nanoscale chemical sensor based on organic thin-film transistors
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Nanoscale chemical sensor based on organic thin-film transistorsLiang Wang, Daniel Fine, and Ananth Dodabalapur Citation: Applied Physics Letters 85, 6386 (2004); doi: 10.1063/1.1842364 View online: http://dx.doi.org/10.1063/1.1842364 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/85/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Hybrid organic/inorganic ambipolar thin film transistor chemical sensor Appl. Phys. Lett. 98, 213504 (2011); 10.1063/1.3583594 Dual-gate organic thin film transistors as chemical sensors Appl. Phys. Lett. 95, 133307 (2009); 10.1063/1.3242372 Ultralow drift in organic thin-film transistor chemical sensors by pulsed gating J. Appl. Phys. 102, 034515 (2007); 10.1063/1.2767633 Planar nanoscale architecture for organic thin-film field-effect transistors Appl. Phys. Lett. 89, 203118 (2006); 10.1063/1.2388569 Fabrication of 70 nm channel length polymer organic thin-film transistors using nanoimprint lithography Appl. Phys. Lett. 81, 4431 (2002); 10.1063/1.1526457
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Nanoscale chemical sensor based on organic thin-film transistorsLiang Wang,a) Daniel Fine, and Ananth Dodabalapurb)
Microelectronics Research Center, the University of Texas at Austin, Austin, Texas 78758
(Received 24 August 2004; accepted 27 October 2004)
Nanoscale organic thin-film transistors were fabricated to investigate their chemical sensingproperties. The use of a four-terminal geometry ensures that the sensor active area is truly nanoscale,and eliminates undesirable spreading currents. The sensor response was markedly different innanoscale sensors compared to large-area sensors for the same analyte–semiconductor combination.The chemical sensing mechanisms in both microscale and nanoscale transistors are brieflydiscussed. ©2004 American Institute of Physics. [DOI: 10.1063/1.1842364]
Fabrication methods for, and the properties of, nanoscaleorganic and polymer field-effect transistors are being inves-tigated by many groups. In recent work, we have reportedthe electrical properties of nanoscale(channel lengths5–50 nm) pentacene transistors.1 One of the more promisingapplications for nanoscale organic transistors is in chemicalsensing. A nanoscale device geometry(in conjunction withthe use of patterned receptors) allows us to detect very smallnumbers of analyte molecules. Such detection will be diffi-cult in large-area devices in which the incremental change inresponse produced by low analyte concentrations is smalland may be lost in noise. Although there have been reportsfor the sensing effects of large-area organic and conjugatedpolymer transistors,2–4 such work has not been reported intransistors of nanoscale dimensions. It is likely that the elec-trical transport and sensing mechanism would be differentfrom that of large-scale devices. In this letter, we fabricatedtransistors with a series of channel lengths from tens of mi-crons to 20 nm. The grain size of the organic semiconductorwas also varied to investigate the role of scale in organictransistor sensing behaviors. Pentacene was chosen as theactive layer due to its relatively high mobility and wide usein organic electronics.5,6
The transistor device structure is shown in Fig. 1(a). Aheavily dopedn-type silicon substrate serves as the gate.Transistors of channel length greater than 1mm utilize the100 nm thermally grown SiO2 layer as the gate dielectricupon which the electrodes were defined by photolithography.For nanoscale transistors(channel lengths below 1mm),windows of 50mm size were etched with buffered oxideetch solution out of the 100 nm field oxide layer and a 5 nmSiO2 layer as the gate dielectric was grown by rapid thermalannealing in oxygen. The electrodes of nanoscale transistorswere patterned by electron-beam(e-beam) lithography on aJEOL JBX-5DII. 3 nm Ti/45 nm Au was then deposited bye-beam evaporation, followed by a lift-off process. Electrodepatterns of channel lengths in series from 36mm down to60 nm with channel width-to-lengthsW/Ld ratio of 10 orgreater were obtained. For smaller channel length devices,we use aW/L of about 2–3. Bottom-contact devices werecompleted by thermally evaporating 300–600 Å of penta-cene(purchased from Aldrich) with a base vacuum of about3310−7 Torr at different growth rates and different substratetemperatures for different grain sizes. These transistors were
operated as chemical sensors with a pentacene layer exposedto the airborne analyte(1-pentanol).
An Agilent 4155C semiconductor parameter analyzerwas used to examine the transient variation of drain currentIds under fixed gate voltage(Vg) and source–drain voltage(Vds) in air at room temperature. A peristaltic pump was em-ployed to drive air as a carrier through a transfer line(TY-GON tube) into a monojet syringe to deliver the saturationvapor of 1-pentanol there to sensing devices. The syringewas fixed onto a three-dimensional micromanipulator. A so-lenoid valve was placed in the middle of the transfer line toswitch the analyte delivery. Within the entireIds measure-
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FIG. 1. (Color online) (a) The structure of a bottom contact device with50 Å SiO2 as gate dielectric.(b) Characterization of a 60 nm channelsW/L=10d used for sensing measurements, measured in a vacuum of 2.4310−3 Torr, calculated mobility=0.021 cm2/V s.
APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 26 27 DECEMBER 2004
0003-6951/2004/85(26)/6386/3/$22.00 © 2004 American Institute of Physics6386 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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ment period of 60 s, the analyte was delivered only duringthe middle 20 s(the shaded area in each figure), to offer acomparison of the current behavior before, during and afterthe exposure to the analyte. The drain current was recordedevery 400 ms. Before sensing measurement, each device wastested with pure air and no sensing effect on drain currentwas detected. After each single sensing measurement, a re-verse bias configuration(high positive gate voltage) was ap-plied to recover the device characteristics. The measuredleakage currents through the gate dielectric were negligiblecompared to the drain currents. After the measurements, allof the devices were examined by a field-emission scanningelectron microscope(SEM) (LEO1530) and no deteriorationin morphology was found.
High growth ratess4.4–7.1 Å/sd at room temperatureduring thermal evaporation will give a pentacene layer ofsmall grains with an average size of 80–140 nm. Nanoscalechannels comprising of larger pentacene grains, with an av-erage size of 250 nm, were achieved by slow growth rates0.5 Å/sd at room-temperature deposition. For channel
lengths greater than a micron, an elevated substrate tempera-ture s80°Cd and medium growth rates1.9 Å/sd during depo-sition resulted in large grains with an average size of 1mm.The mobilities of the transistors were of the order of10−2 cm2/V s, with a highest value of 0.045 cm2/V s ob-served in air for the transistors with pentacene grain size of1 mm and channel length of 36mm. Figure 1(b) shows thecharacterization of a 60 nm channelsW/L=10d with penta-cene grain size of 80 nm, measured in a vacuum. Its mobilitywas calculated to be 0.021 cm2/V s.
In nanoscale transistors, it is technically difficult to pat-tern the active semiconductor within the channel area, andthereby for devices with smallW/L ratios, the spreading cur-rents which travel outside the defined channel cannot be ig-nored. To collect these spreading currents, guard electrodeswere designed at two sides of the channel and kept at thesame potential as the drain, so that the direct current fromsource to drain was collected at the drain.1 The use of thisfour-terminal geometry ensures that the sensor active area istruly nanoscale. We investigated the response ofIds (operatedin saturation region) upon exposure to the saturation vapor of1-pentanol, with a series of channel length and varied grainsizes of pentacene under the same experimental conditions.As shown in Fig. 2(a), while the long channel length devicesall exhibited a decrease in current upon delivery of the ana-lyte, the small channel length devices showed an increase,sometimes by a factor of.5. There are two mechanismsinfluencing sensor behavior: One causing a decrease in cur-rent (dominant in largeL devices) and one causing an in-crease(dominant in smallL devices). The crossover of re-sponse behavior depends on grain size, occurring in theinterval of channel length 150–450 nm for,80 nm grainsize. Under the same condition, when the average grain sizeof pentacene is increased to 250 nm, the sensors exhibits thecrossover behaviors at larger channel lengths(from450 nm to 1mm), as shown in Fig. 2(b). Figure 2(c) showsthe SEM image(after measurements) for a 150 nm channelwhich is smaller than the covering pentacene grains(,250 nm).
FIG. 2. (Color online) (a) Sensing data ofIds (normalized to that measuredjust before the analyte was delivered) for 80 nm pentacene grain size anddifferent nanoscale channel lengths(same W/L of 10), measured atVg
=Vds=Vside=−2.5 V (two side guards were kept at the same potential as thedrain), d sdistance from syringe nozzle to deviced=2 mm, v sanalyte fluxd=45 ml/min.(b) Sensing data of normalizedIds for 250 nm pentacene grainsize, measured at the same conditions as(a). (c) SEM image taken aftersensing measurements of a 150 nm channel with an average pentacene grainsize of 250 nm, scale bar=400 nm. The grains appearing in the figure arepentacene.
FIG. 3. (Color online) Sensing data of normalizedIds under the condition ofVg=Vds=−25 V, v=45 ml/min, andd=2 mm for different microscale chan-nel lengths, with average pentacene grain size of 140 nm and 1mm in (a)and (b), respectively.
Appl. Phys. Lett., Vol. 85, No. 26, 27 December 2004 Wang, Fine, and Dodabalapur 6387
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Figures 3(a) and 3(b) is the sensing responses of longchannel devices with pentacene grain size of 140 nm and1 mm, respectively. For all devices with channel lengths of2 mm or greater,Ids manifested decreasing responses uponanalyte delivery. The amplitudes of decreasing signal for2 mm channels were smaller than those of longer channels.This effect is stronger with larger pentacene grains[Fig.3(b)]. These results are consistent with the reported work forsensing effects dependent on organic grain sizes and channellengths in large scale.2,7,8 The sensing responses shown inFigs. 2 and 3 are reproducible for different devices with thesame channel lengths and grain sizes, indicating that the re-sponse of pentacene transistors to the 1-pentanol vaporchanges from decreasingIds to increasingIds, when the chan-nel length shrinks from micron to 100 nm, with a crossoverhappening in a transition interval of channel length which isrelated to the grain sizes of pentacene.
To examine the influence of analyte delivery on the sens-ing responses, we adjust the analyte fluxsvd and the syringenozzle–device distancesdd. It turns out that for all channellengths and grain sizes, increasingv and decreasingd havesimilar influences, i.e., to increase the amplitude of the sens-ing signal. Also it was found that the sensing behavior wassimilar operated in saturation region or linear region. Figure4(a) gives an example of this sensing test on a 22 nm chan-nel with an average grain size of 80 nm, measured under theoperation in linear region. Figure 4(b) shows the SEM imageof this device taken after sensing measurements.
The modeling for the mechanism of the sensing responsewill be published elsewhere. Here, we briefly propose theconcept of the sensing mechanism of polycrystalline organicthin-film dependent on the channel length relative to grainsizes. In the organic semiconductor layer, both grains andgrain boundaries could be affected by the analyte molecules.Due to their dipole nature, the analyte molecules airborne ongrain boundaries will trap the mobile charge carriers fromthe channel.2,4,7,8 Meanwhile the analyte(1-pentanol) inter-acting with the semiconductor(pentacene) grains will resultin excess holes through chemical processes that are not com-pletely understood. We propose that the overall sensing re-sponse is the result of a combination of these two competingeffects. For a longer channel relative to grain sizes, there areenough grain boundaries inside the channel so that theformer effect is dominant and the overall sensing response isthe current decreasing(mobility reduced by the trapping ef-fect at grain boundaries).2,3 For a shorter channel relative tograin sizes, the latter effect dominates due to very few num-ber of grain boundaries inside channel, which leads to thecurrent increasing by excess charges from the interaction be-tween grains and the analyte.
In summary, we describe the first results of the chemicalsensing properties of nanoscale organic transistors. The sens-ing behavior of these small dimension devices is markedlydifferent from that of larger devices for the same analyte.The differences in the nature of response as a function ofscale are related to two physical effects produced by theanalyte: One dominant at small channel lengths and the otherat larger channel lengths. This work extends the findings ofSomeyaet al.7 and Torsiet al.8 to nanoscale dimensions. Itpoints to scale being a very key element in the sensing pro-cess with different mechanisms dominating at differentlength scales. Further experiments to investigate the interac-tion between the analyte molecule and the organic semicon-ductor as sensing layer are undergoing in this group. Re-cently, there is increasing evidences that the interfacebetween semiconductor and gate insulator is also a factorwhich influences the sensing behavior.9
The authors thank Professor Heinz von Seggern(Tech-nical University of Darmstadt) and Taeho Jung and SuvidNadkarni for helpful discussions. The authors thank the sup-port from grants by NSF NIRT, DARPA, and AFSOR. Theyalso thank the CNM and TMI at the University of Texas-Austin for use of facilities.
1L. Wang, D. Fine, T. Jung, D. Basu, H. von Seggern, and A. Dodabalapur,Appl. Phys. Lett.85, 1772(2004).
2B. Crone, A. Dodabalapur, A. Gelperin, L. Torsi, H. E. Katz, A. J. Lov-inger, and Z. Bao, Appl. Phys. Lett.78, 2229(2001).
3Z.-T. Zhu, J. T. Mason, R. Dieckmann, and G. G. Malliaras, Appl. Phys.Lett. 81, 4643(2002).
4L. Torsi, M. C. Tanese, N. Cioffi, M. C. Gallazzi, L. Sabbatini, and P. G.Zambonin, Sens. Actuators B98, 204 (2004).
5H. Klauk, M. Halik, U. Zschieschang, G. Schmid, W. Radlik, and W.Weber, J. Appl. Phys.92, 5259(2002).
6D. Knipp, R. A. Street, and A. R. Völkel, Appl. Phys. Lett.82, 3907(2003).
7T. Someya, H. E. Katz, A. Gelperin, A. J. Lovinger, and A. Dodabalapur,Appl. Phys. Lett.81, 3079(2002).
8L. Torsi, A. J. Lovinger, B. Crone, T. Someya, A. Dodabalapur, H. E.Katz, and A. Gelperin, J. Phys. Chem. B106, 12563(2002).
9M. C. Tanese, D. Fine, A. Dodabalapur, and L. Torsi(in preparation).
FIG. 4. (Color online) (a) Sensing data of a 22 nm channel withVg=−2 V, Vds=Vside=−0.4 V and v=45 ml/min for different d (nozzle–device distance), “reference” =absence of analyte.(b) SEM image of thedevice in(a) taken after measurements, grain,80 nm, scale bar=100 nm.The appearing grains are pentacene.
6388 Appl. Phys. Lett., Vol. 85, No. 26, 27 December 2004 Wang, Fine, and Dodabalapur
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