Ž .Materials Science and Engineering C 11 2000 117–119www.elsevier.comrlocatermsec

Fabrication of a prototype humidity-sensitive capacitor via layer-by-layerself-assembling technique

Dan Li a, Yadong Jiang b, Yanrong Li b, Xujie Yang a, Lude Lu a, Xin Wang a,)

a Materials Chemistry Laboratory, Nanjing UniÕersity of Science and Technology, Nanjing, 210094, People’s Republic of Chinab Department of Materials Science and Engineering, UniÕersity of Electronic Science and Technology of China, Chengdu, 610054,

People’s Republic of China

Accepted 2 May 2000

Abstract

A prototype capacitor-type humidity sensor was designed and fabricated by a layer-by-layer self-assembling technique. The sensorconsists of a self-assembled electrically conductive polyaniline film and an insulating polyelectrolyte film which act as the top electrodeand the dielectric layer, respectively. The two layers are also linked via a self-assembling process. q 2000 Elsevier Science B.V. Allrights reserved.

Keywords: Polyaniline; Polyelectrolyte; Self-assembly; Humidity sensor

1. Introduction

Ultrathin films and multilayered structures are impor-tant for many applications — including X-ray optics,nonlinear optics, microelectronics — and are under inves-tigation for use as light-emitting devices, chemical and

Ž .biological sensors. The layer-by-layer self-assembling SAtechnique has received great interest for preparing variousultrathin films in recent years, including polyelectrolytes,conducting polymers, organic dyes, inorganic semiconduc-

w xtors, and even fullerenes 1,2 . The technique has beengreatly expected for constructing complicated moleculardevices since the thickness and composition of a film maybe controlled easily at the molecular level using an SA

w xprocess 3 . However, few devices have been directlyprepared by SA technique to our knowledge.

As we know, an electronic device usually consists ofdifferent functional sections. For instance, a capacitor ismade of two electrically conductive layers and an insulat-ing layer, which are used as the electrodes and the dielec-

Ž .tric layer of the capacitor, respectively see Fig. 1 . Inorder to assemble an electronic device using the SA tech-

) Corresponding author. Tel.: q86-25-431-5943; fax: q86-25-431-5518.

Ž .E-mail address: wxin@public1.ptt.js.cn X. Wang .

Ž .nique, three problems must be resolved: 1 a devicestructure containing several layers with different functional

Ž .properties e.g., conductive or insulating must be de-Ž . Ž .signed; 2 these layers may be obtained by SA; 3 these

different layers can also be integrated into a device by SA,i.e., the layers can be self-linked.

w xDecher 1 has fabricated various self-assembled poly-electrolyte films via electrostatic interaction. Some poly-electrolyte films have high electrical resistivity and may beused as a dielectric film. Recently, we successfully fabri-cated electrically conductive polyanilinerpolymeric acidfilms via an SA process based on the doping reaction

w xbetween polyaniline and polymeric acids 4,5 . The processis compatible with Decher’s method, and the polyanilinelayer may be self-linked with the polyelectrolyte layer viaa polymeric acid layer as a bridge. This letter reports howwe design and realize a capacitor sensor using these SAfilms.

2. Design of the sensor structure

The structure of a humidity-sensitive capacitor is simi-lar to the sensor shown in Fig. 1. The dielectric layer ismoisture absorptive. The capacitance of the sensor

´ ´ S0 rCs

d

0928-4931r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0928-4931 00 00186-7

( )D. Li et al.rMaterials Science and Engineering C 11 2000 117–119118

Fig. 1. Schematic diagram of a capacitor.

´ is the vacuum dielectric constant, ´ is the relative0 r

dielectric constant of the dielectric layer. S is the effectivearea of the electrodes, and d is the distance between twoelectrodes. When the sensor is exposed to moisture, ´r

increases due to moisture absorption of the dielectric layer.As a result, the capacitance C increases as increasedhumidity. Thus, the humidity may be detected by measur-ing the capacitance of the sensor.

In order to facilitate fabricating a capacitor using theSA method, we adopted another structure. Its schematicdiagram and the equivalent circuit are shown in Fig. 2.Unlike the sensor in Fig. 1, the bottom electrode is re-placed by two comb-like gold electrodes. The device isequivalent to the series connection of two capacitors,which are connected by the moisture-penetrable top elec-trode. Such a structure has been used in commercialhumidity-sensitive capacitors. The bottom electrodes werefabricated on a silicon wafer via a conventional microfabri-cation technology, with an effective area of about 16 mm2.The distance between two bottom electrodes is 20 mm. In

Žthis sensor, the composite film of poly styrenesulfonic. Ž . Ž .acid PSSA rpoly diallyldimethylammoniumchloride

Ž q.P was used as the dielectric layer, and the conductingŽ .polyaniline PAni rPSSA film was used as the top elec-

trode.

3. Assembly of the sensor

Polyaniline was prepared and purified according to thew xprocedure similar to that in Ref. 6 . Its solution was

obtained by dissolving some emeraldine base powder inŽ .N-methylpyrrolidinone NMP and filtered before use. The

concentration is 0.2 wt.%. PSSA was obtained by flowingŽ . Ž .the solution 1wt.% of poly sodium styrenenesulfonate

Ž .Aldrich, M ;70,000 through the cation-exchange resinwq Ž .column. P Aldrich, high molecular weight was used as

received. They were used as assembling solutions.The assembly process involved the following steps.

First, the silicon wafer with a comb-like electrode wasimmersed into 1% aqueous solution of Pq, assembling amonolayer of Pq on the surface of the silicon wafer

w xaccording to Kotov et al.’s procedure 7 . Then, 40 bilayersof PSSArPq were self-assembled onto the wafer by alter-natively immersing the wafer into the 1% aqueous solutionof PSSA and Pq based on the electrostatic interactionbetween PSSA and Pq. The film was terminated with amonolayer of PSSA. Unlike the procedure described in

w xRef. 1 , PSSA was used, instead of its sodium salt. TheUV–VIS absorption spectra of PSSArPq films with vari-ous bilayers on the quartz slides can confirm the self-as-sembled PSSArPq may be formed by the electrostatic

Ž .interaction see Fig. 3 . Finally, 10 bilayers PAnirPSSAwere sequentially self-assembled onto the wafer by alterna-tively immersing the wafer into the 0.2% polyaniline solu-tion in NMP and 1% aqueous solution of PSSA. Wepreviously reported that a monolayer of polyaniline can beabsorbed onto the polymeric acids and electronically con-ductive multilayered polyaniline films can be made via

w xdoping-induced deposition effect 4,5 . In this sensor struc-ture, the PSSA layer between the polyelectrolyte layer andthe polyaniline layer linked the two layers together byelectrostatic attraction and doping reaction, respectively.

4. Humidity-sensitive property of the sensor

The humidity-sensitive property was investigated in ahome-established system. A series of saturated salt solu-tions were used as various humidity sources. The sensor

Ž . Ž . Ž .Fig. 2. Scheme of the prototype humidity-sensing capacitor 1 and equivalent circuit 2 3 .

( )D. Li et al.rMaterials Science and Engineering C 11 2000 117–119 119

Fig. 3. UV–Vis spectra of self-assembled PSSArPq films with various numbers of layers.

was kept at various humidity for 30 min and the capaci-tance was measured by a CM9601A digital multimeter.The relationship of the capacitance and relative humidity isshown in Fig. 4. As expected, the capacitance increasedwith humidity. It seems to conclude that a capacitor-typesensor has been successfully fabricated and the self-assem-

q Ž .bled PSSArP layer and the polyaniline PAni rPSSAfilm do work as the dielectric layer and the top electrode,respectively. When humidity rose in the environment, thepolyelectrolyte layer adsorbed moisture, which caused theincrease of its dielectric constant. As a result, the capaci-

Ž .tance increased see Section 2 .Ž .The response time of the sensor was very short -5 s .

However, unlike commercial capacitor sensors, the rela-tionship of the capacitance and relative humidity is not

Fig. 4. Relationship of capacitance of the sensor and relative humidity.

linear. The capacitance values at various humidity shownin Fig. 4 are the mean of five measurements. The standarderror is up to 5–30%, indicating that the sensor is notstable and repeatable enough for practical applications.The reason is not clear yet. Efforts to improve its perfor-mance are in progress.

5. Conclusion

A prototype capacitor was designed and fabricated basedon a layer-by-layer SA technique, using conductivepolyaniline layer as the top electrode and polyelectrolytelayers as the dielectric layer. The capacitor showed sensi-tivity to humidity.

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