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Sensors and Actuators B 192 (2014) 565– 571

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l h om epage: www.elsev ier .com/ locat e/snb

umidity, light and temperature dependent characteristics ofu/N-BuHHPDI/Au surface type multifunctional sensor

uhammad Tahira,b, Muhammad Hassan Sayyada, Jenny Clarkb, Fazal Wahaba,akhra Azizc,∗, Muhammad Shahidd, Munawar Ali Munaward, Jamil Ahmad Chaudryd

Faculty of Engineering Sciences, GIK Institute of Engineering Sciences and Technology, Topi 23640, KPK, PakistanCavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 OHE, United KingdomDepartment of Electronics, Jinnah College for Women, University of Peshawar, Peshawar 25120, PakistanInstitute of Chemistry, University of the Punjab, Lahore 54000, Pakistan

r t i c l e i n f o

rticle history:eceived 14 July 2013eceived in revised form 3 October 2013ccepted 16 October 2013vailable online 7 November 2013

eywords:

a b s t r a c t

This paper reports the sensing properties of organic semiconducting material N-butyl-N′-(6-hydroxyhexyl) perylene-3,4,9,10-tetracarboxylic acid diimide (N-BuHHPDI). A multifunctional surfacetype Au/N-BuHHPDI/Au sensor has been fabricated by depositing 140 nm thin layer of N-BuHHPDI onpre-patterned gold (Au) electrodes, using vacuum thermal deposition. The effect of humidity, light andtemperature on the electrical capacitance and resistance of the device has been investigated. The sen-sor responded exponentially toward the variation in humidity, light and temperature. An equivalentcircuit of the multifunctional sensor has also been developed. By using the Tauc’s law, optical band

-Butyl-N′-(6-hydroxyhexyl)erylene-3,4,9,10-tetracarboxylic acidiimide (N-BuHHPDI)ultifunctional sensorptical band gaptomic force microscopy (AFM)esistive–capacitive sensor

gaps (2 eV, 2.18 eV and 3.78 eV) of N-BuHHPDI have been obtained from its UV–vis spectrum. Atomicforce microscopy (AFM) has been employed to study the morphology of N-BuHHPDI thin film. Theenvironmental-conditions-dependent properties make the device useful for its applications in measure-ment of humidity, temperature and light.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Assessment of environmental conditions in our daily life andarious fields can be accomplished by monitoring temperature,umidity, light and toxic gases, etc. present in the air [1,2]. Smallolecular organic semiconductors based sensors are of great sig-

ificance due to their low cost, ease of fabrication and greatertability. These materials have high sensing capability for thearameters such as temperature, radiations and humidity [3–5].ue to strong �-conjugation system and low molecular weight,rganic semiconductors like perylene and its derivatives belong tone of the important class of organic materials which bear promis-ng optoelectronic properties and thus may be used as functionalemiconducting materials [6,7]. They have strong absorption in theisible range [8] and have good chemical and thermal stabilities [9].he n-type behavior of perylene [10] makes it a useful candidate for

ts potential application in different electronic and optoelectronicevices which include solar cells [11], field effect transistors and

ight emitting diodes [12,13]. Most of the perylene derivatives are

∗ Corresponding author. Tel.: +92 91 9216758; mobile: +92 300 5904028.E-mail address: fakhra69@yahoo.com (F. Aziz).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.10.073

insoluble in water and possess excellent hydrophobic properties[14]. This hydrophobic property makes these materials more dis-tinctive candidates for humidity sensing applications as comparedto other water soluble organic materials. The high sublimation tem-perature (700 K) is another convincing characteristic of perylenesfor their potential application as a temperature sensor to monitortemperature at elevated levels. At the same time, the broad UV–visspectrum and good absorption of light recommend perylenesequally useful for light sensing applications. Thus, taking advan-tages of these motivating properties, the potential of perylene hasbeen explored for humidity, temperature and light sensing appli-cations due to the possession of such interesting properties alltogether.

Mainly, sensors are fabricated in two types of geometry; sand-wich type and surface type. Though the sandwich type sensorsare very reliable but expensive, complex and prone to short-ening of the device while, on the other hand, surface typeorganic thin film structures are simple, low-cost, easy to fabricateand versatile alternatives to their counterparts [15,16]. Keeping

these notable properties in view, surface type configurations arebeing extensively studied for the fabrication of various capaci-tive and resistive type sensors employing low molecular organicsemiconductors. Humidity sensors, photocapacitive sensors and

5 Actuators B 192 (2014) 565– 571

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2

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Fs

66 M. Tahir et al. / Sensors and

emperature sensors based on organic semiconducting materialsuch as phthalocyanines, porphyrins, their derivatives and methyled have been reported in the literature [2,16–20].

Since, very little is known about the sensing properties of pery-ene and its derivatives in the available literature. Thus the purposef the present work is to use N-BuHHPDI as an active material forts potential applications in the fabrication of a surface type Au/N-uHHPDI/Au multifunctional sensor. The capacitive and resistiveesponses of the sensor are investigated as a function of relativeumidity (%RH), temperature and amount of illumination. The sen-or has been characterized at normal pressure in the presence ofir to realize its potential use in normal environmental conditions.he humidity sensing properties of the device are correlated to theorphology of N-BuHHPDI thin film.

. Experimental work

The organic semiconductor N-BuHHPDI is used as an active layern the fabrication of Au/N-BuHHPDI/Au multifunctional sensor. The

olecular structure of N-BuHHPDI is shown in Fig. 1. Glass slide haseen used as a substrate and cleaned in acetone for 10 min followedy cleaning in isopropanol for another 10 min using ultrasonic bath.he substrate is further cleaned for 5 min by producing plasmanside the vacuum thermal evaporator at pressure 10−3 mbar. Then00 nm thick Au electrodes were deposited on the substrate suchhat the inter-electrode gap was kept 40 �m. Afterwards, 140 nmhick film of N-BuHHPDI was thermally deposited over the Aunter-electrode gap at 2.3 × 10−5 mbar. Fig. 2 shows the schematiciagram of the fabricated Au/N-BuHHPDI/Au surface type sen-or. For vacuum thermal evaporation of Au and N-BuHHPDI thinlms, Edward Auto 306 Vacuum Coater has been used with in situTM5 quartz crystal thickness monitor. The self-made humidityhamber, fitted with a commercial humidity meter is used forumidity dependent measurements in which humidity is con-rolled through dry nitrogen and wet air. The humidity-resistancend capacitance of the sensor were measured by using “ESCORTLC-133 A” dual display LCR meter. For temperature-dependentapacitive and resistive measurements of the sensor, Karl SussM5 probe station with a thermo chuk ‘Alpha’ series system,odel TP 0315A-TS-2 of Temptronic Corporation, USA, is used.

he temperature–capacitance measurements were carried out inir at normal atmospheric pressure at 0%RH in dark conditions. To

heck the light sensitivity, the fabricated sensor is illuminated with

25 W movable incandescent lamp. By varying distance betweenhe lamp and sensor, light dependent characteristics have been

O

O

N

O

O

N

H C3

OH

Fig. 1. Molecular structure of N-BuHHPDI.

Glass su bstrat eAu Au

N-BuHHPD I

ig. 2. Cross-sectional view of the fabricated Au/N-BuHHPDI/Au surface-type sen-or.

Fig. 3. (a) The AFM image on 2 �m × 2 �m scale measured as height from imagedata (b) phases of the film.

measured using CEM DT-1300 light meter. The morphology ofN-BuHHPDI film has been investigated by AFM NanoScope-IIIaSPM from DimensionTM 3100, Digital Instruments Veeco Metrol-ogy Group at Cavendish Lab, University of Cambridge, UK. The AFMwas operated in the noncontact imaging mode to prevent dam-age to the film. The UV–vis spectrum was recorded with HewlettPackard 8453, UV–vis spectrophotometer using unpolarized lightin the wavelength range of 280–1100 nm, at a temperature of 300 K.

3. Results and discussions

The surface morphology of N-BuHHPDI thin film has been stud-ied using AFM. Fig. 3 shows two dimensional AFM images on a scalesize of 2 �m. To see the fine features in the topography of the film,the scanning rate was kept at lower frequency of 0.8976 Hz whilethe tip height was maintained at 50 nm. A quite uniform distri-bution of grains with random orientations can be seen in Fig. 3.However, shapes and sizes of the grains are slightly different fromeach other. The surface profile also shows some pores and intersti-tials sites among the grains. Since the shape of the grain is nearlyelliptical with an average length and width of 180 nm and 85 nm,respectively, so the average grain size is found to be 15,300 nm2.These pores may be due to the random orientations of grains resid-ing on one another. The film also contains voids, which producediscontinuity in the film and are responsible for absorbing water

vapors causing the capacitance of sensor to increase. Moreover, thesurface of the film has some non-uniform roughness with averageroughness (Ra) value of 217 nm. The roughness of the film is pos-sibly due to the non-uniform growth of some grains during the

M. Tahir et al. / Sensors and Actuators B 192 (2014) 565– 571 567

250 500 750 10000.0

0.2

0.4

0.6

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1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

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2.0x1024

4.0x1024

6.0x1024

8.0x1024

1.0x1025

3.78 eV2.18 eV

h2

h (eV )

2 eV

Fig. 5. Direct band gap measurements for N-BuHHPDI thin film.

0 20 40 60 80 100

0.0

0.3

0.6

0.9

1.2

RH%

C (n

F)

0.0

1.5

3.0

4.5

6.0

R(G

)

CR

Wavelength (nm)

Fig. 4. UV–vis spectrum of N-BuHHPDI thin film.

lm deposition and random orientation of grains. This surface-oughness is considered responsible for enhanced adsorption ofater vapors so as to increase humidity capabilities of the thin film.

ig. 3(a) shows the image on 2 �m × 2 �m scale measured as heightrom image data while Fig. 3(b) shows the phases of the film.

The UV–vis absorption spectrum of the 140 nm thick film of-BuHHPDI, deposited on quartz substrate, is shown in Fig. 4.wo prominent bands, the B-band or Soret-band in the range85–350 nm of UV region and the Q-band, between 400 nm and50 nm in the visible region are observed in the recorded spectra.s can be seen from Fig. 4, the B-band with a sharp peak appearst 302 nm while the Q-band appears at 470 nm and 563 nm withigh-energy peak and low-energy peaks, respectively. The domi-ant peaks in Q-band are interpreted in terms of direct electronicransition between bonding and anti-bonding molecular orbitals21]. The absorption peaks around 470 nm and 563 nm are clearvidence of aggregation in thin films of perylene derivatives [22].he formation of molecular aggregates may be ascribed to effective–� stacking interactions of perylene nuclei [23]. The transitions,

or the two bands in the visible region, may be from the groundtate to two (or more) excited states [24].

In order to calculate optical energy band-gap of semiconductingaterials and nanomaterials, absorption spectra play a significant

ole. Tauc’s law [25] has been applied to measure the optical bandap according to the following expression:

E = (E − Eg)m (1)

is the photon energy, Eg is the band gap energy, d is the thicknessf the film and m is a constant for the transition (m = 1/2 for directransition and m = 2 for indirect transition). The direct transition

odel is applied to calculate optical band gap of the N-BuHHPDIlm. So (˛E)2 is plotted against E, as shown in Fig. 5. The energyand gap is determined by extrapolating linear portion of (˛E)2

ersus E to zero. We have obtained the optical energy band gapalue of 2 eV and 2.18 eV in the visible region for the Q-band and.78 eV in the infrared region for the B-band or Soret band. Thealculated band gap values of N-BuHHPDI are in good agreementith the values reported in Refs. [26–29].

The control and monitoring of humidity is a prime concernecause it significantly affects the properties of various materials,articularly hydrophilic materials. Relative humidity is the ratio ofhe actual amount of water vapors present to the saturated amount

Fig. 6. Capacitance/resistance–humidity relationships for the Au/N-BuHHPDI/Ausurface-type sensor at 100 Hz frequency.

of water vapors in the air, at any given time. Relative humidity canbe expressed as follows:

%RH = actual vapor densitysaturated vapor density

× 100 (2)

Fig. 6 shows capacitance/resistance versus %RH relationshipsfor Au/N-BuHHPDI/Au surface-type sensor. The capacitance andresistance of the sensor against humidity were recorded at 300 Kin dark. As can be seen from Fig. 6, capacitance of the humid-ity sensor increases while the resistance decreases exponentiallywith increasing relative humidity. The capacitance of the Au/N-BuHHPDI/Au surface type sensor has increased by 13 times and theresistance has decreased down by 20 times with increasing lev-els of relative humidity. The capacitive/resistive response of thesensor has been investigated over a range of 0–90%RH. At lowerhumidity levels, i.e. from 0 to 60%RH, the capacitance (C) remainsconstant due to fewer amounts of adsorbed water vapors which areinsufficient to increase the dielectric constant of the organic semi-conductor layer. However, there appears a significant increase inthe capacitance (C) beyond 60%RH, which can be explained by the

expression given below:

C = ε0εrA

d(3)

5 Actuators B 192 (2014) 565– 571

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cmrfipabrasmswtIaltlubeqctcrlor

wilmioiBltsaosac

0 6000 12000 18000 24000 30000

0.00

0.05

0.10

0.15

0.20

0.25

C (n

F)

Illumination (lux)

(a)

(b)

Fig. 7. (a) Capacitance–light graph for the Au/N-BuHHPDI/Au surface-type sensor

68 M. Tahir et al. / Sensors and

here εr is the relative dielectric constant, ε0 is permittivity ofree space, A is the active area of the sensor and d is the dis-ance between the plates of the capacitor. The capacitance increasesxponentially from 100 pf to 1300 pf in the range 60–90%RH. On thether hand the sensitive region for resistive response lies between

and 60%RH, while beyond 60%RH, the device becomes insensi-ive for variation in the resistance. Accumulation of more waterapors within the humidity chamber results in more adsorptionf moisture in the pores of organic film which in turn increase εr

f the organic semiconductor and hence give rise to the enhancedapacitance. The capacitive and resistive response to %RH showsysteresis. The upward and downward arrows show increasingnd decreasing humidity levels, respectively. A very minute changeccurs in the capacitance as compared to resistance of the sen-or indicating that the fabricated sensor is more effective forapacitance response than resistance. The capacitive and resistiveesponse can be combined for sensing a broad range of humidityor commercial applications.

Numerous factors, such as porosity of thin film, dielectriconstant of water, active sensing area and polarizability of theaterial, are involved in the capacitance/conductance variation in

esponse to the humidity. At higher humidity levels and greaterlm thickness, sensitivity of the sensor is reduced because theores of the thin film get filled with water molecules. The rel-tive permittivity of small molecular organic semiconductor liesetween 4 and 8 [30], while that of water is ∼80 [31,32]. Atoom temperature (300 K), the dielectric constant of water remark-bly increases capacitance/conductance of the Au/N-BuHHPDI/Auensor, as a result of large difference between the relative per-ittivities of water and active film of N-BuHHPDI. When the

ensor is exposed to humid environment, a solid surface adsorbsater molecule and results in the formation of two barely dis-

inguishable layers, i.e. chemisorption and physisorption layers.n the beginning, no water molecules are present in the pores ofctive material; rather they contain only dry air. The chemisorptionayer is formed at lower humidity levels while the physisorp-ion layer occurs at higher humidity levels. In the chemisorptionayer and the initial physisorption layer, the water molecules arenable to move freely as they are doubly bonded. However, singlyonded molecules in the successive physisorption layers, can beasily polarized. Hence the increased dielectric constant and conse-uently the enhanced capacitance are inevitable [32]. The increasedontact area and reduced gap between the electrodes increasehe physisorption of water molecules, which in turn increases theapacitance/conductance of the sensor. Due to the aforementionedeasons the capacitance/conductance of a low dielectric material,ike N-BuHHPDI, show a remarkable increase. The humidity-resultsbtained for Au/N-BuHHPDI/Au sensor are consistent with thoseeported in the literature [1,2,16–18].

The N-BuHHPDI possesses good light absorption propertieshich are clear from the UV–vis spectrum shown in Fig. 4. Tak-

ng advantage of this property, the sensor has been studied foright sensing applications. Fig. 7(a) shows capacitance versus illu-

ination relationship of the sensor, at a temperature of 300 K. Anncandescent lamp has been used to illuminate the sensor in a rangef ‘0’ lx (dark condition) to 27,000 lx (illumination). The amount ofllumination was varied by changing the distance between Au/N-uHHPDI/Au sensor and the lamp. The schematic diagram of the

ight–capacitance measurement setup is shown in Fig. 7 (b). Ini-ially, the lamp was illuminated from 60 cm height above theensor. The distance was varied in a fixed step size of 2 cm withn average exposure time of 5 s to get the readings stable. It can be

bserved from the graph that the capacitance remains almost con-tant up to 7000 lx of incident light and increases between 7000 lxnd 25,000 lx. Beyond 25,000 lx the sensor goes into saturation. Theapacitance of the sensor increases 35 times when the illumination

at 100 Hz frequency, (b) schematic diagram of the light–capacitance measurementsetup.

is varied from dark (0 lx) to 25,000 lx. When Au/N-BuHHPDI/Ausurface type multifunctional sensor was investigated as a photode-tector, it exhibited better results as compared to the porphyrinbased multifunctional sensor reported [16]. Charge carrier concen-tration increases with the increase in the amount of illumination,which in turn increases the polarizability due to transfer of chargecarriers [4]. This exponential increase in the charge carrier concen-tration may be attributed to the band–band excitation [30,33].

It is well known that capacitance depends on the polarizabilityof the active material [34]. There are many sources of polarizabil-ity, such as electronic polarizability (˛e), ionic polarizability (˛i)and dipolar polarizability (˛dip), which are responsible for change

in capacitance of the active layer. Electronic polarizability arisesdue to relative displacement of orbital electrons of an atom ora molecule [35]. It is present in most of the materials but has a

M. Tahir et al. / Sensors and Actuators B 192 (2014) 565– 571 569

300 330 360 39 0 42 0 45 0 48 0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

C (n

F)

Tempera ture (K)

Ft

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ip

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0 50 100 150 200 250 300

0.6

0.7

0.8

0.9

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1.1 C R

Time (s)

C (n

F)

0.40

0.45

0.50

0.55

0.60

0.65

0.70

R (M

)

temperature, respectively. The capacitance, C , and resistance, R ,

ig. 8. Capacitance–temperature relationship for the Au/N-BuHHPDI/Au surface-ype sensor.

ery negligible effect. It is assumed that the perylene derivativeay contain some charge-transfer complexes, which give rise to

he ionic polarizability [16]. Certain dipoles are also present in therganic material, which are arranged in a random fashion. Whenn electric field is applied, these dipoles change their orientationnd give rise to dipolar polarizability. In humidity–capacitanceeasurements of the material, the dipoles of the water molecules

dsorbed by the perylene active layer result in dipolar polarizability17].

The capacitance increases while resistance decreases due to risen the relative permittivity of the material. The polarization andolarizability are related by the following relation [35]:

= N˛E (4)

here P is polarization, ̨ is polarizability and E is electric field.If we take all polarizabilities of the organic material into account

hen the total polarizability (˛n) under normal (dark and dry) con-itions may be written as:

n = ˛e + ˛i + ˛dip + ˛tn (5)

When the device is exposed to illumination or humidity envi-onment, the total polarizability (˛) of sample is given by:

= ˛e + ˛i + ˛dip + ˛t (6)

here ˛t is polarizability under illumination or humid conditionsue to the transfer of electron/holes charge carriers.

Fig. 8 shows capacitance versus temperature relationship of theensor in the range of 300–460 K. The capacitance almost staysonstant up to 400 K showing stability of the N-BuHHPDI againstemperature and beyond 400 K up to 460 K, it sharply increases.s can be seen from Fig. 8, the capacitance increased 5 times forlevating temperatures. However, the porphyrin based multifunc-ional sensor exhibited 3 times increase in the capacitance withising temperature [16].

The temperature sensing mechanism of Au/N-BuHHPDI/Au sur-ace type multifunctional sensor can be well understood from theffect of temperature on the mobility and conductivity (�) of N-uHHPDI thin film. When temperature is increased the mobility of

-BuHHPDI film increases which in turn give rise to conduction of

he charge carriers. Since resistivity and conductivity are inverselyroportional to each other (� = 1/�), therefore, resistance of

Fig. 9. Response and recovery time as a function of capacitance and resistance forthe Au/N-BuHHPDI/Au surface-type sensor at 100 Hz frequency.

N-BuHHPDI decreases with increase in temperature, as describedby Eq. (7) [15].

� = �0exp(

E

kT

)(7)

where � is the resistivity at the absolute temperature T, �0 is thepre-exponential factor, E is activation energy for conduction andk is Boltzmann constant. The possible reason for the increase incapacitance of the sensor may be ascribed to the higher value ofconductance due to large mobility and conductivity of the chargecarriers in the organic film as they attain sufficient energy dueto rise in temperature. A very small hysteresis is observed in thecapacitance showing consistent behavior of the sensor towardincreasing and decreasing temperature.

The response and recovery characteristics of the Au/N-BuHHPDI/Au sensor corresponding to humidification and desicca-tion are shown in Fig. 9. The response time and recovery time of thesensor were measured as follows: initially the sensor was placedin a sealed chamber with 30%RH and then the humidity level wasquickly raised to 90%RH, thereby enabling the sensor to adsorbwater vapors and hence increasing the capacitance. The samplewas exposed to the ambient conditions and the response time of60 s was recorded. The recovery time of the sensor was achievedby first exposing it to a humidity level of 90%RH and then immedi-ately to 30%RH. The sensor recovered itself to equilibrium state bydesorbing water vapors, in nearly 70 s which is considered as therecovery time.

The Au/N-BuHHPDI/Au surface type multifunctional sensor hasbeen studied for the effect of humidity, light and temperature andhas shown sensitivity toward all the investigated parameters. Allthe three sensors can be fabricated on a single substrate, whichwould minimize the overall cost effect. However, the interactionof different effects may be reduced by having separate casing withappropriate window for each sensor.

According to Nahar et al. [36], schematic diagrams for detailedand equivalent circuits of the Au/N-BuHHPDI/Au surface type mul-tifunctional sensor are shown in Fig. 10(a) and (b), respectively. Inthe schematic diagram of Fig. 10(a), Ca and Ra represent the capac-itance and resistance due to the dielectric of air filled in the porearea. The Ceff and Reff denote the capacitance and resistance due tothe dielectric of N-BuHHPDI affected by relative humidity, light and

g g

are due to the dielectric of the glass substrate. It is assumed thatthe properties of organic semiconductor N-BuHHPDI are dominantover other properties; therefore, an equivalent circuit is redrawn

570 M. Tahir et al. / Sensors and Actuat

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ig. 10. Schematic diagram of electrical equivalent circuit model of Au/N-uHHPDI/Au surface type multifunctional sensor: (a) detailed and (b) simplified.

n Fig. 10(b), by taking the capacitance and resistance of the activeaterial into account and neglecting the effects of air and glass

ubstrate.

. Conclusions

In the present work, the effect of humidity, light and tempera-ure on the Au/N-BuHHPDI/Au surface type multifunctional sensors investigated. The sensor exhibited exponential response towardhese environmental conditions by detecting variation in capaci-ance and resistance. It is observed that capacitance increases andesistance decreases with increasing level of humidity. The increasen the capacitance can be ascribed to increased dielectric constantf the active layer due to adsorption of water vapors on the thin-lm surface. On the other hand the resistance is greatly affectedy the change in the dipole moment due to dissociation of waterolecules adsorbed. The material is stable at high temperatures

nd, therefore, can be used for temperature sensing applicationsrom 300–460 K. Due to hydrophobic nature of N-BuHHPDI, it isoncluded that N-BuHHPDI is one of the promising candidates forumidity sensing applications. The broad absorption spectrum ofhe material makes it equally good for light sensitivity. The surfaceoughness of the thin film is evaluated by atomic force microscopy.he measured rms roughness of the N-BuHHPDI film shows thathe film surface is not smooth.

cknowledgements

The authors are thankful to Ghulam Ishaq Khan Institute of Engi-eering Sciences and Technology (www.giki.edu.pk) and Cavendishaboratory, Cambridge University UK (www.cam.ac.uk) for theacilities provided. The authors also highly acknowledge the rolef Higher Education Commission of Pakistan (www.hec.gov.pk) forranting PhD fellowship to Mr. Muhammad Tahir and Mr. Fazalahab.

eferences

[1] F. Aziz, M. Hassan Sayyad, K. Sulaiman, B.H. Majlis, K.S. Karimov, Z. Ahmad, G.Sugandi, Characterization of vanadyl phthalocyanine based surface-type capac-itive humidity sensors, Journal of Semiconductors 31 (1–6) (2010) 114002.

[2] Z. Ahmad, M.H. Sayyad, M. Saleem, K.S. Karimov, M. Shah, Humidity-dependentcharacteristics of methyl-red thin film-based Ag/methyl-red/Ag surface-typecell, Physica E 41 (2008) 18–22.

[3] F. Aziz, M. Hassan Sayyad, K. Sulaiman, B.H. Majlis, K.S. Karimov, Z. Ahmad,G. Sugandi, Influence of humidity conditions on the capacitive and resistiveresponse of an Al/VOPc/Pt co-planar humidity sensor, Measurement Science

and Technology 23 (2012) 014001.

[4] M.H. Sayyad, M. Shah, K.S. Karimov, Z. Ahmad, M. Saleem, M. Maroof Tahir,Fabrication and study of NiPc thin film based surface type photocapacitors,Journal of Optoelectronics and Advanced Materials 10 (2008) 2805–2810.

[

ors B 192 (2014) 565– 571

[5] M. Saleem, M.H. Sayyad, K.S. Karimov, M. Yaseen, M. Ali, Synthesis andapplication of Ni(II) 5,10,15,20-tetrakis(4′-isopropylphenyl) porphyrin in asurface-type multifunctional sensor, Phys. Scr. 82 (2010) 015703–015708.

[6] G. Fu, N. Xia, Y. Wang, M. Yang, G. Zhang, W. Wang, Preparation of belt-likeaggregates of a perylene derivative, Materials Letters 63 (2009) 409–411.

[7] B. Liang, Y. Zhang, Y. Wang, W. Xu, X. Li, Structures and properties of 1,7-disubstituted perylene tetracarboxylic diimides: the substitutional effect studybased on density functional theory calculations, Journal of Molecular Structure917 (2009) 133–141.

[8] Y.-S. Ma, C.-H. Wang, Y.-J. Zhao, Y. Yu, C.-X. Han, X.-J. Qiu, Z. Shi, Perylene diimidedyes aggregates: optical properties and packing behavior in solution and solidstate, Supramolecular Chemistry 19 (2007) 141–149.

[9] D. Volpati, A.E. Job, R.F. Aroca, C.J. Constantino, Molecular and morphologi-cal characterization of bis benzimidazo perylene films and surface-enhancedphenomena, Journal of Physical Chemistry B 112 (2008) 3894–3902.

10] J.Y. Kim, I.J. Chung, C. Lee, Y.C. Kim, J.K. Kim, J.-W. Yu, Mobility of electrons andholes in an n-type organic semiconductor perylene diimide thin film, CurrentApplied Physics 5 (2005) 615–618.

11] L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R.H. Friend, J.D.MacKenzie, Self-organized discotic liquid crystals for high-efficiency organicphotovoltaics, Science 293 (2001) 1119–1122.

12] F.J. Céspedes-Guirao, S. García-Santamaría, F. Fernández-Lázaro, A. Sastre-Santos, H.J. Bolink, Efficient electroluminescence from a perylenediimidefluorophore obtained from a simple solution processed OLED, Journal of PhysicsD: Applied Physics 42 (2009) 105106.

13] M. Tahir, M.H. Sayyad, F. Wahab, F. Aziz, M. Shahid, M.A. Munawar, Perylenediimide: synthesis, fabrication and temperature dependent electrical charac-terization of heterojunction with p-silicon, Physica B: Condensed Matter 426(2013) 6–12.

14] M. Tahir, M.H. Sayyad, M. Shahid, J.A. Chaudry, M.A. Munawar, Fabrication ofAl/N-BuHHPDI/ITO Schottky barrier diode and investigation of its electricalproperties, in: International Conference on Advances in Electrical and Elec-tronics Engineering (ICAEEE’2012), Pattaya, Thailand, 2012.

15] M. Saleem, M.H. Sayyad, K.S. Karimov, M. Yaseen, M. Ali, Surface-type multi-functional sensor based on 5,10,15,20-tetrakis(40-isopropylphenyl) porphyrin,Journal of Material Science 44 (2009) 1192–1197.

16] M. Saleem, M.H. Sayyad, K.S. Karimov, M. Yaseen, M. Ali, Cu(II)5,10,15,20-tetrakis(4′-isopropylphenyl) porphyrin based surface-typeresistive–capacitive multifunctional sensor, Sensors and Actuators B:Chemical 137 (2009) 442–446.

17] F. Aziz, M. Hassan Sayyad, K. Sulaiman, B. Majlis, K.S. Karimov, Z. Ahmad, et al.,Influence of humidity conditions on the capacitive and resistive response of anAl/VOPc/Pt co-planar humidity sensor, Measurement Science and Technology23 (2012) 014001.

18] F. Aziz, K. Sulaiman, K. Karimov, M. Muhammad, M. Sayyad, B. Majlis, Investi-gation of optical and humidity-sensing properties of vanadyl phthalocyaninethin films, Molecular Crystals and Liquid Crystals 566 (2012) 22–33.

19] M. Shah, K.S. Karimov, M. Sayyad, Organic semiconductor nickelphthalocyanine-based photocapacitive and photoresistive detector,Semiconductor Science and Technology 25 (2010) 075014.

20] M. Shah, M.H. Sayyad, K.S. Karimov, Fabrication and study of nickel phthalo-cyanine based surface type capacitive sensors, Proceedings of World Academyof Science, Engineering and Technology 43 (2008) 392.

21] A.T. Davidson, The effect of the metal atom on the absorption spectra of phthalo-cyanine films, The Journal of Chemical Physics 77 (1982) 168.

22] C. Kohl, T. Weil, J. Qu, K. Müllen, Towards highly fluorescent and water-solubleperylene dyes, Chemistry: A European Journal 10 (2004) 5297–5310.

23] A. Pucci, F. Donati, S. Nazzi, G.U. Barretta, G. Pescitelli, L.D. Bari, et al., Associationphenomena of a chiral perylene derivative in solution and in poly(ethylene)dispersion, Reactive and Functional Polymers 70 (2010) 951–960.

24] E.H.A. Beckers, S.C.J. Meskers, A.P.H.J. Schenning, Z. Chen, F. Würthner, P. Marsal,et al., Influence of intermolecular orientation on the photoinduced chargetransfer kinetics in self-assembled aggregates of donor–acceptor arrays, Journalof the American Chemical Society 128 (2005) 649–657.

25] J. Tauc, A. Menth, States in the gap, Journal of Non-Crystalline Solids 8–10 (1972)569–585.

26] J.B. Bodapati, Huriye Icil, Highly soluble perylene diimide and oligomericdiimide dyes combining perylene and hexa(ethylene glycol) units: synthesis,characterization, optical and electrochemical properties, Dyes and Pigments 79(2008) 224–235.

27] Y. Zhang, Z. Xu, H.-X. Qiu, G.-Q. Lai, Y.-J. Shen, Synthesis, spectroscopic andelectrochemical properties of the perylene–tetrathiafulvalene dyads, Journalof Photochemistry and Photobiology A: Chemistry 204 (2009) 32–38.

28] S. Asir, A.S. Demir, H. Icil, The synthesis of novel, unsymmetrically substituted,chiral naphthalene and perylene diimides: photophysical, electrochemical, chi-roptical and intramolecular charge transfer properties, Dyes and Pigments 84(2010) 1–13.

29] C. Kufazvinei, M. Ruether, J. Wang, W. Blau, A blue light emitting perylenederivative with improved solubility and aggregation control: synthesis, char-acterisation and optical limiting properties, Organic Electronics 10 (2009)674–680.

B, Robert E. Florida Publishing, Malabar, FL, 1983.31] S.H. Xiao, H.J. Xu, J. Hu, W.F. Jiang, X.J. Li, Structure and humidity sensing prop-

erties of barium strontium titanate/silicon nanoporous pillar array compositefilms, Thin Solid Films 517 (2008) 929–932.

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the University of Punjab, Lahore, Pakistan, and PhD in Analytical Chemistry from

M. Tahir et al. / Sensors and

32] L.Y. Li, Y.F. Dong, W.F. Jiang, H.F. Ji, X.J. Li, High-performance capacitive humiditysensor based on silicon nanoporous pillar array, Thin Solid Films 517 (2008)948–951.

33] M.A. Omar, Elementary Solid State Physics: Principles and Applications, Pear-son Education Pte. Ltd., Singapore, 2002.

34] F. Gutman, L.E. Lyons, Organic Semiconductors, Robert E. Krieger PublishingCompany, Malabar, FL, 1980.

35] K.C. Barua, Introduction to Condensed Matter Physics, Narosa Publishing HousePvt. Ltd., 2007, pp. 196.

36] R. Nahar, V. Khanna, A study of capacitance and resistance characteristics ofan Al2O3 humidity sensor, International Journal of Electronics Theoretical andExperimental 52 (1982) 557–567.

iographies

uhammad Tahir received his MSc degree in Physics with Gold Medal from theohat University of Science and Technology, KPK, Pakistan in 2007 and MS degree

n Engineering Sciences (Applied Physics), from Ghulam Ishaq Khan (GIK) Institutef Engineering Sciences and Technology, Topi, KPK, Pakistan in 2009. Currently, hes Lecturer at the Department of Physics Abdul Wali Khan University Mardan, KPK,akistan. At the same time, he is PhD scholar at the Faculty of Engineering Sci-nces, GIK Institute of Engineering Sciences and Technology. He joined Cavendishaboratory, Department of Physics, University of Cambridge, UK as a visiting PhDesearch scholar in 2011. His research interests are amplified spontaneous emissionn oligomer crystals, organic semiconductor based sensors, electronic and optoelec-ronic devices.

uhammad Hassan Sayyad received his MSc degree in Physics with specializationn electronics in 1982 from Government College Lahore, Pakistan and PhD in Appliedhysics from Dublin City University, Ireland in 1994. Currently, he is Professor ofhysics and Professor of Engineering at Faculty of Engineering Sciences, Ghulamshaq Khan Institute of Engineering Sciences and Technology, Topi. Prof. Sayyad’sesearch interests are in the area of organic electronics, optoelectronics and laserblation.

enny Clark received her MSc degree in Physics from University College London, UKn 2003 and PhD degree in Physics from University of Cambridge, UK in 2007. During007–2009, she had been a Postdoctoral Research Fellow at Politecnico di Milano,

taly. Currently, she is Dorothy Hodgkin Royal Society Fellow at the Cavendish Lab,niversity of Cambridge, UK. Jenny’s research interests are the photophysics of

ors B 192 (2014) 565– 571 571

organic materials for solar cells and steady-state and time-resolved spectroscopyof conjugated polymers and related materials.

Fazal Wahab received his MSc degree in Physics from University of Malakand, KPK,Pakistan in 2007 and MS degree in Engineering Sciences (Applied Physics) fromGhulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi, KPK,Pakistan in 2009. He is currently pursuing his PhD degree in the same institute.He worked as a visiting research fellow at Laboratory of Emerging Sciences, BrownUniversity, Providence, RI, USA. His research interests are organic semiconductorPhysics and devices.

Fakhra Aziz received her MSc degree in Electronics from University of Peshawar,KPK, Pakistan in 1992 and MPhil degree in Electronics from Quaid-i-Azam Uni-versity Islamabad, Pakistan in 1998. She got PhD degree in Engineering Sciences(Applied Physics) in 2012 from GIK Institute of Engineering Sciences and Technology,Pakistan. Currently, she is Assistant Prof. at the Department of Electronics, JinnahCollege for Women, University of Peshawar, Pakistan. Her research interests are thinfilms, organic semiconductor based sensors, electronic and optoelectronic devices.

Muhammad Shahid received his MSc degree in chemistry with specialization inAnalytical chemistry from the University of Punjab, Lahore, Pakistan in 1997andPhD in Chemistry in the field of Organic Synthesis from the University of Pun-jab, Lahore, Pakistan in 2013. Currently, he is Lecturer in chemistry at Governmentdegree College Township Lahore. His research interests consist of synthesis of hightechnology colorants. These colorants have applications in semiconductor basedsensors, electronic and optoelectronic devices.

Munawar Ali Munawar received the MSc degree in Organic Chemistry with GoldMedal in 1982 from The Islamia University Bahawalpur, Pakistan and PhD in OrganicChemistry from University of Wales, UK in 1996 and Post Doc. from University ofFlorida, Gainesville, USA in 2009. He was a member of Royal Society of Chemistry,UK (1996–2008). Currently, he is Professor of Organic Chemistry in the Institute ofChemistry, University of Punjab, Lahore, Pakistan. Prof. Munawar’s research inter-ests include Synthesis of biological active compounds, organic colorants, etc.

Jamil Ahmad Chaudry received the MSc degree in Analytical chemistry in 1972 from

University of Aberdeen UK in 1982. He has been the Director of Institute of Chemistryand Dean of Science at the University of Punjab, Lahore, Pakistan. Currently, heis acting as a Vice Chancellor at Garrison University Lahore, Pakistan. Prof. Jamil’sresearch interests are in the area of analytical and inorganic chemistry.