Organic Electronics 24 (2015) 96–100

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Organic Electronics

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Response enhancement mechanism of NO2 gas sensing in ultrathinpentacene field-effect transistors

http://dx.doi.org/10.1016/j.orgel.2015.05.0221566-1199/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: jiangch@nanoctr.cn (C. Jiang).

Misbah Mirza a,b, Jiawei Wang a,b, Liang Wang a, Jun He a, Chao Jiang a,⇑a CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Centre for Nanoscience and Technology, No. 11 Zhongguancun Beiyitiao,Beijing 100190, Chinab University of Chinese Academy of Science, No. 19A Yuquan Road, Beijing 100049, China

a r t i c l e i n f o

Article history:Received 11 March 2015Received in revised form 13 April 2015Accepted 16 May 2015Available online 18 May 2015

Keywords:PentaceneUltrathin filmTop contact configurationNO2 gas sensor

a b s t r a c t

By optimization of structural and physical–chemical properties at a precision level of molecules, organicsensing devices seek to realize the state of the art in monolayer based device applications. In our work,pentacene ultrathin film transistor has been integrated into the implement of a gas sensor based on anincrease in its mobility and a shift of its threshold voltage. A limit of detection of pentacene monolayerfield-effect transistor was found to be at sub ppm level when it is applied for detection of NO2. Comparedwith a thick layer sensor device, the pentacene monolayer NO2 sensor has boosted up sensing responsewith three orders of magnitude. An enhancement of high selectivity and response mechanism can beunderstood with a reasonable description emphasizing on the 2D transport characters and a series ofgauzy interplay for monolayer pentacene film with NO2 analyte.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, much research efforts have been devoted to gassensors due to the fact of severe air pollution. NO2, a main pollu-tant arising from combusted fuel, can cause photochemical smog,acute pulmonary, acid rains, edema, and irritation of eyes and iseven suspected to cause cancer [1–3]. To date, commercial NO2sensors are usually bulky, have low selectivity and high powerconsumption. Therefore, it is highly desirable to develop novelgas sensors that are endowed with efficient manufacturing, highthroughput, and good device performance such as high sensitiv-ity/selectivity, light weight and low power consumption.

As a sensor alternative, organic field effect transistor (OFET) hasbeen demonstrated and continuous demand for electronic sensortechnology is a motivating initiator for its development. A typicalOFET gas sensor is achieved by non-covalent bond interactionbetween analytes and organic semiconductors. The field effectmobility, on/off ratio, threshold shift and change in conductivityused to characterize the sensing response of an OFET sensor for agiven gaseous analyte constitute the output parameters.Therefore, an enhancement interaction between the electronicsensors with them is foreseen as a core element in intriguingOFETs capable of detecting and delivering such responses.

Recently, the investigation of ultrathin transistors has got atten-tion as an attractive research subject especially in sensor technol-ogy due to its prominent properties such as quick adsorption,diffusion and desorption thus facilitating high performance sensorswhich are significantly desired for uses. Furthermore, it provides achance to clarify response mechanism with a deep insight intophysics of device as the conductive channel locates at the interfacebetween organic semiconductors and the dielectric [4]. The ultra-thin transistors-based sensors allow fabrication of devices bydirect investigation of film morphology effect on device physics,thus opening the door for enhanced sensitivity and less recoverytime, low cost and accelerating the interaction between conductivechannel and the analyte. Zhang et al. fabricated the ultrathin filmorganic transistors by spin coating method [5], Yang describedthe ultrathin organic transistors for chemical sensing [6], Li et al.reported high performance ammonia sensor using ultrathinorganic semiconductor film [7]. Hetero-structure of PTCDI-Ph andultrathin film of p-6P based NO2 sensor in organic filed effect tran-sistors had shown a good sensitivity [8]. However, few deepresponse mechanisms were discussed in these works and therewere few reports for connecting the highly sensitive OFET-basedsensors with its two dimensional (2D) transport characters.

This paper aims to implement novel organic semiconductor gassensors and decipher the structural chemistry of pentacene ultra-thin film and proposed mechanism for interaction between NO2gas analyte and pentacene ultrathin film. The threshold shift, drain

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M. Mirza et al. / Organic Electronics 24 (2015) 96–100 97

current and mobility variations as a consequence of NO2 analyteare described more relevantly. The limit of detection of our ultra-thin pentacene device is 100 ppb (at sub ppm level), which is muchhigher than pentacene thick layer counterpart. The selectivity ofour ultrathin pentacene device is not taken as for granted, ratherwe discriminated our monolayer devices for CO, CO2, N2O andNH3 gases as well.

2. Experimental

Pentacene (99% pure) purchased from Sigma Aldrich was usedwithout further purification. All reagents (ethanol, acetone, HCl,H2O2) used were analytical grade. The used deionized water wasprepared using Milli-Q Millipore (Bedford, MA, USA) purificationsystem and the resistivity of water was about 18 MO cm�1. Theelastomeric polymer (PDMS) stamp replication from SU-8 patternwas prepared using conventional photolithography as illustratedin our previous work [16]. After cleaning Si wafer with ethanol,acetone and HPM (HCl:H2O2:H2O = 1:1:6) a negative photo-resistof SU-8 was spin coated. The substrate was heated for few minutes.After exposing UV through photo-mask, the SU-8 thin film wasdeveloped. An elastomeric polymer (PDMS) was poured ontoSU-8 mold. PDMS was peeled off from SU-8 mold after the curingprocess. An 80 nm thick gold electrodes were evaporated on poly-meric stamp as metal electrodes with channel length L = 100 lmand width W = 3000 lm. In the fabrication of device, heavily dopedsilicon with thermally grown SiO2 of thickness 300 nm was used assubstrate. Toluene solution of polystyrene (PS) was spin coated onSiO2 as hybrid dielectric layer. After that pentacene monolayer filmof 1–4 nm was deposited by Auto 306(BOC Edward Co.) with adeposition rate of 1.25 nm/min evaporated at a vacuum pressureof 10�5 Pa. The morphology of pentacene monolayer film was char-acterized by a Nanoscope III (Vecco Co.) atomic force microscopywith tapping mode. The schematic diagram of device for NO2

Fig. 1. (a) The schematic diagram of monolayer pentacene device. (b) AFM image of monmonolayer film at VDS = �60 V respectively.

sensor is shown in Fig. 1a. The sensing chamber was kept at1 atm pressure to provide inert atmosphere of N2. The dilutedNO2 gas was introduced into the sensing chamber using mass flowcontroller (MFC). The current–voltage characteristic of pentacenemonolayer OFETs were measured by Keithley 4200 SCS instrumentat room temperature.

3. Results and discussion

The carrier transport in OFETs involves charge migrationthrough conducting channel within a grain and across the grainboundaries. OFETs are interfacial devices and dielectric layer inter-facial properties influence the carrier transport hence its mobility.The charge carrier mobility in OFETs is normally determined bythe molecular orientation, grain size and grain boundaries espe-cially located at the dielectric/semiconductor interface [9–13].Each of them usually plays a vital role in the OFET sensing pro-cesses. The modulation in mobility and drain current occurs whenanalyte interact with conductive region. The interacted analytemay act as either dopants or traps for charge carrier [14]. For chargetransport in OTFT, a continuous and highly ordered semiconductinglayer is prerequisite, while for gas sensors, ultrathin thickness of theactive layer is accessible so that it can reduce the time of absorp-tion/desorption of analyte with a good electrode contact from thedevice conductive channel [15]. It is noteworthy that continuousultrathin film growth is rather a challenging task. We have reportedthe fabrication of novel top contact pentacene ultrathin field effecttransistor in a controllable manner (as explained in our previouswork) [16]. To avoid the penetration of direct evaporation of goldinto monolayer film we fabricated flexible electrodes.

The schematic diagram of monolayer pentacene device isshown in Fig. 1a. The charge carrier mobility evaluated from trans-fer curve in the saturation regime (as shown in Fig. 1c) can beobtained using the equation [17]:

olayer (thickness of 1.6 nm) pentacene film. (c and d) Transfer and output curves of

98 M. Mirza et al. / Organic Electronics 24 (2015) 96–100

IDS ¼lCiW

2LðVGS � V thÞ2 ð1Þ

where W and L are channel width and length while VGS is gate volt-age, Vth is threshold voltage, Ci is dielectric capacitance per unitarea, l is mobility and IDS is drain current. The mobility of our topcontact ultrathin pentacene device is 0.038 cm2/Vs and thresholdvoltage is �35.5 V and on/off is 106. Typical transfer (VDS = �60 V,VGS = �60 V) and output curves of ultrathin pentacene device areillustrated in Fig. 1c and d, respectively. AFM image as shown inFig. 1b indicates morphology of pentacene ultrathin film.

Fig. 2a shows the transfer characteristics at room temperaturefor various concentration of NO2 gas analyte. The plots showincrease in drain current with increasing the concentration ofNO2 analyte. The sensitivity response of these devices to NO2 ana-lyte was evaluated by using the following equation:

%DS ¼ INO2 � IN2IN2

� �� 100 ð2Þ

The concomitant change in threshold voltage and mobility areused to characterize the sensing property. It is anticipated thatthese effects result in change of drain current. The change inthreshold voltage and mobility with different concentration ofNO2 analyte exposure is illustrated in Fig. 2b. The sensitivityincrease with respect to various concentrations is plotted inFig. 2c along with increase in drain current. The sensitivity pre-sents an increase over 1300% by enhancing on current Ion andmobility change with even exposure of a small amount of 2 ppmNO2. When the exposure amount of NO2 increases to 10 ppm, thesensitivity keeps on increasing to around 3000% and then goes tosaturation with further NO2 exposure. The sensing response ofthick layer pentacene devices is 103 orders less than ultrathindevices. The enhanced mobility in our ultrathin devices, by con-trolling the gas ambience, could provide opportunities to improvethe performance of OFETs.

The limit of detection is expressed as a concentration corre-sponding to the smallest signal that can be detected with

Fig. 2. (a) Transfer curves of monolayer film exposed with different concentrationconcentration of NO2. (c) The sensitivity percentage change and drain current change fo

reasonable certainty for a given analytical procedure. UsingFig. 2c for concentration dependent sensitivity response of themonolayer pentacene OFET, we can extract the limit of detectionusing following equation [18].

YLOD ¼�abþ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2b2 � ½b2 � g2Db2�½a2 � g2Da2�

q

½b2 � g2Db2�

8<:

9=; ð3Þ

In this study, the value of g = 2.58 corresponding to 99% confi-dence level is shown in Fig. 2d. The g is the confidence factor usedin statistical calculation to estimate mean and standard deviationmore reliably that is why also named as reliability factor. The valueof limit of detection is 100 ppb (sub-ppm level). To the best of ourknowledge, this sensitivity is the highest for any pentacene basedsensors because of its ultrathin thickness.

To confirm the reproducibility of our ultrathin OFET sensors,nine cycles were carried out at 0.5 ppm concentrations (shown inFig. 3a). The excellent performance under varying NO2 concentra-tions and good reproducibility make our sensor suitable for a traceanalysis and quantitative detection. The response/recovery timefor 0.5 ppm NO2 is less than 20 s.

We also accomplished experiments for CO, CO2 N2O and NH3gases. The CO, CO2, N2O gases did not show any response for mono-layer pentacene devices but NH3 showed response as described inour previous work. Remarkably the sensitivity of NO2 gas is thehighest among all gases we analyzed (Fig. 3b) which providepotential strategy to suppress the poor selectivity for organic semi-conductor sensors.

To study the response mechanism for such an ultrathin NO2sensor, understanding of the decisive factors of transport in organicfield effect transistors is imperative. As our device is of few mono-layer, prospect of NO2 analyte to reach on these interfaces istremendous. The high sensitivity response of ultrathin devicescan be attributed to a synergy enhancement effect combining astrong electron affinity of NO2 and a pure 2D transport characterwhere grain boundaries are highlighted.

of NO2 gas analyte. (b) Threshold voltage and mobility change with respect tor different concentration of NO2. (d) Limit of detection calculation.

Fig. 3. (a) Time response of nine successive cycles of NO2 sensor of pentacene monolayer device. (b) Pentacene monolayer device selectivity for five different gases.

M. Mirza et al. / Organic Electronics 24 (2015) 96–100 99

NO2 being a strong electron acceptor absorb electron thusgenerates holes while interacting with pentacene film. Therefore,carrier concentration of devices becomes high in the devices whichare exposed to NO2 molecules. The initially injected carriers firstfill the traps under the transport band or transport energy leveland then move to flow current. The carrier concentration is foundto be mobility dependent by power law l (n) a nd which isconsistent with energetic disorder model [19]. The charge carriermobility is dependent on carrier concentration [20–22] whichmeans higher the carrier concentration larger is the charge carriermobility.

Another key point is the large sensitivity of the ultrathindevices. As proved in our previous work of electric field dependentmobility [23], in pentacene based monolayer transistors the grainboundaries play a vital role in the charge transport progress. Thecharge carriers will inevitably go into grain boundaries withoutupper grains acting as a bridge over grain boundaries, which makethe transport feature of grain boundary more dominate. A largeamount of trap states are distributed in grain boundaries as theyare sites of much more disorder. As illustrated in Fig. 4, in mono-layer devices the gas molecules interact directly with the grainboundaries, generating large number of holes. These holes enterquickly into localized states making the extra holes to transportsmoothly through the energy level with higher density of states,

Fig. 4. (a) The schematic images of the monolayer device with DOS. (b) Monolayer

thus increasing the charge carrier mobility. Conversely in thicklayer devices, the grain boundaries in first monolayer are coveredby the grains of upper layer. The charge carriers present in the firstmonolayer will go through a specific path to connect to grains inthe second layer, making the grain boundaries less important incharge transport. Additionally, the increase in number of holes willnot have much influence on the mobility within the grains as thegrains near the conductive channel are not directly exposed tothe gas analyte, in a consequence not influencing the grain bound-aries too.

The above analysis can be further consolidated by the analysisof trap density. During the exposure of NO2 analyte large amountof deep traps in the grain boundaries are massively filled, or equiv-alently the NO2 analyte reduces the trap density, decreasing thetrapping probability of injected holes by gate voltage, which canbe reflected by sub threshold swing (SS):

SS ¼ @Vg=@ðlog10IdÞ ð4Þ

From the sub-threshold swing the effective trap density can beextracted by equation [24]:

N ¼ Cq

qSSkT ln 10

� 1� �

ð5Þ

device with NO2 analyte exposure. (c) Thick layer with NO2 analyte exposure.

Fig. 5. Sub-threshold swing (SS) and trap density change for various concentrationof NO2.

100 M. Mirza et al. / Organic Electronics 24 (2015) 96–100

where k is Boltzmann’s constant, T is absolute temperature, q iselectronic charge and C is the dielectric capacitance. It is clear fromFig. 5 that the exposure of NO2 analyte decreases the trap density.

The trap density extracted from SS in N2 atmosphere is1.13E12 cm�2 while for 2 ppm NO2 is 4.23E11 cm�2. The trap den-sity for different concentration of NO2 are evaluated and shown inFig. 5. The trap density shows an abrupt change with NO2 expo-sure. We also carried out experiments for thick layer pentacenefilm but the sensitivity was poor (three orders of magnitude less)as compared to monolayer counterpart. The increase in mobilityand positive shift of threshold voltage in ultrathin pentacene filmcan also be understood by change in density of trap states due toNO2 exposure.

4. Conclusion

In conclusion, NO2 gas sensing with ultrathin pentacene filmwith near one whole monolayer coverage has shown excellent sen-sor performance including high sensitivity, fast response/recoverytime, good selectivity, low concentration detection capability,excellent reproducibility. The ultrathin pentacene devices allowdirect adsorption of the gas molecules in the conductive channeland decrease the carrier trap density, thus increasing the mobility.More physical explanation is resorted to the combination of strongelectron affinity ability of NO2 and grain boundary dominated 2Dtransport. This study may pave a boulevard towards developinghighly sensitive room temperature NO2 semiconductor sensorand enhanced mobility provides promising route in improvingthe performance of OFETs.

Acknowledgements

This work was supported by the National Basic ResearchProgram of China (Grant 2011 CB932800) and the NationalNatural Science Foundation of China (Grants 21432005, 11074056and 11104042).

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Response enhancement mechanism of NO2 gas sensing in ultrathin pentacene field-effect transistors1 Introduction2 Experimental3 Results and discussion4 ConclusionAcknowledgementsReferences