Tunable Multicolor Ordered Patterns with Two Dye Molecules

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By Wenchong Wang , Chuan Du , Hai Bi , Yinghui Sun , Yue Wang , Christian Mauser , Enrico Da Como , Harald Fuchs , and Lifeng Chi *

Since their invention in 1987 by Tang et al, [ 1 ] small-molecule organic light-emitting diodes (OLEDs) have been the subject of intense scientifi c and technological investigation for applications ranging from lighting [ 2 ] and displays [ 3 ] to sensors. [ 4 ] Although monocolor OLEDs are suffi cient for some applications, color integration on a single substrate could greatly enhance their technological impact, especially for full-color displays. Mostly, the challenge lies in the production of user-defi ned multicolor patterns with suffi cient resolution, owing to the absence of pat-terning techniques such as photolithography as applied for inor-ganic semiconductors. Several techniques have been developed, such as subsequent deposition with shadow masks, [ 5 ] vapor jet printing, [ 6 ] reconfi guration of devices by thermal imaging, [ 7 ] and dip-pen lithography. [ 8 ] These techniques, although having been successfully applied to device fabrication, suffer from insuffi cient resolution, poor scalability or complicated multi-step processing. Currently, a technique that combines effi cient, simple fabrication with tunable physical properties is still a big challenge for organic semiconductors. In previous publica-tions, we proposed two strategies to pattern organic molecules by intentionally introducing nucleation centers under vacuum conditions, resulting in organic patterns with high resolution and uniformity over large area. [ 9 , 10 ] Here we demonstrate that two kinds of molecules can be controlled to nucleate on a prede-fi ned area simultaneously. Importantly, we show the capability of tuning the photoluminescence/fl uorescence color of the organic nuclei on defi ned locations by varying the mixing ratio of well selected molecules. Furthermore, tunable multicolor pat-terns can be fabricated by controlling the local concentration of one type of molecule into clusters of the other.

Usually, colored patterns are composed of different mol-ecules with different emission colors. [ 5 ] However, realization of multicolor emissions with fewer molecules is of great advantage

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DOI: 10.1002/adma.201000129

[∗] Dr. W. C. Wang, C. Du, Dr. Y. Sun, Prof. H. Fuchs, Prof. L. F. Chi Physikalisches Institut and Center for Nanotechnology (CeNTech)Universität Münster, 48149 Münster (Germany)E-mail: chi@uni-muenster.deH. Bi, Prof. Y. WangState Key Laboratory of Supermolecular Structure and MaterialsCollege of ChemistryJilin UniversityChangchun, 130012 (P.R. China)C. Mauser, Dr. E. Da ComoPhotonics and Optoelectronics GroupDepartment of Physics and CeNSLudwig-Maximilians-Universität80799 Munich (Germany)

in simplifying both fabrication and characterization of devices. It is well known that the absorption and emission spectra of dye molecules in liquid solutions depend on both the aggre-gation states of the molecules and the dipole moments of the surrounding solvent molecules. [ 11 , 12 ] For a long time, it was dif-fi cult to apply these effects in liquid systems to device fabrica-tion. Recently, solid solvation has attracted increasing interest. A number of researchers have sought to develop tuning of the emission by introducing dopants that change the local electric fi eld in the fi lm. For example, a shift of several tens of nanom-eters of the wavelength of maximum emission can be continu-ously tuned by doping [2-methyl-6-[2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl]-4H-pyran-4-ylidene]propane-dinitrile (DCM2) in aluminum tris(8-hydroxyquinoline) (Alq 3 ) and N , N ′-dipenyl- N , N ′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) as a result of the polarization effect. [ 13 , 14 ] More recently, colored patterns that rely on controlling local molecular aggre-gate states were demonstrated by depositing a single molecule on a polymer-patterned quartz substrate. [ 15 ] However, use of light-emissive small molecules, instead of polymers, will open a novel way to tune optical properties and be favorable for OLED fabrication. For this study, we chose N , N ′-bis(1-naphthyl)- N , N ′-diphenyl-1,1′-biphenyl′-4,4′-diamine (NPB, Figure 1 a) and N , N ′-di[( N -(3,6-di- t -butyl-carbazyl)) n -decyl] quinacridone (DtCDQA, Figure 1 b). The former is a blue-light-emitting, classic hole-transport material widely used in high-performance OLEDs, [ 16 ] while the latter is an orange-light-emitting material with high quantum yield. Figures 1 c,d show photoluminescence (PL) spectra of the two molecules as a powder and dilute solutions. The PL spectrum of NPB shows the typical blue emission with a peak at 446 nm in the powder state; the peak shifts to 464 nm in dilute CH 2 Cl 2 solution due to the polarization effect. [ 13 ] DtCDQA powder has an emission maximum at 581 nm, while in dilute solution DtCDQA displays maximum emission at 548 nm, which corresponds to the monomer band, together with 588 and 636 nm for the aggregation bands. [ 17 , 18 ] This change in spectroscopic properties upon aggregation has been intensively investigated. It originates from intermolecular chromophore interaction in the aggregate that couples the optical transitions of the individual molecules. [ 19 , 20 ]

In a previous publication, we proposed a method to pat-tern organic molecules on mesoscopic scales by using organic molecular beam deposition (OMBD) under ultrahigh vacuum (UHV) conditions, and demonstrated the high quality pat-terning of organic molecules with full site-selectivity and high uniformity by adjustment of the growth parameters. [ 9 ] The tech-nique can be extended to control the nucleation of two kinds of molecules on a predefi ned area, such as the Au-patterned SiO 2 schematically shown in Figure 2 a. The molecules, sublimed

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Figure 1. Molecular structure of a) NPB and b) DtCDQA. Photoluminescence spectra of c) NPB and d) DtCDQA in the powder state and CH 2 Cl 2 at a concentration of 1 × 10 −6 g mL −1 .

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from two independent crucibles and deposited on the substrate, can diffuse along the surface and prefer to nucleate on the pre-patterned Au positions due to the larger binding energy with predefi ned patterns.

We prepared a series of samples on Au dot arrays patterned on SiO 2 (0.5 μm in diameter, 1.3 μm in periodicity) with dif-ferent NPB and DtCDQA mixing ratios. The substrate tempera-ture and growth rate were kept at 140 °C and 0.2 nm min −1 , respectively. Atomic force microscopy (AFM) topography measurements show that both NPB and DtCDQA nucleate exclusively on Au dots. As an example in Figure 2 b, we show an AFM image of NPB and DtCDQA mixing ratio of 9:1. Fur-thermore, the organic nuclei have an excellent uniform height distribution of 113.7 ± 1.1 nm, demonstrating greatly improved uniformity in comparison with that on bare SiO 2 with a height distribution of 114.2 ± 15.2 nm (see Supporting Information S1). More importantly, the concentration of DtCDQA in NPB strongly infl uences the emission color, as shown in the fl uores-cence microscopy images of Figure 2 c. The colors can be tuned from deep blue of pure NPB to sky blue, green, and yellow-green by increasing DtCDQA concentration to 5%, 10%, and 20%, respectively, up to the orange of pure DtCDQA.

We further performed PL measurements to investigate the origin of the color tuning. Figures 3 a and 3 b show the PL spectra of different mixing ratios of NPB and DtCDQA molecules,

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together with the spectra of the pure substances, on patterned substrates as shown in Figure 2 b. Pure NPB and DtCDQA have single emission peaks, at 460 and 583 nm, respectively, which are quite similar to their powder states. The co-deposited sam-ples exhibit DtCDQA-concentration-dependent PL. Besides the NPB emission at 455 nm, the co-deposited samples also con-tain emissions centered at 527 nm (DtCDQA monomer) and 557 nm (DtCDQA aggregate) at low concentration (less than 20%). At high concentration (more than 33%), the spectra are dominated by emission peaked at 583 nm, similar to that of the pure DtCDQA pattern. The presence of emission at 557 nm indicates the existence of the DtCDQA aggregate state in NPB, even at the low concentration of 2%. We note that the emission intensity of NPB decreases rapidly as the DtCDQA concentra-tion increases and becomes very weak when the concentra-tion is over 30%, indicating energy transfer between the two molecules. We fi tted the PL spectra with Gaussians to obtain the individual emission intensity and plotted the relative inten-sity ratios for emissions of DtCDQA monomer ( I DM ) and DtCDQA aggregate ( I DA ) to NPB ( I NPB ) as a function of DtCDQA concentration, shown in Figure 3 c. At low DtCDQA concentra-tion (less than 20%), both I DM / I NPB and I DA / I NPB increase with the concentration, suggesting increased formation of DtCDQA monomer and aggregate states in NPB. The steep slope of I DM / I NPB over I DA / I NPB indicates that more DtCDQA molecules

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Figure 2. a) Schematic evolution of patterned growth with two molecules on a patterned substrate. The molecules, sublimed from a crucible and deposited on the substrate (top left, side view), can diffuse along the surface (right, top view) and prefer to nucleate on the prepatterned Au posi-tions due to larger binding energy (left bottom, side view). b) Morphology of co-deposited NPB and DtCDQA (mixing ratio of 9:1) as viewed by AFM, c) Fluorescence microscopy images of co-deposited NPB and DtCDQA with different mixing ratio. Scale bars: 1 μm.

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exist in the monomer state due to the large number of NPB mol-ecules surrounding them. However, the two ratios behave very differently as the concentration rises to more than 20%. The I DA / I NPB increases faster, while, in contrast, the I DM / I NPB shows a sudden decrease. This can be explained considering that, at high concentration, the molecules have a large probability of forming aggregate states, and more aggregate state emission results in less monomer state emission. The PL spectra of Figures 3 a and 3 b were replotted on the calculated 1931 Com-mission Internationale de L’Eclairage ( x , y ) coordinates (CIE x , y ) to illustrate the range of color achieved, as shown in Figure 3 d. A PL color tunable from blue to green and then to orange can clearly be traced, which is in good agreement with the fl uores-cence microscopy images of Figure 2 c.

The dependence of PL on DtCDQA concentration in NPB enables us further to fabricate tunable multicolor pat-terns based on the two molecules. To achieve this, we create DtCDQA patterns with different thickness on predefi ned areas by a two-step growth process. Using a Au-patterned SiO 2 substrate, we fi rst selectively grew DtCDQA mol-ecules on Au areas with controlled height at high substrate temperature. The substrate temperature was optimized, depending on the pattern dimensions and growth rates, to grow the molecules exclusively on Au. [ 9 ] The molecules were then deposited with a certain thickness at reduced substrate temperature, which prevents the molecules from diffusing into the Au area. This two-step growth process can create

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organic patterns with height h 1 on the Au area and h 2 on the SiO 2 area, as schematically shown in Figure 4 a. Two pattern designs, Au lines and dots on single-crystalline SiO 2 , were prepared to demonstrate that surface occupancy ratio of Au can be a parameter for multicolor tuning. Experimentally we deposited 5 nm DtCDQA on the prepatterned SiO 2 at a substrate temperature of 180 °C, followed by deposition of 1 nm DtCDQA at a substrate temperature of 50 °C. In the area pat-terned with Au lines (with 1 μm width and 5 μm periodicity), a DtCDQA pattern with h 1 = 32 nm and h 2 = 1 nm was achieved, as indicated by the AFM image and line profi le in Figure 4 b. The fl uorescence microscopy image displays only the orange color of DtCDQA over Au lines because the emission from 1 nm DtCDQA on SiO 2 is undetectable (see Supporting Infor-mation S2). After AFM and fl uorescence microscopy measure-ment, the sample was returned to UHV again to deposit 10 nm NPB at 100 °C. A double color pattern, with orange on the Au area and green on the SiO 2 area, can be achieved, as shown in Figure 4 d. The substrate temperature was optimized to be low enough to keep both DtCDQA and NPB from diffusing into the Au area but still to activate the dilution of DtCDQA in NPB. The close-packed nuclei on SiO 2 in the AFM image shown on the right-hand side of Figure 4 d provide direct evidence for the diffusion of molecules over a short distance. The orange color originates from the aggregate state of DtCDQA, owing to the large number of molecules there, and the green color from the monomer state of DtCDQA, owing to the ca. 10%

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Figure 3. a,b) Photoluminescence spectra of NPB and DtCDQA co-deposited on a patterned substrate. c) Relative intensity of DtCDQA monomer emission ( I DM ) and aggregate emission ( I DA ) to NPB emission ( I NPB ) with different mixing ratios. d) CIE chromaticity diagram showing chromaticity coordinates of PL spectra in (a,b). The numbers in brackets are the mixing ratio of the two molecules.

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mixing ratio with NPB (1 nm DtCDQA in 10 nm NPB). The appearance of green light is further evidence of the diffusion and dilution of DtCDQA into NPB. Moreover, a blue-green color pattern can be achieved by further depositing 100 nm NPB at 100 °C, [ 21 ] as shown in the fl uorescence microscopy image of Figure 4 f. The AFM image ( Figure 4 f, right) shows that the close-packed nuclei cross-linked together after depo-sition of 100 nm NPB. The further deposited NPB dilutes DtCDQA on the Au area to form monomers, leading to emis-sion of green light. On the SiO 2 area, the concentration of DtCDQA is less than 1%, therefore NPB emission will domi-nate the PL spectra and give rise to a blue color.

In the Au-dot-array area (with 1 μm diameter and 3 μm periodicity) of the same sample as in the paragraph above, the two-step DtCDQA growth process, that is 5 nm at 180 °C and 1 nm at 50 °C, created the DtCDQA pattern with h 1 = 90 nm and h 2 = 1 nm shown in the AFM image and the corresponding line profi le in Figure 4 c. The different h 1 for a different pattern can be attributed to the surface occupancy ratio of Au (8.7% for the dot array, which is about 0.43 that of the 20% of the line array). That is to say, h 1 can be controlled by surface occupancy ratio of the Au area, besides the number of molecules depos-ited. The same as in Figure 4 b, an orange on Au and green on SiO 2 double color pattern can be obtained by depositing 10 nm DtCDQA at 100 °C, as shown in the fl uorescence microscopy image of Figure 4 e. However, the color on Au remains orange

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on the further deposition of 100 nm NPB due to the large amount of DtCDQA (about 45% of DtCDQA). As discussed previously, the DtCDQA concentration on SiO 2 is less than 1% and NPB emission will dominate the PL emission, resulting in an orange-blue color pattern ( Figure 4 g). The right-hand parts of Figures 4 e and 4 g are the corresponding morphology viewed by AFM. In all the cases, by using two dye molecules we can realize tunable multicolor patterns with resolution in the sub-micrometer range.

In conclusion, we have demonstrated that one type of mole-cule can be diluted into clusters of another on predefi ned areas by extending the patterned growth technique to more than one kind of molecule. This top-down assisted bottom-up strategy provides a versatile way to defi ne organic semiconductors on designed locations together with photoluminescence color tuning. Additionally, tunable multicolor patterns can be real-ized by controlling the local mixing ratio of molecules. This strategy provides a simple way to defi ne primary color emit-ting areas with a resolution in the sub-micrometer regime. Although the designed prepatterns shown in this work were fabricated on SiO 2 with electron beam lithography (EBL), the method can be extended to other surfaces such as indium tin oxide (ITO), and other patterns fabricated with parallel methods such microcontact printing and nanoimprinting, thus it will allow the patterning of organic patterns with tun-able color over large areas.

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Figure 4. a) Schematic representation of DtCDQA distribution on patterned surface by a two-step growth technique. b,c) Left: AFM images of 5 nm DtCDQA grown at 180 °C followed by 1 nm DtCDQA at 50 °C on a Au line array (b, line width 1 μm, periodicity 5 μm) and dot array (c, diameter 1 μm, periodicity 3 μm) on SiO 2 . Right: Line profi les along the lines marked in the left-hand AFM images. d,e) Left: Fluorescence microscopy images of 10 nm NPB deposited on samples shown in (b,c) at 100 °C Right: Corresponding AFM images. f,g) Left: Fluorescence microscopy images of further 100 nm NPB deposited on samples shown in (d,e) at 100 °C. Right: Corresponding AFM images.

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Experimental Section All the patterns were generated by EBL on heavily doped Si(100) with 300 nm thermally grown oxide. After EBL, the substrates were coated with 4 nm Cr adhesion layer and 6 nm Au layer by thermal evaporation, followed by lift-off of the poly(methylmethacrylate) PMMA resist. After sequential ultrasonic lift-off and cleaning in chloroform, acetone, ethanol, and deionized water, the prepatterned substrate was introduced into UHV and degassed. The organic molecules were sublimed from a quartz crucible and deposited on the substrate. The growth rate was controlled by crucible temperature and monitored by a quartz microbalance. The as-grown samples were analyzed ex situ by AFM and fl uorescence microscopy/spectroscopy.

Acknowledgements This work was supported by the Transregional Collaborative Research Centre TRR 61 by DFG, the National Natural Science Foundation of China (50733002), the Major State Basic Research Development Program (2009CB939700), and the 111 Project (B06009). E.D.C. is grateful to the BMBF for funding through the project “TOPAS 2012”.

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Supporting Information Supporting Information is available online from Wiley InterScience or from the authors.

Received: January 12, 2010 Revised: March 1, 2010

Published online: April 26, 2010

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