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Journal of Luminescence 121 (2006) 132–136

www.elsevier.com/locate/jlumin

AFM studies on formation of new phase responsiblefor enhanced photoluminescence in light-emitting

small molecular thin films

Vivek Kumar Shuklaa,�, Satyendra Kumara, Dinesh Devab

aDepartment of Physics and Samtel Centre for Display Technologies, Indian Institute of Technology Kanpur, Kanpur-208016, IndiabAFM Lab, Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India

Received 6 September 2005; accepted 14 November 2005

Available online 23 February 2006

Abstract

Tris(8-hydroxyquinoline) metal complex (AlQ3) is a widely used light-emitting material in organic light-emitting

devices (OLEDs). The environmental stability is still a major problem with OLEDs and needs further improvements. In

this report, we examine the optical properties of the thin films of this material and the effect of environment on it. The

annealing in different ambients were done with the aim to understand the physical phenomenon involved in AlQ3

thin films. An enhanced photoluminescence intensity is observed for the samples annealed in oxygen near 100 1C.

Sudden change in roughness which may be as a consequence of change in surface morphology due to phase change and

fraction of new phase is estimated by phase images taken by Atomic Force Microscopy (AFM). The enhanced

photoluminescence is understood in terms of formation of a new phase.

r 2005 Elsevier B.V. All rights reserved.

Keywords: Organic light-emitting devices (OLEDs); Photoluminescence (PL); Tris(8-hydroxyquinoline) aluminium (AlQ3); Atomic

force microscopy (AFM)

1. Introduction

Since first report on small molecule basedorganic light-emitting devices (OLEDs) [1], tris(8-hydroxyquinoline) aluminium (AlQ3) has becomearchetype material used as light-emitting layers in

e front matter r 2005 Elsevier B.V. All rights reserve

min.2005.11.003

ing author. Tel.: +91 512 2597 654;

90 914

ess: vkshukla@iitk.ac.in (V.K. Shukla).

OLEDs. However, ambient instability is a majorproblem with these devices [2–4]. Moreoverthermal stability is a critical issue for devicelifetime. The reasons for device degradation arecrystallization of organic layers [2,3], cathodeoxidation and photo-instability of organic layers.

Thin films of AlQ3 were prepared by thermalevaporation on silicon and glass substrates. Thinfilms were annealed at different temperatures invarious ambients of air, nitrogen and oxygen with

d.

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and without moisture. The effect of light has beenalready investigated on these organic thin films[5–7].

AAC mode AFM experiments were carried outto study the surface topography and phaseimaging of surfaces. In AAC mode, a cantileverwith relatively high spring constant is oscillatednear its resonant frequency. The oscillating tip ismade to approach the sample until it strikes thesurface during each period of oscillation. Changein amplitude gives topographic image. Phaseimaging is an added advantage of AAC modewhere phase lag between approach and retractionis measured. Change in phase lag indicates thechange in surface property beyond topography.Topography and phase imaging is done simulta-neously. Cluster and particle size is measured usingtopographic images while fraction of new phase isestimated by phase images.

The increase in photoluminescence intensity isobserved for the samples annealed in oxygen near100 1C. This enhancement is understood due to anew phase formation.

2. Experimental

Thin films of AlQ3 were deposited on Si andglass substrates (kept at room temperature) at adeposition rate of less than 2 A/s by thermalevaporation of powder in specially designedcrucibles at a base pressure of �10�6 Torr.Thickness of the film during the growth wasmonitored using a digital thickness monitor. Thefilm thickness was also measured using Alphastepprofiler and by ellipsometry. The thicknesses in allthe cases are in good agreement.

Photoluminescence data at room temperaturewere recorded using a spectrofluorometer (Fluo-rolog 3, Jobin Yvon) with front face detectinggeometry, in which the emitted signal is collectedat �221 with respect to the normal at the surface ofthe sample, coincident with the excitation lightdirection.

AFM measurements were conducted usingmolecular imaging (MI), USA make AFM equip-ment in noncontact, acoustic AC (AAC) mode.Advantage of AAC mode is that phase image is

also obtained with topography images. Cantileversused for AAC mode NSC 12 (c) were fromMikroMasch having force constant �4.5N/m andfrequency �150kHz. Images of all the sampleswere collected for scan area of 1500� 1500 nm2

with 512 data points in each line. The scan speedwas kept around 1 lines/s. The images were taken atthe various locations of the sample and all theimaging were done in air at room temperature.SPIP software has been used to analyze thecollected images.

3. Results and discussion

The optical properties of thin films of AlQ3

deposited at room temperature were studied. Theeffects of moisture, oxygen, nitrogen and air withtemperature were investigated.Different samples prepared on Si substrate

(100 nm thick) were annealed in different ambi-ents, i.e. vacuum (�3� 10�5mbar), dry oxygen,humid oxygen (65% relative humidity), nitrogenand in air for 15min each and PL spectra wererecorded immediately at room temperature. Smallchange in initial PL peak intensity of differentsamples was due to variation in film thicknesses(Fig. 1).No appreciable change in the PL intensity as

well as in the PL peak position was found whenthese films were annealed in vacuum up to 200 1C,which shows that annealing in vacuum does notaffect the PL properties of these films.Annealing the sample with dry O2 retains its

intensity unchanged up to �80 1C. However, anincrease in PL intensity is observed if annealedbetween 100 and 120 1C. Above this temperature,there is a monotonic decrease in intensity. Theseresults show that some new phase of AlQ3 isformed when film is annealed in presence ofoxygen which is responsible for enhancement ofPL intensity. [8]. The initial PL peak position(�525 nm) remains unchanged below criticaltemperature (�100 1C) and above this temperatureit falls monotonically down to 505 nm on anneal-ing at 175 1C.Experiment of annealing the sample with humid

O2 (65% RH) at different temperature shows that

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500

505

510

515

520

525

530

PL

pea

k (n

m)

20 40 60 80 100 120 140 160 180 200 220

Annealing Temperature (°C)

Annealing in AirAnnealing in VacuumAnnealing in Dry O2

Annealing in Humid O2

Annealing in N2

Fig. 2. The photoluminescence peak positions of AlQ3 films

on Si substrate annealed for 15min at each temperature in

various ambient of vacuum, nitrogen, oxygen (dry and humid)

and air.

20 40 60 80 100 120 140 160 180 200 220

PL

pea

k In

ten

sity

(a.u

.)

Annealing Temperature (°C)

Annealing in AirAnnealing in VacuumAnnealing in Dry O2Annealing in Humid O2Annealing in N2

Fig. 1. The photoluminescence peak intensity of AlQ3 films on

Si substrate annealed for 15min at each temperature in various

ambients of vacuum, nitrogen, oxygen (with and without

humidity) and air.

V.K. Shukla et al. / Journal of Luminescence 121 (2006) 132–136134

its intensity remains unchanged up to �120 1C.This can be understood as the balance betweenincrease due to oxygen reaction and decrease dueto moisture content reaction with temperature [9].After annealing for 15min at this temperature(�120 1C), there is a sudden fall in the peakintensity, which is similar to annealing in dryoxygen, but this fall is more in comparison to incase of dry O2. The initial PL peak (�525 nm) falls(blue shifts) monotonically up to critical tempera-ture and then suddenly goes down to 505 nm onannealing at 175 1C (Fig. 2).In case of annealing in air, after annealing at

100 1C, the intensity enhances and starts fallingdown after annealing above this temperature (evenat 120 1C), the red shift is observed after annealingat 120 1C. Both PL intensity decrease and red shiftabove critical temperature may be explained byformation of aggregates or crystals which promoteformation of excimers or ground state complexes[10].The results for the sample annealed in nitrogen

ambient shows that the intensity remains approxi-mately unchanged while the PL peak positionshifts to 520 nm till �120 1C, and suddenly falls to510 nm when annealed at 175 1C.It is well known that PL emission intensity in

AlQ3 films decreases with annealing temperature

which is due to chemical reaction with water athigher temperature. At higher temperature, the8-hydroxyqunoline is produced which is highlyvolatile above 90 1C [9]. Hence, chemical reactionwith moisture decreases the concentration ofemission centers which are responsible for decreasein emission intensity as shown in the case ofannealing in air and humid oxygen.

The annealing in vacuum experiment shows thatthere in no appreciable change in PL peak positioneven after annealing at 175 1C. Hence the possibi-lity of blue shift due to increased crystallinity as aresult of only annealing is ruled out. The forma-tion of new phases in presence of ambient isproved in these cases.

The AFM experiments (in situ) have beencarried out to investigate the changes in morphol-ogy on annealing the sample in air at differenttemperatures (50, 75, 100, 125 1C) for 15min andAFM images were recorded immediately aftercooling down to room temperature. Suddenincrease in surface roughness has been observedwhen annealed at 75 1C (Fig. 3). This change inroughness is attributed to the formation of a newphase.

To understand the mechanism of change in PL,AFM phase images were recorded at roomtemperature on as-deposited AlQ3 films as well

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as films treated with oxygen at �100 1C for15min. Fig. 4(a) and (b) show phase images of as-deposited and annealed film in oxygen at 100 1C.Scale for both the phase images are identical as canbe seen in the color scale given at the bottom of theimages. No contrast in phase image is seen in as-deposited sample. On the other hand phase image

0 25 50 75 100 125 150

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

Rm

s ro

ug

hn

ess

(nm

)

Annealing temp (°C)

Fig. 3. RMS roughness as a function of annealing temperature.

AFM data are taken immediately after cooling down to room

temperature.

900

800

800

700

600

600

500

400

400

300

200

200

100

00

0 5 10 15

nm

nm

v

Pha

se fl

atte

ned

Scanned ↑→(a) (

Fig. 4. AFM phase images of as-deposited film on Si substrate

of annealed sample shows clear features indicatingthe presence of a new phase (Fig. 4(b)).

4. Conclusion

Photoluminescence measurements results showthat new phases of AlQ3 is formed when thin filmis annealed in the presence of oxygen which isresponsible for enhanced PL intensity. Phaseimaging by AFM shows clear features of a newphase formation. Hence enhanced PL intensityafter annealing AlQ3 film near critical temperature(�100 1C) in presence of dry oxygen is due toformation of a new phase.

Acknowledgements

Financial support from the Department ofScience and Technology, New Delhi, SamtelGroup of Industries and IIT Kanpur is gratefullyacknowledged. Authors thank Professor AsimaPradhan for photoluminescence measurements.

900

800

700

600

500

400

300

200

100

0

nm

8006004002000nm

0 5 10 15v

Pha

se fl

atte

ned

Scanned ↑→b)

(a) and annealed at 100 1C for 15min in dry oxygen (b).

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