomic- UK. force microscope (AFM) nanolithography of P3HT or PQT-

0957-4484/10/075301+07$30.00 1 © 2010 IOP Publishing Ltd Printed in the UK Nanotechnology 21 (2010) 075301 J Kettle et al 12 [13]. The SSD is a planar device, based on a single-layered structure with ohmic contacts also formed within the plane of the semiconductor nanochannel [14]. This is in contrast to conventional multi-layered vertical diode structures that are either based on a pÐn doping junction or a barrier structure. Recently, we have demonstrated that organic SSDs could be fabricated using NIL, which would enable higher throughput and greater reproducibility than what is achievable using AFM lithography [15]. Planar nanotransistors, commonly referred to as an in- Figure 1. Schematic diagram illustrating the low temperature plane gate (IPG) transistor, were first introduced in an nanoimprint lithography process utilized in this paper for fabricating IPG transistors. Stage (1) involves spin-coating the PMMA, P3HT AlGaAs/GaAs two-dimensional electron gas (2DEG) by and PBMA layers, (2) involves embossing at low temperature to Wieck et al [16, 17]. In contrast to conventional FETs, where produce a thickness contrast, while at stage (3) the insulating lines in the metal gate is placed on top of the channel and gate insulator, the P3HT are formed by argon milling through the residual PBMA IPG transistors have a layout with the source, drain, current layer and P3HT. Finally, we removed the PBMA material (4). channel and one or two gates lying in the same plane. IPG transistors can realize control of the electric field parallel to the substrate (commonly, this is an inorganic two-dimensional molecular weight 337 000; glass transition temperature (Tg) ◦ electron gas), which results in a strong potential confinement 13Ð35 C[23] purchased from Sigma-Aldrich) dissolved in in the 1D channel. This planar device architecture can be anisole (7.5 wt%), a thermoplastic for geometric patterning, utilized to avoid contact resistance problems found for 3D was spin-coated onto the substrate at 3 krpm for 2 min and ◦ OFETs with nanoscale channel lengths. IPG transistors and baked for 30 min at 80 C in order to evaporate the solvent. We similar devices have now been realized in several material tested a range of polymer/solvent combinations and observed systems including SiGe/Si [18], GaAs/AlGaAs [19], silicon- that the minimum decrease in final film conductivity was on-insulator [20], indium tin oxide [21] and diamond-based obtained using this formula. materials [22]. Additionally, IPG based on n-type and p-type Imprinting was done using an in-house manufactured channels have been realized on a single substrate [23]. hydraulic hand press. Heating is achieved using cartridge This paper aims to demonstrate that IPG transistors can heaters, connected to a Eurotherm 301 PID temperature be fabricated into organic semiconductors using regioregular controller with a K-series thermocouple. During imprinting, P3HT. We have developed a new approach to patterning both the mold and PBMA-coated substrates were heated to polymer semiconductors at the nanoscale, which is described 60 ◦C, which is above the glass transition temperature range in more detail. Furthermore, we demonstrate the suitability of PBMA, enabling the polymer to become rubbery. The of P3HT-based IPG transistors as highly sensitive chemical temperature is raised at an initial rate of 30 ◦Cmin−1.Prior detection sensors. to the compression of the substrate and shim, the temperature was held at 60 ◦C for 10 min, to ensure an even distribution of 2. Experimental details heat across the sample surface and metallic heating elements. The mold was then pressed into the PBMA resist using a force The device structure used in these studies is shown in the of 1 kN and held at this force for 10 min to create a thickness schematic diagram in figure 1. For the device, n++ Si contrast in the PBMA. The nanoimprint templates, used in substrates with a 300 nm dry oxide layer were prepared this work, were fabricated into a silicon master by Qudos Ltd, using conventional solvent cleaning techniques. Wafers were in the UK and replicated into nickel by 3-DAG AG. After supplied by Si-Mat GmbH from Germany. The root mean 10 min hold time under high force, the sample was cooled square (RMS) surface roughness of this material was measured to room temperature using rapid air cooling. The advantage at 1.1 nm over a 20 µm × 20 µm area after cleaning. of using this embossing system is that it is possible to cool A thin layer (∼8 nm) of polymethylmethacrylate (PMMA, the substrate and stamp to room temperature without releasing Aldrich, weight average molecular weight 950 000) was spin- the force. This was a concern when working with PBMA coated (0.5 wt% in anisole) onto the substrate to improve the thermoplastic, due to the low Tg and is discussed further in the morphology of the P3HT and reduce the pin-hole density. results section. The mold was then separated from the PBMA ◦ The PMMA layer was hard baked at 220 ◦C, enabling the resist after cooling to 20 C to ensure that the thickness contrast PMMA films to be resistant to dissolution by trichlorobenzene in the PBMA was preserved. The residual thin resist in the (TCB). The use of a hard-baked PMMA buffer layer has been compressed region was removed by anisotropic argon milling previously shown to improve device stability by Sun et al [24]. (25 W, 25 sccm, 20 mTorr). This process created isolating lines Regioregular P3HT (regioregular ratio >94%, weight in the P3HT film, separating the gate from the source and drain. average molecular weight 55 kDa) was purchased from Rieke Argon milling was preferred over the conventional oxygen (O2) Metals Inc., dissolved in TCB at a concentration of 2.5 wt%, plasma etching technique, due to fears that oxygen etching was spin-coated onto the PMMA-coated substrate at 3 krpm would heavily dope the P3HT, which would later inhibit device for 4 min and annealed in nitrogen for 1 h at 120 ◦C. performance. Finally, the PBMA was stripped from the sample Subsequently, polybutylmethacrylate (PBMA; weight average surface by placing the sample for a short time in an acetone

2 Nanotechnology 21 (2010) 075301 J Kettle et al

Figure 2. AFM image of the active area of the IPG, where the darkened areas indicate isolating trenches. DrainÐsource current can be controlled by the left or right gate and additionally back-gated via the n++ Si substrate. The surface morphology of the P3HT demonstrated very low surface roughness, which was attributed to a PMMA buffer layer, which reduced grain boundaries. Image size is 8 µm × 8 µm. bath and gold contact pads were added. The performance of the fabricated OFETs was measured using an Agilent E5280B- F Precision Source/Monitor Unit Modules with Cascade probe station.

3. Results

The IPG transistor fabricated using P3HT, incorporating the Figure 3. Electrical performance showing (a) transfer characteristics and (b) output characteristics (with VDS =−80 V) of the P3HT PMMA buffer layer, is shown in figure 2. The transferred IPGÐFET. Note that only the side gate output characteristics are features have similar dimensions to those in the mold, shown. indicating that the nanoimprint process has high fidelity during pattern transfer. Conventionally, in NIL, release of the imprint is done at could be done at low temperatures. Early attempts were ◦ 10Ð50 CbelowTg of the imprint material to preserve pattern made to imprint using conventional NIL polymers such as definition. However, this is not practical when working with PMMA and using the mr-I 7000 series. One of the limiting PBMA as the Tg is much lower than that of conventional NIL issues we discovered when using these materials was that polymers. Releasing at a temperature above Tg should result in embossing must be done at a temperature much greater than ◦ some loss of the pattern definition in the PBMA, as the material the Tg of P3HT (Tg ∼ 50 C). As a consequence, we remains in a rubbery state. However, we see no evidence that observe significant lateral movement of the P3HT film in areas pattern definition is lost. We believe this is due to two factors: which are compressed as a result of features in the stamp. (1) Tg is not a single defined temperature which separates the This prevents us from achieving a continuous film of P3HT glassy and rubbery states but is in fact a temperature range after embossing. Using PBMA, good pattern uniformity was dependent upon the observation frequency and the molecular achieved over the entire 2 cm × 2 cm imprint lithography field. weight of the polymer; and (2) we are using high molecular The IPG transistors fabricated have a geometrical channel weight PBMA where the extent of polymer chain entanglement width Wgeo = 380 nm, an insulating trench width, Wt = ◦ is high and at 20 C the polymer chains will be in a range of 400 nm and an etch depth equal to the thickness of the P3HT + physical states from an entangled glassy polymer through to a (25 nm). Using low power Ar milling, the SiO2 on the rubbery entangled polymer [25]. Hence, flow of the polymer substrate surface acts as an etch stop layer, preventing further chains requires highly cooperative chain disentanglement and depth increases. is very slow on the timescale of the experiment, resulting in Figure 3(a) shows the measured room temperature output the retention of pattern definition. This is supported by the characteristics of the P3HT IPG transistors with different gate observation that pattern definition is lost when releasing at T = voltages, VGS, applied to its left-hand in-plane gate (with 35 ◦C where all polymer chains are in the rubbery state and the right in-plane gate left floating). In the measurements, flow is greatly increased on the timescale of the experiment. the source contact (bottom contact from figure 2)ofthe It is important to note that PBMA was preferred over transistor was grounded and a drain voltage, VDS, was applied conventional NIL polymers such as PMMA as processing to the drain contact (top contact from figure 2). Generally,

3 Nanotechnology 21 (2010) 075301 J Kettle et al electrical measurements of the effect of the right gate of the IPG transistors show similar currentÐvoltage characteristics to that of the left gate. The devices shown in this work are p-channel, enhancement-mode FETs. The negatively biased IPG (in-plane gate) creates an electric field, which is directed in the plane of the P3HT towards the channel; this directly influences the charge carrier density in the channel. Thus, increasing the negative bias enables current to pass through the so-created thin, conductive channel. The negative drain voltage consequentially attracts holes within the channel towards the drain. For the device, the measured current ID is plotted against VDS at 40 V step increments of VGS, varying from −100 to 20 V in steps of 40 V. The device operates up to a regime VDS ≈ VGS, which is important in several applications and for device integration. The device can be pinched-off at room Figure 4. Change in threshold voltage (V ) and onÐoff ratio temperatures and the sourceÐdrain currents saturate reasonably TH (ION/IOFF) performance of the P3HT IPGÐFET with (dashed line) well. and without (solid line) a PMMA buffer over a 10 day period. In To study the devices quantitatively, we also measured the between measurements, devices were stored in dark with ambient air transfer characteristics of the IPG transistors, with V = and a humidity level of 40Ð50%. An AFM topography scan of the DS device without a buffer layer is shown as an inset to the figure. −80 V. In these measurements, the source contact was again grounded, but the voltage VD applied to the drain and the current I were measured as a function of the voltage V DS GS V , when the device is operated by the back gate; the applied to its left gate. Here, we see an approximately TH reason for this difference is twofold. Firstly, insulating parabolic curve (when plotted on a linearÐlinear plot) in the trenches between the gate and the channel are relatively wide gate voltage range of V  V  −80 V (where V is TH GS TH (W = 300 nm) and no dielectric infill has been used in this the threshold voltage) and thus the transistor shows a behavior g work. By utilizing a stamp with smaller lateral dimensions similar to a conventional FET. and/or dielectric material fill in the trenches, the field effect Viewing the electrical measurements of the side gate of would be enhanced and the ability to switch the device on the devices shown in figure 2, specifically the transfer curve and off would increase. Secondly, in organic semiconductor of the P3HT-SiO2 device (figure 3(b)), shows a threshold transistors, the off-current is crucially affected by the doping. ( / ) voltage, VTH, measured at 30 V. The onÐoff ratio ION IOFF As initially the areas on the P3HT films most affected by . × 3 was measured at 3 84 10 . Due to the choice of substrate doping are the top and trench sidewall surfaces, it is likely ++ used in this work (n silicon with 300 nm SiO2 top surface, that this will cause increased off-current when operated from acting as a dielectric), the device can also operate as a wire the side gate. However, with back-gating, the dominant transistor similar to those reported by Chou et al [26]. The conduction mechanism occurs along the film at the P3HTÐ device operates largely in enhancement-mode. A negative substrate interface and would therefore not be as heavily VGS is necessary to increase the conductance in the channel affected by doping. This is confirmed from the experimental and attract majority holes carriers within the channel towards data shown in figure 3. the dielectric/gate substrate, forming a conductive channel. Despite recent improvements in material mobility, it In order to contact the gate, direct probing was done using remains very challenging to fabricate P3HT-based organic tungsten probes to the n++ silicon surface. A diamond cleaver transistors in ambient conditions due to the undesirable was used to scratch away the top 300 nm thick SiO2 sample, so interactions of P3HT with atmospheric oxygen and moisture, that momentarily probing onto the underlying Si surface can which results in doping and charge trapping [27, 28]. Recent be achieved. Surprisingly, the performance of this contact is work by Sun et al showed that, by incorporating a PMMA very robust and reproducible, as contact resistance is minimal. dielectric layer when fabricating bottom gate FETs, a smoother As good practice, we would re-scratch the surface after every P3HT film could be obtained [24]. Consequentially, this measurement. Overlaid on the side gate transfer characteristics reduced the density of pin holes in the film, thus inhibiting in figure 3(b) is the back gate performance of the device. In water and air from penetrating into the film and degrading the these measurements, the source contact was again grounded electrical performance of the device. − and VDS was fixed at 80 V (within the saturation region), but Showninfigure4 are the onÐoff ratio (ION/IOFF) and the current IDS was measured as a function of the voltage VGS threshold voltage (VTH) as a function of time over a 15 day applied via the back gate (n++ silicon substrate), rather than period for the device shown in figure 2. A device without the side gate. When operated from the back gate, electrical the PMMA buffer layer is included in the graph to give a performance is improved; we observe a reduced threshold relative comparison; the topography measured using AFM of voltage of VTH = 15.7 V and an increased onÐoff ratio, this device is given as an inset into figure 4. For this device, 3 ION/IOFF = 7.02 × 10 . the process conditions were the same as the device fabricated When comparing the back gate to the side gate with a buffer layer. However, the P3HT was spin-casted performance, the device exhibit increased ION/IOFF and lower directly onto the bare SiO2 substrate at 3 krpm for 4 min and

4 Nanotechnology 21 (2010) 075301 J Kettle et al then annealed in nitrogen for 1 h at 120 ◦C. Subsequently, the PBMA thermoplastic was spin-coated onto the substrate at 3 krpm for 2 min and the process follows as previously discussed. Between measurements, the devices were stored in the dark with ambient air and a humidity level of 40Ð50%. It is immediately apparent that the device with a PMMA buffer layer remains relatively stable over the time; we see that ION/IOFF decreases by only 30% of the initial value over 10 days. However, this decrease is minor compared to the rapid degradation experienced by the without-buffer-layer device. Since it took approximately 20 min to transfer the devices to the measurement set-up, it is very likely that the initial doping has already had a significant influence, which explains why the first measured ION/IOFF of the device without a buffer layer is much lower than the with-buffer-layer device. With the no-buffer-layer device, we see that the doping rate decreases rapidly after 1 day and ION/IOFF remains approximately constant for a further 2 days, after which the device ceases operation due to high gate leakage. To gain a fuller understanding into the stability, the film surfaces can be studied using the AFM images. When considering the micrographs, the surface roughness (RA)of the P3HT film on the PMMA buffer layer was measured at 0.6 nm and the maximum rms peak-to-valley fluctuation was Zmax = 5.2 nm. This is an indication that the surface morphology of the film is likely to be amorphous in nature [29]. In being so, the likelihood of pinholes in the film is reduced, due to weaker crystallinity, hence oxygen and water cannot penetrate into the film and reduce device performance [26]. Conversely the device without a buffer layer was measured at 2.84 nm and the maximum peak-to-valley fluctuation rms was Zmax = 16.33 nm. As a result of the rougher morphology, a higher density of pinholes is likely, which would result in increased doping levels and the poor stability observed in figure 5. Previous studies have confirmed this effect, showing that increasing the density of grain boundaries between the Figure 5. Chemical sensing experiments comparing with the source and drain in conventional bottom gate OFETs results sensitivity of a conventional bottom gate OFET. A schematic of the in more hole carries trapped in the grain boundaries caused by device used to compare the sensitivity to that of the P3HT IPG water residing in the grain boundaries [32]. transistor is shown in (a), indicating the (i) Au source/drain contacts, (ii) P3HT layer, (iii) PMMA buffer layer and (iv) 300 nm SiO2/Si substrate. Device dimensions are 2000 µm and 60 µm. The 4. Chemical sensing experiments normalized (b) on-current and (c) mobility response to IPA vapor analyte are shown. The analyte is applied to the device at t = 180, Sensors based on organic semiconductors such as P3HT 570, 930 and 1290 s. Data for both devices is normalized to the initial measurement prior to analyte exposure. Both devices exhibit are very suitable for vapor detection applications, owing to good reversibility at t = 390, 750, 1100 and 1470 s. their ability to rapidly absorb and desorb analytes at room temperature [30]. One of the major advantages of organic- semiconductor-based sensors is that the chemical constitution and high surface area make organic-semiconductor-based IPG of the semiconductor can be varied by altering the end/side transistors very interesting for use in chemical sensing FET groups to recognize a range of gaseous analytes [31]. Gas sensors based on OFETs show changes in the channel sensors (ChemFETs). conductivity but also other parameters such as carrier mobility, In order to evaluate the sensitivity and applicability of threshold voltage and on/off ratio. The changes in these the P3HT IPGÐFET as ChemFET sensors, four analytes were parameters can be related quantitatively to vapor concentration used; methanol, acetone, IPA and TCE. The performance by calibration. Furthermore, as IPG transistors have a very of these devices was compared with that of a conventional high surface area to bulk ratio of the channel, chemical 3D bottom gate FET, a schematic of which is shown in analytes should strongly influence the electrical properties of figure 5 (inset), fabricated using the same materials as the device. The combination of multi-parameter response the IPGÐFET. Initially the devices were tested prior to

5 Nanotechnology 21 (2010) 075301 J Kettle et al analyte exposure. As such, even though we are not doing Table 1. Summary of sensing results comparing a P3HT-IPG quantitative measurement of the device sensing sensitivity, the transistor with a conventional bottom gate OFET; four different simultaneous measurement of both 2D and 3D devices allow analytes were tested, IPA, acetone, TCE and methanol. us to study whether the 2D nanodevices indeed have higher Analyte On-current Mobility sensitivity. In order to compare performance between the IPGÐFET Bottom gate FET IPGÐFET Bottom gate FET bottom gate FET and IPGÐFET, the relative performances were normalized against this initial value. We exposed the device IPA 0.44 0.96 0.45 0.98 to the analyte odor by placing a cotton bud nearby, with the Methanol 0.52 0.83 0.40 0.92 TCE 0.55 0.83 0.56 0.86 analyte accurately dispensed onto it. The analyte exposure was Acetone 0.89 1.28 0.64 0.98 started at t = 0s,andatt = 30 s, the response of the device was re-measured, having allowed sufficient time for the analyte to absorb into the semiconductor. Subsequently, the cotton bud transistor channel showed an enhanced response upon analyte was removed from the proximity of the device and we waited exposure. However, the increase reported is much smaller a further 180 s (t = 210 s), before re-measuring the device. At than that observed on changing from a conventional 3D-OFET t = 210 s, both devices showed good reversibility. This was architecture to the IPG device structure. repeated 4 times to obtain an average response. Whilst P3HT IPGÐFETs have been demonstrated as In conventional ChemFET sensors, four properties highly sensitive ChemFET sensors, it is worth discussing the (IDS(ON),µ, ION/IOFF and VT ) may be deduced for each limitations of such devices. Firstly, these devices are not transistor to study the response to analytes. Whilst we have suitable for a wide range of organic semiconductor material; looked at the effect of analytes on the performance of several the materials must be high mobility (µ>0.001 cm2 V−1 s−1) transistor parameters, we have found that the on-current and to produce a measurable sensor performance, which makes mobility offer the most reliable data from sensing tests on the the device architecture unsuitable for materials such as IPG transistors. The IDS(OFF) values were found to be highly polytriarylamines (PTAA), which possess a low mobility. variable because of their low magnitude, adversely causing Furthermore, when attempting to fabricate devices using the ION/IOFF measurement to be very sensitive to noise. different materials, intermixing between the different polymer Additionally, the extrapolation procedure used to calculate VT layers can become problematic, so solvent and concentration of makes this value very variable. Therefore, the sensing results the thermoplastic must be optimized for each semiconductor. for ION/IOFF and VT are excluded from these discussions. The Also, in order to ensure consistency between devices, process normalized response of the mobility and on-current to IPA conditions must be accurately optimized and controlled to vapor for both the bottom gate FET and IPGÐFET are shown in ensure no process variations. Such process variations can figure 5. Upon exposure to the analyte, both the mobility and result in dimensional variations, which have a large effect on on-current decrease. However, there is a remarkable difference subsequent device performance. Finally, the large sourceÐ when comparing the bottom gate FET and the IPGÐFET. drain resistance makes the device incompatible with direct Whilst for the bottom gate FET the reductions in normalized integration with previous real-time Op-Amp chemical sensor on-current and normalized mobility are only 5% and 8%, circuits [16]. respectively, the corresponding normalized responses for the However, the results of this paper indicate that P3HT IPGÐ IPGÐFET are 60% for on-current and 50% for mobility. We FET can operate as highly sensitive ChemFET devices. Such have studied systematically the response of the IPG device devices are known to operate at high frequencies due to their compared to the bottomgate FET upon exposure to four low parasitic capacitances [33], so could be potentially utilized different analytes using this experimental technique; the results as highly responsive devices. are summarized in table 1. In all cases, the results are very conclusive; the IPGÐFET shows improved sensitivity when 5. Conclusions compared to the bottom gate FET. The increase in sensitivity may result from increased absorption of the vapor molecules In conclusion, we report the fabrication of in-plane gate (IPG) into the conducting channel for the IPGÐFET, due to the transistors using organic semiconductors, in this case P3HT. increased relative surface area of the channel region. A figure In order to achieve this, we developed a new approach to of merit for the relative performance of the device can be patterning polymer semiconductors at the nanoscale, which devised by considering the on-current of each device (ION) as involves low temperature embossing using PBMA. Although a function of surface area of the channel (As). This figure of the transistor dimensions and material thickness have not been merit represents the possible change in electrical performance fully optimized, the current performance is very encouraging per unit surface area of the channel. For the bottom gate for sensor applications. The reproducibility of the devices −2 FET, ION/As = 8.95 A m . However, for the IPGÐFET, reported here is already sufficient for integration applications. −2 this figure of merit is much greater, ION/As = 1363 A m , Finally, we demonstrated the feasibility of using P3HT IPGÐ which reflects the difference in sensing performance. This FETs as highly sensitive chemical sensors. Qualitative is a similar effect as observed by Torsi et al, who studied measurements showed that altering the device geometry in the effect of nanosized grain boundary density upon sensor this manner enhanced the normalized device response when response in oligothiophene devices [32]. They found that compared to a conventional bottom gate FET for four different devices with a larger number of grain boundaries in the analytes.

6 Nanotechnology 21 (2010) 075301 J Kettle et al Acknowledgments semiconductor memory operating at room temperature Appl. Phys. Lett. 83 1881Ð3 [15] Kettle J, Whitelegg S, Song A M, Madec M B, Yeates S and The authors would like to thank Dr Marco Palumbo, Dr Yi Turner M L 2009 Fabrication of poly(3-hexylthiophene) Luo, Dr Yanming Sun, Dr Peng Bao and Mr Mal McGowan for self-switching diodes using thermal nanoimprint lithography their assistance. We thank the UK Home Office for supporting Proc. EIPBN Conf. (Marco Island, May 2009) this work through the project ‘Low Cost Sensor Arrays Using [16] Wieck A D and Ploog K 1990 In-plane-gated quantum wire Organic Semiconductors’ and the Royal Society for the Brian transistor fabricated with directly written focused ion beams Appl. Phys. Lett. 56 928 Mercer Feasibility Award. This work has been supported by [17] Nieder J, Wieck A D, Grambow P, Lage H, Heitmann D, Nano ePrint Ltd. Klitzing K v and Ploog K 1990 One-dimensional lateral-field-effect transistor with trench gate-channel insulation Appl. Phys. Lett. 57 2695 References [18] Tobben D, de Vries D K, Wieck A D, Holzmann M, Abstreiter G and Schaffler F 1995 In-plane-gate transistors [1] Anthony J E 2008 The larger acenes: versatile organic fabricated from Si/SiGe heterostructures by focused ion semiconductors Angew. Chem. Int. Edn 47 452Ð83 beam implantation Appl. Phys. Lett. 67 1579 [2] Rincon Llorente G, Dufourg-Madec M-B, Crouch D J, [19] Wieck A D and Ploog K 1992 High transconductance Pritchard R G, Ogier S and Yeates S G 2009 High in-plane-gated transistors Appl. Phys. Lett. 61 1048 performance, acene-based organic thin film transistors [20] Farhi G, Saracco E, Beerens J, Morris D, Charlebois S A and Chem. Commun. 21 3059 Raskin J-P 2007 Electrical characteristics and simulations of [3] Sirringhaus H, Kawase T, Friend R H, Shimoda T, self-switching-diodes in SOI technology Solid State Inbasekaran M, Wu W and Woo E P 2000 High-resolution Electron. 51 1245Ð9 inkjet printing of all-polymer transistor circuits Science [21] Kettle J, Perks R M and Hoyle R T 2009 Fabrication of highly 290 2123 transparent self-switching diodes using single layer indium [4] Sekitani T, Noguchi Y, Zschieschang U, Klauk H and tin oxide Electron. Lett. 45 79 Someya T 2008 Organic transistors manufactured using [22] Sumikawa Y, Banno T, Kobayashi K, Itoh Y, Umezawa H and inkjet technology with subfemtoliter accuracy Proc. Natl Kawarada H 2004 Memory effect of diamond in-plane-gated Acad. Sci. 105 4976Ð80 field-effect transistors Appl. Phys. Lett. 85 139 [5] Huang J, Xia R, Kim Y, Wang X, Dane J, Hofmann O, [23] Reuter D, Seekamp A and Wieck A D 2004 Fabrication of Mosley A, de Mello A J and Bradley D D C 2007 Patterning two-dimensional n- and p-type in-plane gate transistors from of organic devices by interlayer lithography J. Mater. Chem. the same p-doped GaAsÐInGaAsÐAlGaAs heterostructure 17 1043Ð9 Physica E 21 872Ð5 [6] Hwang H S, Zakhidov A A, Lee J, Andre X, DeFranco J A, [24] Sun Y, Lu X, Lin S, Kettle J, Song A and Yeates S 2009 Fong H H, Holmes A B, Malliaras G G and Ober C K 2008 Polythiophene-based field-effect transistors with enhanced Organic Electronics Dry photolithographic patterning process for organic air stability doi:10.1016/j.orgel.2009.10.019 electronic devices using supercritical carbon dioxide as a [25] Brandrup J and Immergut E H Polymer Handbook 3rd edn solvent J. Mater. Chem. 18 3087Ð90 (New York: WileyÐInterscience) [7] Chan J, Huang X Q and Song A M 2006 Nondestructive [26] Guo L, Krauss P R and Chou S 1997 Nanoscale silicon field photolithography of conducting polymer structures J. Appl. effect transistors fabricated using imprint lithography Appl. Phys. 99 0237101 Phys. Lett. 71 1881 [8] Leising G, Stadlober B, Haas U, Haase A, Palfinger C, [27] Sainova D, Janietz S, Asawapirom U, Romaner L, Zojer E, Gold H and Jakopic G 2006 Nanoimprinted devices for Koch N and Vollmer A 2007 Improving the stability of integrated organic electronics Microelectron. Eng. 83 831Ð8 polymer FETs by introducing fixed acceptor units into the [9] Zhang J, Ming Li C, Chan-Park M B, Zhou Q, Gan Y, Qin F, main chain: application to poly(alkylthiophenes) Chem. Ong B and Chen T 2007 Fabrication of thin-film organic Mater. 19 1472Ð81 transistor on flexible substrate via ultraviolet transfer [28] Chabinyc M L, Street R A and Northrup J E 2007 Effects of embossing Appl. Phys. Lett. 90 243502 molecular oxygen and ozone on polythiophene-based [10] Loo Y, Willett R L, Baldwin K W and Rogers J A 2002 thin-film transistors Appl. Phys. Lett. 90 123508 Additive, nanoscale patterning of metal films with a stamp [29] Majewski L A, Kingsley J W, Balocco C and Song A M 2006 and a surface chemistry mediated transfer process: Influence of processing conditions on the stability of applications in plastic electronics Appl. Phys. Lett. 81 562 poly(3-hexylthiophene)-based field-effect transistors Appl. [11] Ahn S H and Guo L J 2008 High-speed roll-to-roll nanoimprint Phys. Lett. 88 222108 lithography on flexible plastic substrates Adv. Mater. [30] Mabeck J T and Malliaras G G 2006 Chemical and biological 20 2044Ð9 sensors based on organic thin-film transistors Anal. Bioanal. [12]AustinMD,GeH,WuW,LiM,YuZ,WassermanD, Chem. 384 343Ð53 Lyon S A and Chou S Y 2004 Fabrication of 5 nm linewidth [31] Li B et al 2006 Volatile organic compound detection using and 14 nm pitch features by nanoimprint lithography Appl. nanostructured copolymers Nano Lett. 6 1598Ð602 Phys. Lett. 84 5299Ð301 [32] Torsi L, Lovinger A J, Crone B, Someya T, Dodablalapur A, [13] Majewski L A, Balocco C, King R, Whitelegg S and Katz H E and Gelperin A 2002 J. Phys. Chem. B Song A M 2008 Fast polymer nanorectifiers for inductively 106 12563Ð8 coupled RFID tags Mater. Sci. Eng. B 147 289 [33] Wedge D C et al 2009 Real time vapour sensing using an [14] Song A M, Missous M, Omling P, Peaker A R, OFET-based electronic nose and genetic programming Samuelson L and Seifert W 2003 Nanoscale two-terminal Sensors Actuators B 143 365Ð72

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