Oxidation of Rubrene Thin Films: An Electronic Structure Study

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Oxidation of Rubrene Thin Films: An Electronic Structure Study Sumona Sinha, Chia-Hsin Wang, Manabendra Mukherjee, Tapas Mukherjee, and Yaw-Wen Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/la503357t • Publication Date (Web): 10 Nov 2014 Downloaded from http://pubs.acs.org on November 19, 2014

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Oxidation of Rubrene Thin Films: An Electronic Structure Study Sumona Sinha‡, C-H Wang§, M. Mukherjee*‡, T. Mukherjeeǁ and Y-W Yang§, † ‡

§

ǁ

Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata – 700064, India National Synchrotron Radiation Research Center, Hsinchu, Taiwan-30076

Physics Department, Bhairav Ganguly College, Kolkata-700056, India

†

Department of Chemistry, National Tsing-Hua University, Hisnchu, Taiwan 30013

ABSTRACT The performances of organic semiconductor devices are crucially linked with their stability at the ambient atmosphere. The evolution of electronic structures of 20 nm thick rubrene films exposed to ambient environment with time has been studied by UV and X-ray photoemission spectroscopy (UPS and XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory (DFT). XPS, NEXAFS data and DFT calculated values suggest the formation of rubrene-epoxide and rubrene-endoperoxide through reaction of tetracene backbone with oxygen of ambient environment. Angle dependent XPS measurement indicates entire probed depth of the films react with oxygen by spending only about 120 min in ambient environment. The HOMO peak of pristine rubrene films almost disappear by exposure of 120 min to ambient environment. The evolution of valence band (occupied states) and NEXAFS (unoccupied states) spectra indicates that the films become more insulating with exposure as HOMO-LUMO gap increases on oxidation. Oxygen induced chemical reaction completely destroys the delocalized nature of the electron distribution in the tetracene backbone of rubrene.

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The results are relevant to the performance and reliability of rubrene based devices in the environment. ■ INTRODUCTION In recent years, rubrene (5,6,11,12-tetraphenyl-tetracene) has emerged as a promising material for organic field effect transistors (OFET) because OFETs based on rubrene single crystal currently hold the record of hole mobilities (~ 40 cm2/Vs) among organic semiconductors 1. Even the mobility of rubrene thin-film transistors (TFT) obtained as high as 2.5 cm2/Vs, is comparable with that of hydrogenated amorphous silicon TFT 2. Significant efforts have been made for making the performance of rubrene based TFTs better 3-5. In addition, few studies were carried out to understand transport and electronic properties of rubrene TFT 6-8. But the key issue regarding the wide-spread production of organic electronic devices is the long term stability in ambient operating conditions. It is found that the device performance of vacuum evaporated organic semiconducting thin films is degraded by ambient environment

9-11

. Therefore, the real

atmospheric effect on the electronic structure with time is immensely important to study for making more developed realistic devices. It has long been known that rubrene solution converts into rubrene-endoperoxide in the presence of oxygen and UV light

12

. Few studies are done on

the oxidation of rubrene single crystal where authors have shown that the trap states within the band-gap are created due to oxidation of rubrene molecules in the single crystal phase

13, 14

.

However it is well-known that TFT is more convincing than single crystal based one for practical device applications. The researchers have previously reported their works by focusing the change of electronic structure of vacuum deposited π conjugated organic semiconductors by introducing oxygen at various pressures in the chamber

15-18

. But the concerned issue about the worldwide

manufacturing of OTFTs is the degradation with time in ambient operating environments 2


19-21

.

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On the other hand, lower injection barrier between the electrode and the active organic layer is needed to facilitate a greater charge across the interface. Among all the metals used in the devices, gold is a primary choice for the electrode material due to its noble property and high work-function; enabling an Ohmic contact for the electrode/organic interfaces 6, 22- 23. Therefore, it is highly desirable to have a comprehensive investigation about the effects of air, moisture and ambient light on the electronic structure of vacuum deposited rubrene thin films grown on gold substrate. Motivated by this vital issue, in this report, the effects of successive ambient environment exposure to the electronic structures of 20 nm thick vacuum evaporated rubrene thin films on clean Au substrate have been studied using ultra-violet photoemission (UPS). Whereas the X-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure spectroscopies (NEXAFS) were employed to find out the origin of change of electronic structures of rubrene thin films with time of exposure to ambient environment. While photoemission spectroscopy provides information about the occupied states, NEXAFS is useful to map unoccupied states of the system

24, 25

. Moreover, experimental evidence has been rationalized by referring to the

results of density functional theory (DFT) calculations carried out on the isolated molecules. Rubrene molecule comprises of a tetracene backbone and four phenyl side groups

26

. We have

shown that rubrene-epoxide and rubrene-endoperoxide are formed through reaction of tetracene backbone with oxygen of ambient environment. The oxygen induced chemical reaction results in the attenuation of the highest occupied molecular orbital (HOMO) peak of the pristine rubrene with its complete disappearance within 120 min of exposure. It is also revealed that the oxidation destroys the delocalized nature of the molecular orbitals of rubrene and increases the HOMOLUMO gap with exposure time. The results are relevant to the reduction of charge carrier 3


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mobility of rubrene based OTFT by several orders of magnitude with spending time in ambient environment. ■ EXPERIMENTAL SECTION The X-ray (XPS) and ultra-violet (UPS) photoelectron spectroscopy experiments have been carried out using a Omicron Multi-probe (Omicron NanoTechnology, UK) ultrahigh vacuum (UHV) system (base pressure ~ 5.0 x 10-10 mbar), which is equipped with two light sources. Photons with energy of 21.2 eV from a He discharge lamp was used for UPS measurements. All UPS measurements were done by applying a sample bias of −8.0 V. A monochromatized Al Kα source provides photons with energy of 1486.6 eV for XPS. In angle dependent XPS experiments, the sample was rotated to change the electron emission angle (take-off angle) by keeping the X-ray source and the analyzer fixed. The organic vapor deposition chamber (base pressure ~ 9.0 x 10-9 mbar) was directly attached to the analysis chamber allowing the samples to be studied without breaking the vacuum. The clean gold substrate was obtained by Ar+ sputtering on 100 nm thick Au coated Si wafer (Sigma Aldrich) until the C 1s and O 2p XPS signals were vanished and consistent values of the work function was obtained. Rubrene films of thickness of about 20 nm were grown on clean Au substrate by thermal evaporation of rubrene powder (Acros Organics, 99%) at 180oC from a resistively heated quartz crucible of an organic material effusion cell (MBE-Komponenten GmbH, Germany). The nominal thickness of the rubrene films were calibrated by monitoring the evaporation rate with a quartz crystal microbalance (Sycon Instruments. Inc, USA). The evaporation rate for all the films was about 0.05 Å/s. The exposure of rubrene films to ambient air (temp ~ 25±1oC, relative humidity ~ 40±5%, light: laboratory florescent lamp) was carried out by transferring the sample from UHV to air side with exposure time set at 2, 5, 10, 25,120, 480, 1440 min successively. Generally, 4


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florescent lamp emits some ultra-violet radiation along with visible light 27, 28. Earlier researchers have shown that illumination increases the oxidation of rubrene thin films. They also used a source of radiation in the UV to visible range

29, 30

. After each exposure, the sample was

characterized using XPS and UPS spectroscopies. The 1440 min exposed rubrene thin films were heated twice at 70oC and 80oC for 10min in UHV chamber. After heating, these samples were also investigated by using XPS and UPS spectroscopy. A set of angle-dependent measurements were carried out on 120, 480 and 1440 min exposed samples to probe the thickness dependent profile of oxygen attacked carbon sites at various take-off angles from 90o to 20o. XPS peak positions and intensities were determined by using Peakfit data evaluation software. The curve fitting of the core XPS lines was carried out using Gaussian–Lorentzian sum (GL) functions. By following the same manner of sample preparation, near edge x-ray absorption fine structure spectroscopy (NEXAFS) experiments were carried out at BL24A beam line of the storage ring of the National Synchrotron Radiation Research Center, Taiwan. The NEXAFS spectra at the C-K and O-K absorption edges were acquired in the total electron yield (TEY) detection mode. The data were collected with X-ray incidence perpendicular and electric field vector parallel to the sample surface. The base pressure of deposition and analysis chambers were 1.0 x 10-8 and 1.0 x 10-10 torr respectively.

All the depositions and characterizations were performed at room

temperature. ■ COMPUTATIONAL METHODS The equilibrium geometry, electronic structure and X-ray absorption spectra of the free molecule were calculated by DFT with the computer code STOBE-deMon RPBE exchange/correlation functional was applied

34, 35

31- 33

. A gradient corrected

. To calculate the equilibrium geometry

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and the X-ray absorption spectra, we used all-electron triple-ζ valence plus polarization (TZVP) atomic Gaussian basis sets for carbon and oxygen centers, while the hydrogen basis sets were chosen to be of the double-ζ (DZVP) type

36

. The starting geometries for the optimization

procedure were obtained using Avogrado (http://avogadro.cc/). To calculate X-ray absorption spectra, the Slater transition state method was applied

37, 38

. In this case the optimized geometry

obtained from the geometry optimization calculation was kept fixed and absorption spectra were calculated. In order to obtain an improved representation of relaxation effects in the inner orbitals, the ionized center was described by using the IGLO-III basis

39

. A diffuse even

tempered augmentation basis set was included at the excitation center to account for transitions to unbound resonances. The NEXAFS spectra were generated through a Gaussian convolution of the discrete spectra with a broadening of 0.5 eV. ■ RESULTS AND DISCUSSION Figure 1 shows the series of valence bands of pristine and multistep air exposed samples by UPS. It can be observed from Figure 1, the lower energy region of the VB is drastically affected with increasing exposure time of the deposited rubrene film in the air. The first peak at the lowest binding energy of the VB or HOMO of the pristine rubrene is attenuated and the sharpness of the other peaks which are located at higher binding energy region (2-5 eV) of the VB is also reduced

6


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with successive exposure to air. With exposure to air the position of the peaks of the VB

VB by UPS Intensity (Arb. Unit)

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o

1440 mins+ 80 C 1440 mins 120 mins 10 mins Pristine

0

2

4 6 8 Binding Energy (eV)

10

12

Figure 1. The VB spectra of rubrene thin films before and after exposure to air and heated in UHV chambers. The vertical scales are adjusted to better show the spectral features. are also shifted to the higher binding energy side. In addition, a new peak corresponding to the excitation of O 2s orbital

40

appears at higher binding energy region of the VB (~ 26 eV,

observed from XPS) at larger exposure to air (data not shown). The lower binding energy cut-off of HOMO provides the value of hole injection barrier from Au to HOMO of rubrene, the most important parameter for the device purpose. The evolution of HOMO part of the VB with exposure time in air is plotted in figure 2 (a). It can be clearly observed from figure 2 (a) that the intensity of HOMO (of pristine rubrene) at ~1.45 eV is being dramatically attenuated by exposure to air and it is almost vanished after 120 min exposure to air. The higher binding energy cutoff (HBEC) of the UPS spectrum is related to the work-function of the sample. Figure 2 (b) shows the series of HBEC of pristine as well as air-exposed samples. The HBEC is gradually shifted to the higher binding energy region with increasing exposure time. It is interesting to note the changes of the 1440 min air exposed sample after it was heated at 70oC for 7


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(a)

Change in HOMO

(b)

o 1440 min+80 C Heated HBEC

o 1440 min+70 C Heated 1440 min + 80oC heated

Intensity (Arb unit)

Intensity (Arb. unit.)

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1440 min 480 min 120 min

25 min

1440 min + 70oC heated

1440 min 480 min 120 min 25 min 10 min 5 min 2 min Pristine

10 min 5 min 2 min Pristine 2.5

2.0

1.5

1.0

0.5

0.0

16

Binding Energy (eV)

17

18

19

Binding Energy (eV)

Figure 2. (a) HOMO and (b) HBEC of before (Pristine) and after multistep air exposed rubrene thin films. The vertical scales are adjusted to better show the spectral features. 10 min in UHV chamber. A shifting of VB spectra (figure 1) and HBEC (figure 2 (b)) towards lower binding energy was observed for the heated sample. Further shift to lower binding energy was observed when the sample was further heated at 80oC for 10 minutes in UHV chamber. The changes in the VB strongly indicate that the ambient exposure greatly affects the rubrene thin films and the effects are reduced with UHV heating. To find out the origin of the exposure induced change of UPS spectra, NEXAFS experiments on carbon K- edge was carried out. 8


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Figure 3 shows a series of C K-edge NEXAFS spectra taken at normal incidence geometry for the rubrene thin films before and after exposure to atmosphere. One can easily observe from the

1440 min

Intensity (Arb Unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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25 min

10 min

α

βγ ηζ

Pristine

280 285 290 295 300 305 310 315 320

Photon Energy (eV) Figure 3. Carbon K-edge NEXAFS spectra taken at normal-incidence geometry before (pristine) and after multistep air exposure of 20 nm thick rubrene thin films. The vertical scales are adjusted to better show the spectral features.

figure the three well defined peaks, namely α, β and γ at 284.18, 285.09 and 285.68 eV respectively of π* region of C K-edge NEXAFS of pristine rubrene thin films. The peak positions are close to the previously reported data

26

. With increase of the exposure time, the

intensity of α and γ peaks are significantly attenuating. In fact, the signature of α and γ peaks are almost vanished after 25 minutes exposure. The gradual diminishing of lowest unoccupied molecular orbital (LUMO) or the first peak (α) of NEXAFS spectra along with disappearing of 9


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HOMO peak (fig 1) shows that HOMO-LUMO gap increases with exposure. This indicates that the rubrene films become more insulating with ambient exposure 24, 25. The peaks labeled α and γ are assigned to transitions from carbon atoms at the tetracene backbone whereas the peak labeled β is mostly associated to the excitations within the four phenyl side groups of rubrene

26

. The

gradual diminishing of α and γ peaks indicates that the tetracene backbone of rubrene was very much affected with the exposure to ambient environment. A series of XPS survey scans before and after air exposure is shown in figure 4. It can be easily observed from the figure that O 1s peak has appeared and the intensity of the peak increases with exposure time. Furthermore, N 1s peak, at around 400 eV, was not observed in our scans. This suggests that the possibilities of nitridation of rubrene can be ruled out.

C 1s

XPS Survey Scan

O 1s

Intensity (Arb. Unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1440 min

120 min 10 min Pristine

1200

1000

800

600

400

Binding Energy (eV) 10


200

0

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Figure 4. XPS survey spectra of pristine, 10 min, 120 min and 1440 min air exposed rubrene thin films. The vertical scales are adjusted to better show the spectral features.

C 1s core level XPS spectra of pristine and multi-stepped exposed rubrene thin films are shown in figure 5(a). The spectrum for the pristine rubrene film is dominated by a sharp peak located at 284.5 eV. An additional low intensity peak, located around 286.3 eV (blue shaded region of figure 5 (a)) is found to grow stronger with air exposure. The binding energy of the small peak is (a) 3000 2000 1000

(b)

C 1s

Raw Data Fitted Data Rubrene New

400 300 200 100 0

1440 min

0 3000

225

0

150

Counts

1000

3000 2000

25 min

1000

3000 2000

0

25 min

75

0

0 288

75

10 min

1000

1000

120 min

160 120 80 40 0

0

2000

1440 min

120 min

2000

3000

Raw Data Fitted Data Lower B.E. Peak Higher B.E. Peak

O 1s

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 25

Raw Data Fitted Rubrene

287

286

5 min

0 536

Pristine 285

284

283

535

534

533

532

531

530

529

528

Binding Energy (eV)

282

Binding Energy Figure 5. (a) C 1s and (b) O 1s core level XPS spectra of pristine and after multistep air exposed rubrene thin films. The vertical scales are adjusted to better show the spectral features.

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in the range for carbon atoms bonded to oxygen, therefore, we conclude that this peak corresponds to the formation of chemical bonds between oxygen and carbon atoms of rubrene molecules. It may be noted that for C atoms directly bonded to oxygen in hydroxyl (-COH) configurations the peak in XPS occurs at ~285.6 eV (shifts of ~1-1.5 eV to higher BE relative to sp2 peak). Furthermore, the C=O double-bond emission occurs at even higher BE range and arises from >C=O (∼287.5 eV) followed by COOH (∼289 eV). Hence, it can be presumed that the peak appeared at ∼286.3 eV in our case is due to the formation of epoxy (C-O-C) and/or endoperoxide (C-O-O-C) groups as it should have a larger BE compared to hydroxyl groups 41-44

15,

. To estimate the fraction of rubrene molecules that has been oxidized, we have plotted the

ratio of reacted to non-reacted carbon intensity with exposure time in figure 6 (a). It can be observed from the figure that the oxidization of rubrene molecules was rapid upto 120 mins exposure. After that, the reaction gradually saturates. It can also be observed (points at 1440 mins in figure 6 (a)), that there is negligible change in intensity ratio due to heating upto 80oC which indicates that oxidized rubrene molecule was not dissociated due to heating at 80oC in UHV chamber. For gathering more deterministic information, O 1s core level spectra were also collected after successive exposure. It can be observed from figure 5 (b), the intensity of O1s peak at about 533.1-532.9 eV, increases with longer exposure time in ambient environment. An additional peak was required at about 531.5 eV to fit the O 1s XPS data, for the 25 min exposed sample. The intensity of this additional component of O 1s spectra also increases with exposure time. Presence of strong O 1s peak at about 533.1-532.9 eV indicates that epoxide and/or endoperoxide compounds are predominant products due to oxidation of rubrene thin films 15, 30.

12


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0.08

(a) IO(Weak)/ IO(Main)

0.10 IC(Reacted)/ IC(Nonreacted)

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0.08 0.06 0.04

for exposure time 1440 mins air exposed +UHV heated

0.02

(b)

0.06 0.04 0.02

for exposure time 1440 mins air exposed +UHV heated

0.00

0.00 0

Figure 6. (a)

500 1000 Exposure time (mins)

I C ( React ) I C ( Nonr e act )

and (b)

1500

I O (Weak ) I O ( Main )

0

500 1000 Exposure time (mins)

1500

vs. exposure time of rubrene thin films. C (React) and C

(Nonreact) indicate reacted and non-reacted carbon. O (weak) and O (main) indicate weak and main components of O 1s spectra.

No noticeable O 1s peak around 534.7 eV corresponding to the chemisorbed/intercalated water molecules in sp2 hybridized solid 41, 44 was observed, which indicates that water molecules were negligible in the films. To explore the origin of weak component of O 1s peak at 531.5 eV more clearly, the intensity ratio of weak to main oxygen peak with exposure time is plotted in figure 6 (b). We can observe from the figure that the ratio suddenly increase from 10 mins exposure, then the increase is gradual. Whereas the ratio decreases drastically due to heating. As the oxidized rubrene compound was not dissociated due to heating at 80oC (figure 6 (a)), this change of intensity ratio with heating suggests that the weak component of O 1s spectra arises from physisorbed oxygen within rubrene films. Moreover, 20 nm rubrene films become thinner by partial evaporation during the heating at 80oC and signal from Au substrate was observed in XPS spectra (figure S1, Supporting Information). It may be noted that the peak position of the weak 13


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component is identical to that obtained from air exposed Au substrate (figure S2, Supporting Information)

45

. This indicates that oxygen molecules diffuse through amorphous rubrene thin

films and get physically absorbed on Au substrate on longer exposure in air which may give rise to the shift of HBEC towards higher binding energy. The HBEC shift may be partially ascribed to the new oxidized compounds 14 that are different from pristine rubrene. In order to explore the mechanism of the observed dependence of work function with exposure to air for our sample, we investigated the change of work function with exposure for the pristine Au substrate. We find that work function of the clean Au substrate changes from 5.48 eV to 4.52 eV after a long air exposure (figure S2, Supporting Information). The clean Au surfaces are inert and the surface contaminants brought about by exposing it to air may be attached to the surface defects like kink, step, void, etc. In this case, the situation is even more complicated because Au film is polycrystalline; grain boundary can also act as trapping centers. This type of shift of HBEC was also reported by other researchers 16, 46.

0.08

(a)

(b) IC(React)/IC(Nonreact)

0.07

IO(Total)/IC(Total)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.06

0.05

0.04 120 min 480 min 1440 min

0.03

0.07

0.06

0.05

120 min 480 min 1440 min

0.04 90

80

70

60

50

40

30

20

90

80

Take Off Angle (degree)

70

60

50

40

30

Take Off Angle (degree)

14


20

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Figure 7. (a)

IO (Total ) IC (Total )

and (b)

I C ( React ) I C ( Nonr e act )

vs. take off angle of 120 min, 480 min and 1440 min

exposed rubrene thin films. The vertical scales are adjusted to better show the spectral features. C (React) and C (Nonreact) indicate reacted and non-reacted carbon.

If oxygen was to diffuse through rubrene layer and reach Au substrate, then it can be expected that reacted oxygen will be present at all depth of the rubrene film. To check this, we have performed angle dependent XPS experiments on 120, 480 and 1440 min exposed rubrene thin films. The C 1s and O 1s core level data were collected for these three samples at different take off angles (90-20°). In figure 7 (a), we have plotted the ratio of calculated total oxygen to total carbon intensity of exposed rubrene thin films with take-off angle. Assuming inelastic mean free path of electron to be ∼3nm here the probing depth for XPS is ∼9nm at 90°and ∼3nm at 20° 47, 48

. No systematic behavior of the ratio indicates that oxygen was diffused equally at least up to ~

9 nm in rubrene layer. To access the quantity of reacted oxygen atoms with thickness in the rubrene layer, the ratio of reacted to non-reacted carbon intensity of 120, 480 and 1440 min exposed rubrene films with take-off angle is plotted in figure 7 (b). It also shows nonsystematic change of the ratio that confirms that the reacted oxygen is homogeneously present at least upto about 50% depth of the films. Since there was no reduction of oxygen intensity upto the measured depth, it was assumed that the film was porous enough to allow oxygen to diffuse upto the substrate. Next, O K-edge NEXAFS spectra at normal X-ray incidence geometry was collected for 1440 min exposed film to get information for the specific bonding configurations of oxygen atoms. 15


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Unfortunately, O K-edge NEXAFS database in the literature is not rich enough to deconvolute and assign all the peaks

41, 49-53

. Upon judicious review of the available literature we have

deconvoluted the peaks as shown in Figure 8 (a). However the assignment of all peaks could not be made. In order to substantiate the experimental findings and to

(a)

Experimental 538.1 eV

536.8 eV

535.1 eV

532.8 eV

For 1440 mins Exposed Rubrene Thin Films 530.9 eV

O- K edge


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Flat Conformation 540.1 eV

Raw Data Fitted Data

Rub epoxide Rub endo-peroxide

(b)

Calculated

530

535 540 545 Photon Energy (eV)

550

Figure 8. Ο K-edge (a) NEXAFS spectra at normal geometry for 1440 min exposed rubrene thin films and (b) calculated O K-edge angle integrated XAS spectra for epoxide (blue) and endoperoxide (red) in rubrene molecule (with flat tetracene backbone). Vertical bars show the ionization thresholds. understand the situation in a better way, we have performed theoretical investigations. As shown in figure 8 (b), O K-edge XAS spectra for random molecular orientation was calculated for 16


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rubrene epoxide and endoperoxide molecules. The incorporation of oxygen atoms on particular carbon sites of rubrene molecule was done following literature for our calculation 54, as shown in figure 9. In this context, it can be mentioned that rubrene molecule has two stable conformations with flat and twisted backbone respectively as obtained from our geometry optimization calculation. It was observed that rubrene molecule with twisted backbone has lower total energy compared to that with flat backbone. The energy difference obtained from our calculation was about 285 meV. The number is close to the previously reported value 26.

Figure 9. (a) Pristine rubrene molecule; oxygen incorporations in rubrene molecule: (b) epoxide (c) endoperoxide (note the bending of the backbone). Color code: hydrogen (white), carbon (gray) and oxygen (red). It was observed earlier by various researchers that rubrene molecules are mainly in flat conformation at 20 nm thick deposited films on Au substrate

26, 55

. Hence, we have considered

rubrene molecule in flat conformation for theoretical calculations. It is imperative to discuss about the origin of peaks of calculated XAS spectra for their assignment. The peak located at about 530.9 eV of calculated endoperoxide (figure 8 (b)) corresponds to transition for O1s to LUMO orbital whereas the subsequent small peak at about 532.5 eV is due to the transitions into LUMO+1 and LUMO+2 orbitals. The next noticeable peak of endoperoxide at around 535.1 eV, 17


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just below the ionization threshold (shown by the vertical lines in the figure) corresponds to the excitations into the Rydberg-type final state orbitals 56. The peak located at around 532.8 eV of calculated epoxide spectra is due to the excitations into LUMO+3 and LUMO+4 orbitals whereas the cross sections for excitations into LUMO, LUMO+1 and LUMO+2 orbitals of epoxide molecules are small. Furthermore, the peaks at around 536.8, 538.1 and 540.1 eV are arising due to the excitations into σ* orbitals for both the molecules.

The matching of

experimental and theoretical spectra clearly suggests that the sample contains both epoxide and endoperoxide molecules. It should be mentioned here that as there may be some preferential molecular orientation on the substrate and as the sample may contain some molecules with twisted backbone, that are not considered in the calculation, the matching of spectral intensity is not expected. For further verification of the oxidation products of rubrene, density of states (DOS) for rubrene pristine, epoxide and endoperoxide were calculated and plotted in figure 10 (a). Our calculation has revealed that the HOMO peak of pristine rubrene at around 1.45 eV is decreased and shifted towards higher binding for rubrene epoxide whereas this peak is almost disappeared for rubrene endoperoxide. The extended VB DOS is shown in figure S3, supporting documents. Calculated spectra are in good agreement with the observed change of the VB spectra with exposure (figure

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(b) Intensity (Arb. Unit)

Total DOS for VB

(a)


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Pristine Rubrene Rubrene Epoxide Rubrene Endoperoxide

5

4

3 2 1 Binding Energy (eV)

0

Pristine Rubrene Rubrene Epoxide Rubrene Endo- Peroxide

-1

288

286

Calculated C 1s

284

282

Binding Energy (eV)

Figure 10. Calculated (a) DOS and (b) C 1s spectra for rubrene pristine, epoxide and endoperoxide molecule. Appropriate (equal) rigid shift in the energy scales ware given for calculated spectra to match with experimental values. 1(a)). In figure 10 (b) we have plotted the calculated core level DOS for rubrene pristine, epoxy and endoperoxide. Although the DOS does not predict the energy of the XPS core level correctly due to large relaxation effects as a result of core ionization, it can be observed from the calculated DOS that additional peaks at higher binding energies appear for epoxide and endoperoxide molecules, which correspond to the observed low intensity broad additional peak at around 286.3 eV as shown in figure 5(a). We have also approached to study the relative change of HOMO-LUMO gap due to formation of rubrene epoxide and endoperoxide by DFT calculation. Though the present form of DFT calculation usually underestimates of HOMOLUMO gap, the qualitative changes are reliable58,

59

. It can be observed (from figure S4,

Supporting Information) that the HOMO-LUMO gap increases for rubrene epoxide and endoperoxide compared to pristine rubrene. Within the framework of DFT, we further studied the electron density distributions for HOMO and LUMO for pristine and oxidized rubrene molecules. 19


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Figure 11. Electron density distribution (a, c, e) for HOMO orbitals and (a, c, e) for LUMO orbitals of rubren pristine, epoxide and endoperoxide molecule respectively. Figure 11 shows that the HOMO and LUMO of rubrene molecule is delocalized and distributed over the tetracene backbone, whereas, the electron density of HOMO and LUMO for epoxide molecule is disrupted near the epoxy bonds and the delocalized nature of the electron distribution of HOMO and LUMO is completely destroyed for endoperoxide molecule. This indicates that the electron transport along the tetracene backbone of the oxidized rubrene molecule would be significantly reduced. ■ CONCLUSIONS In conclusion, we have studied the evolution of electronic structures of rubrene thin films exposed to ambient environment with exposure time. Initial C K-edge NEXAFS data suggests 20


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that tetracene backbone has been gradually destroyed with exposure. Further XPS, O K-edge NEXAFS and DFT results indicate that rubrene-epoxide and rubrene-endoperoxide through reaction of tetracene backbone with oxygen of ambient environment are formed. The depth profile study of oxygen bonded rubrene species through angle dependent XPS spectroscopy indicates that entire probed depth of the films have reacted with oxygen by spending only 120 min in ambient environment. The oxygen induced chemical reaction destroys the delocalized nature of the molecular orbitals of rubrene and increases the HOMO-LUMO gap. Due to oxidation of rubrene, HOMO of pristine thin film was attenuated and the sharpness of the other higher binding energy peaks of the VB were reduced with the position of VB peaks shifted towards higher binding energy. The HOMO peak of pristine rubrene film was almost completely disappeared by passing 120 min in ambient environment. The observed reduction of work function of rubrene/Au interfaces was attributed to the physisorption of diffused oxygen to polycrystalline Au substrate through rubrene layers. Our results justify the reduction of charge carrier mobility of rubrene based OTFTs by several orders of magnitude with exposure to ambient environment. ■ ASSOCIATED CONTENT *S Supporting Information 1) XPS survey scan before and after 80oC UHV heating of 1440 mins exposed rubrene thin films on Au substrate. 2) XPS survey scan before Ar+ sputtering, corresponding O 1s and C 1s peaks and shift of HBEC for Ar+ sputtering of Au substrate. 3) Calculated DOS of extended VB for rubrene pristine, epoxide and endoperoxide molecule. 21


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4) Calculated DOS of occupied and unoccupied states for rubrene pristine, epoxide and endoperoxide molecules. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGEMENTS The work is partially supported by the India-Taiwan program in Science and Technology. S. S. thanks UGC, Govt. of India for financial support through fellowship. M.M. and T.M. thankfully acknowledge Luca Pasquali (Università di Modena e Reggio Emilia, Itally), K. Hermann (FritzHaber-Institut der Max-Planck-Gesellschaft, Berlin, Germany) and L.G.M. Pettersson (Stockholm University, Sweden ) for valuable e-mail discussions about use of the StoBe code. ■ REFERENCES 1. Yamagishi, M.; Takeya, J.; Tominari, Y.; Nakazawa, Y., Kuroda, T.; Ikehata, S.; Uno, M.; Nishikawa, T.; Kawase T. High-Mobility Double-Gate Organic Single-Crystal Transistors With Organic Crystal Gate Insulators. Appl. Phys. Lett. 2007, 90, 182117. 2. Hsu, C. H.; Deng, J.; Staddon, C.; R.; Betton, P. H. Growth Front Nucleation of Rubrene Thin Films for High Mobility Organic Transistors. Appl. Phys. Lett. 2007, 91, 193505.

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Oxidation of Rubrene Thin Films: An Electronic Structure Study Sumona Sinha, C-H Wang, M. Mukherjee, T. Mukherjee and Y-W Yang

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Oxidation of Rubrene Thin Films: An Electronic Structure Study

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