附件一
行政院國家科學委員會補助專題研究計畫
;
成 果 報 告
□期中進度報告
碟狀分子材料之配向及其光電特性的異向性之研究
計畫類別：
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個別型計畫 □ 整合型計畫
計畫編號：NSC 96－2221－E －110 －042
執行期間： 96 年 8 月 1 日至 97 年 7 月 31 日
計畫主持人：鄭文軍
共同主持人：
計畫參與人員：博士研究生 – 兼任助理: 江征晏
碩士研究生 – 兼任助理: 王信評
成果報告類型(依經費核定清單規定繳交)：
;
精簡報告 □完整報告
本成果報告包括以下應繳交之附件：
□赴國外出差或研習心得報告一份
□赴大陸地區出差或研習心得報告一份
;
出席國際學術會議心得報告及發表之論文各一份
□國際合作研究計畫國外研究報告書一份
處理方式：除產學合作研究計畫、提升產業技術及人才培育研究計畫、列
管計畫及下列情形者外，得立即公開查詢
□涉及專利或其他智慧財產權，□一年□二年後可公開查詢
執行單位：国立中山大学光電工程研究所
中 華 民 國 97 年 7 月 28 日
摘要
碟狀分子材料被視為最富潛力的有機半導體及新型光電材料。為提高碟狀分子材料元件 的性能﹐對碟狀分子堆疊的控制以獲得所需的分子配向極為重要。本計畫的主要課題是研究 碟狀分子與固體表面的相互作用﹐以及基板表面特性對碟狀分子組裝的影響。我們的精力集 中在對基板表面自由能的觀察和研究﹐及其對碟狀分子堆疊的影響。裸玻璃﹐ITO 鍍層玻璃﹐ 鐵氟隆﹐聚醯亞胺被用着基板. 這些基板提供具有不同的表面能的表面。我們還採用氧氣電 漿轟擊表面以對基板表面能進行調整。我們的研究發現基板表面能對碟狀分子在該基板上的 堆疊具有重要的功效。在一個表面自由能較低的表面上﹐碟狀分子一般以“邊緣朝上＂ 錨 定的方式堆疊﹐並可期望得到平面配向的柱狀態﹔而在表面能較高的基板上﹐分子則以“碟 面朝上＂方式堆疊﹐並可形成垂直配向的柱狀態。溫度在分子堆疊過程中亦相當重要。樣品 降溫速度過快將產生局部熱力學紊亂引起柱體堆疊的破損而導致分子定向失敗。基板溫度的 昇高則可使碟狀分子柱偏離基板法向而傾斜。 關鍵詞﹕碟狀分子 分子堆疊 自組裝 表面能 表面錨定 柱狀態配向 AbstractMaterials that consist of discotic molecules are thought to be potential materials for organic semiconductors and photonic applications. It is primary important to achieve desired molecular alignment in the columnar phase in order to get good performance of the devices. The main object of the project is to study the molecule-surface interactions in materials consist of discotic molecules, and the effect of the surface characteristics of the substrates on the molecular assembly of the discotic materials. As to the surface characteristics of the substrates, we focused on the surface free energy and its effects on the self-assembly of the discotic molecular systems. Bare glass plate, Indium-Tin-Oxide (ITO) coated glass, PTFE, and polyimide have used as substrates. These substrates of different materials provide surface with different surface free energies. The effects of O2 plasma on modifying surface free energy of a substrate were studied. It has been
found that surface free energy of a substrate to play a very important role in determining the molecular anchoring of the discotic molecules, and the orientation of the columnar phase. On a surface with low energy, the discotic molecules will assemble with the edge-on anchoring, and the homogenous alignment of discotic columns can be expected, whereas a substrate which possesses a high free energy favours the face-on anchoring and may lead to the homeotropic alignment of the columns. For a homeotropically aligned columnar phase, a decrease in the surface free energy of the substrate will cause an increase in the tilt of the columns from the normal of the substrate.
Key words: discotic molecules, molecular stack, self-assembly, surface free energy, surface
I. Introduction
Discotic mesogens that consist of disc-like molecules can form columnar phases through self-assembly. The chamecophysical properties of the disc-like molecules can be easily modified by changing their chemical components and structures. In the columnar phases, the disc-like molecules spontaneously form one-dimensional molecular stacks and further assemble into two dimensional lattices. The orientation in columnar phases can be frozen into ordered glassy states [1]. In the columnar phase, the π−π conjugation aggregates along the normal of disc, i.e. in axial direction of the column, and the barrier in the direction is rather low, so the charge carriers can easily drift and form a high density disperse current in the columns along the axis, whereas the barrier in the transversal direction can be very high thus only a small tunnelling current can be formed between columns. Thus a well aligned columnar phase is a one-dimentional conductor. It has been reported [2,3] that the axial mobility of charge carriers can reach 10-2 ~ 1 cm2V-1s-1, and the axial conductivity can be 1010 fold greater than the transversal conductivity. These remarkable characteristics make them very attractive materials in opto-electric and photonics applications. One of the attractive applications is the use of the discotic materials in fabrication of molecular wires [4].
To exploiting techniques for the preparation of molecular wires and explore the possibilities of the uses of them in organic optoelectronic devices and micro-bio-sensors motivate us to conduct the researches reported in this project.
II. A brief review of background in this research field
In 1990s, it was seen a great leap in researches on discotic organic semiconductors. Adam et al. [5] reported that the mobility of charge carriers could reach 0.1 cm2 V-1s-1 in a highly ordered columnar phase. Later, An et al. claimed that they had obtained a charge mobility as high as 1.3 cm2V-1s-1 in a liquid crystal phase perylene diimide [6]. This marks that the era of organic semiconductor is coming. One of the main barriers for the uses of discotic materials is that a practical technique for aligning discotic molecules to form a desired molecular stacking is still not available.
In early stage, molecular growth of discotic molecules in a magnetic field [7], and shearing sustained discotic materials [8] were tried to achieve oriented columnar phases. These methods are not adopted because of their complexity and poor alignment results. Molecular epitaxial growth technique has been applied for discotic molecule alignment. Highly ordered discotic materials can be produced on Au (111) and Cu (111) [9]. Taylor et al. reported face-on stack of discotic columns can be achieved on mica substrate [10,11]. However, this technique demands a very high quality substrate, and requires costly equipment. Other techniques, including those used for aligning liquid crystals consist of rod-like molecules, have been tried and are found to be not quite applicable because of the high viscosity of discotic materials.
producing homogenous alignment of discotic columns [12,13]. In this technique, a thin layer of PTFE is coated onto a substrate, which is pre-heated to 300°C, by pressing and rubbing a PTFE rod on the substrate. A low surface free energy of the PTFE favours the edge-on molecular anchoring for many discotic materials. The problems with this technique are difficulties in obtaining large area discotic layer with uniform alignment.
A very attractive method with great potential is the one that is developed by Monobe et al [14-16]. They used a 6 ~ 8 μm wavelength laser beam to illuminate discotic material thin layer. The material in the region exposed to the laser will be heated to a high temperature, and reduce its viscosity, and the molecules in the region will be driven to aligned in accordance to the polarization of the laser beam.
Although the techniques mentioned above can more or less show their ability to align discotic molecules, none of them can be used in practical applications due to their low reliability, high cost, or complexity. On the other hand, despite the substantial activities related to the material processing and the device applications of discotic materials, the fundamental molecular assembly principles that determine the macroscopic orientation of discotic molecules are still poorly understood.
III. Objectives
To insight into the self-assembly systems, find out the factors that govern molecular stacking, and produce electronic and photonic devices with good performance motivates us to carry out work proposed by this project.
Initially, the proposed project includes two phases of work: the study of the effects of surface characteristics and energetic states of environment on the molecular stacking of discotic molecules, and the control of orientation of discotic molecules and construction of columnar phase for the product of molecular wires. What presented in this report is the work carried out in the first phase of the research. The primary objectives of the project are to investigate the surface characteristics of the substrates and interfacial behaviors of the discotic molecules.
IV. Methodology and experimental set up
In general, discotic molecules tend to stack face-to-face through a self-organization which governed by the π−π interaction, and form a columnar structure[17]. In an assembly of discotic molecules, two basic types, the face-on and the edge-on, of molecular anchoring of the discotic columns on a substrate are generally found. In the face-on anchoring, the discotic molecules stack with the normal of the disc faces perpendicular to the substrate surface, whereas in the edge-on anchoring, the normal of the disc faces is parallel to the surface of the substrate. A columnar phase, in which discotic columns aggregate to have their axes parallel to the normal of the substrate over entire observed region, is referred to as the homeotropically aligned columnar phase. The
homogeneous alignment of columnar phase is defined as the phase in which all axes of the columns orient in the same direction and parallel to the substrate surface (c.f. Fig. 1).
(a) (b)
Fig. 1. Schematically showing (a) the homeotropic alignment of molecular stack and (b) the homogeneous alignment of molecular stack of discotic molecules.
The discotic materials used were two in house synthesized discotic compounds HDBP-8 (2,3,6,7,11,12-hexaoctyloxy-dibenzo[a,c]phenazine) and LC10 (2,3,8,9,14,15- hexakisdecyloxydiquinoxazlino[2,3-a:2′3′-c]phenazine), both were synthesized and provided by Prof. C. W. Ong of Chemistry Department of NSYSU. The chemical structures of the material are shown in Fig. 2. Phase transition sequences of the compounds were determined by means of both differential scanning calorimetry and optical microscopy, and are listed in Fig. 2. Both compounds exhibit the hexagonal columnar phase (Colh) in a wide temperature range with a
marked supercooling behavior at Col/Crystal transition.
(a) (b)
Fig. 2. Chemical structure and phase sequence of (a) 2,3,6,7,11,12- hexaoctyloxy-dibenzo[a,c]phenazine (HDBP-8), and (b) 2,3,8,9,14,15- hexakis(decyloxy)-diquinoxazlino-[2,3-a:2’3’-c]phenazine (LC10).
To better understand DLC/substrate interfacial interactions, surface energy of the substrates was evaluated by means of sessile drop measurement. For this purpose contact angles between standard reference liquids and relevant substrates were measured, and the substrates were characterized for surface energy by application of the Owens-Wendt theory [18]. The evaluation of the surface free energy of the substrates was performed using KRÜSS DSA100 surface tension meter.
(Linkam Scientific Instrument Ltd). The alignment states of the samples were examined by means of optical microscopy, that was performed using a polarizing microscope (Axioscop 40, ZEISS). Photomicrographs were taken using a digital camera (PowerShot A620, Canon) that is installed in the microscope.
V. Results and achievement
5.1. Effects of surface free energy of substrates
The surface free energy of the substrate is a decisive factor that can govern the way the anchoring of the discotic molecules on the surface. For rod-like molecules, Porte [19] proposed a classic mode that states the pretilt angle θ, which is the angle the long molecular axis makes with the substrate surface, is determined by the relative magnitude of the surface energies of the substrate γs and the liquid crystal γL,
Δγ = γL – γs. (1)
The condition of Δγ > 0 makes θ = π/2, and induces the homeotropic alignment; the condition of Δγ < 0 makes θ = 0, and induces the homogeneous alignment. If we take disc normal as the molecular axis for discotic molecules and let θ be the angle between surface normal and molecular axis, the results of our observation in the present studies are of agreement with Porte’s theory.
Fig. 3. Alignment of discotic columns on a solid surface. θ is the tilt angle of the column made against the normal of the surface of the substrate, and φ is azimuthal angle of the column on the surface.
Firstly, we treated the glass surface to modify the surface free energy of the substrates by O2
plasma bombardment. The fluxes of oxygen and argon were controlled at 800 ml/min and 80 ml/min, respectively. The chamber pressure was maintained at 1.6 mbar. Figure 4 shows the variation of surface free energy with the duration of bombardment. The surface free energy increases with an increase in bombarding time. We found that on a surface with high surface energy, the discotic molecules stack to form columns with face-on anchoring, whereas a surface with low free energy favors edge-on molecular anchoring. As surface energy decreases, the discotic columns tilt from the normal of the substrate. A short period of plasma bombardment was
0 5 10 15 20 25 30 60 62 64 66 68 70 72 Su rface free e ner gy (mN/ m ) Bombarding time(min)
Fig. 4. Effect of O2 plasma bombardment on the surface free energy.
found to be unable to produce an energy template that can effectively support monodomain homeotropically aligned columnar phase, and as a result, the discotic layer is broken into domains (c.f. Fig. 5). Extending the period of the plasma bombardment is helpful for achieving a monodomain discotic layer [20].
Fig. 5. Photomicrograph of a layer of LC10 sandwiched between substrates treated by 15 min O2 plasma bombardment. The mosaic texture of the sample indicates
domains formed in the sample.
Surfaces with different free energies can be easily prepared by coating the glass substrates with different materials. Table 1 shows the surface free energies of substrates covered with different materials.
Table 1. Surface free energies of different substrates
Substrate Glass ITO Polyimide PTFE
Surface free energy / mJ m-2
53.32 40.98 39.01 38.45
The discotic molecules in a droplet standing on glass surface are in edge-on with discotic columns in a radial arrangement as shown by the depicted molecular stacking model in the Fig.6(a). On ITO surface, which possesses a lower free energy, the discotic molecules are organized in the edge-on orientation. As illustrated in Fig. 6(b), the discotic columns are assembled with axes of the columns parallel to the substrate surface.
When sandwiched between ITO and PTFE coated substrates, HDBP-8 exhibits uniform striped textures indicating a uniform homogeneous alignment of the discotic molecules (c.f. Fig.7).
(a) (b)
Fig. 6. (a) The central radial texture and the molecular stacking model of HDPB-8 on glass surface. The disc-like molecules stack in edge-on orientation, and the discotic columns are arranged in a radial structure. (b) Optical texture of HDBP-8 on ITO, and possible molecular structure of the material. The photographs were taken when the sample temperature was 130°C.
In summary, on surfaces, which possesses a higher free energy, the face-on anchoring of discotic molecules is usually observed, whereas on a surface, with a lower surface free energy will favor the edge-on anchoring. For a homeotropic aligned columnar phase, when the surface free energy is reduced, the columns will tilt from the normal of the substrate, and finally lay parallel to the surface, i.e. become homogeneously aligned, if the surface free energy is very low.
(a) (b)
Fig. 7. Optical texture of HDBP-8 sandwiched (a) between ITO glass substrates, and (b) between rubbed PTFE coated glass plates.
5.2. Surface free energy of polyimide modified by mechanical rubbing
We clearly showed that, as illustrated in Fig. 8, surface free energy of polyimide decreases with an increase in rubbing strength. The surface free energy is found to vary azimuthally after the polymer surface is rubbed. The surface energy measured along rubbing direction is different from that measured against the rubbing direction.
We introduce a concept of the in-line anisotropy of surface energy for the analysis of the orientational behaviour of molecules in a deformed surface energy template. We found that by properly rubbing a polymer, the in-line anisotropy of surface energy can be effectively reduced or eliminated, and this is in favour of producing defect free ferroelectric liquid crystal layers [21]. This is significant as it may lead to opening the use of ferroelectric liquid crystals in display applications. We believe that it will also be useful when we consider the alignment of the columnar
phases.
(a) (b)
Fig. 8. (a) Surface free energy of polyimide thin film against rubbing strength. (b) Azimuthal variation of surface free energy of a rubbed polyimide thin film.
5.3. The effects of temperature of environment
To produce an ordered molecular stack of a columnar phase on planar substrates, the speed of temperature dropping of the sample be controlled below 0.3°C min-1 [22]. A fast change in
temperature will cause turbulence leading to breaking or destroying the ordered molecular stacking.
Fig. 9 shows the variation of surface free energy of ITO coated glass substrate with temperature. The surface free energy of ITO glass drops markedly as temperature increases. The orientation of the discotic columns and found that the tilt of the columns from the substrate normal increased with substrate temperature. This provides a very useful guideline for the alignment of discotic materials. 20 40 60 80 100 44 46 48 50 52 54 56 S urf ace Fr ee E nergy ( m N/ m ) Temperature (oC )
Fig. 9. Temperature response of surface free energy of ITO glass.
VI. Summary and suggestions 6.1. Achievements achieved
i) The energetic states of the substrates have significant effects on the stack of discotic molecules
The surface free energy of the substrate is a decisive factor that governs the way that the discotic molecules anchor on the surface. On a surface with low energy, the discotic molecules will assemble with the edge-on anchoring, and this imposes a homogenous alignment of discotic
columns, whereas a substrate which possesses a high free energy favours the face-on anchoring. For a homeotropically aligned columnar phase, a decrease in the surface free energy of the substrate will cause an increase in the tilt of the columns from the normal of the substrate.
Some preliminary results have been reported [21,23]. Further discussions have been presented in manuscripts [20,25], and have been submitted for publishing.
ii) Surface free energy can be modified by mechanical rubbing
Rubbing breaks the two-dimensional topographical uniformity of the polymer surface and causes changes in the surface free energy of the polymer thin films. The surface free energy is found to decrease with an increase in the rubbing strength, and exhibit azimuthal non-uniformity. One paper [24] has been published, and another manuscript [25] has been submitted for publishing.
iii) The effects of the temperature of the substrate
Surface energy of the substrate is found to decrease with the increase in temperature, and this leads to an increase in the tilt of the discotic columns from the normal of the substrate. This provides another guideline for the control of the orientation of the columnar phase. A manuscript on this theme is writing up.
6.2. A self-assessment
We have learnt the effects of surface characteristics on the molecular stacking of the discotic molecules, and have an insight into the interfacial region of the discotic materials and solid surface system. Over 95% of the targets proposed by the project have been achieved. In addition, we have studied the effects of surface characteristics of the substrates on interfacial behaviours for a wide range of mesogenic materials. We are now reaching the frontier in this research area. The knowledge about the surface characteristics of the substrates and the interfacial are very useful and helpful.
6.3. Suggestions for future work
Based on the achievements, further researches that should be conducted in future are suggested as follows.
i. Modify surfaces for achieving desired anchoring of discotic molecules. The surface modification can be achieved by using new surfactants, engineering surfaces into suitable topographic patterns, or through a photo-processing.
ii. We are proposing to use laser annealing discotic layer to achieve desired molecular conformation directly. This is a new approach in controlling assembly of discotic materials, and would open a new research direction in this field.
References
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molecules in the columnar phase, Proc. Asia Dis. 2007, 2, 1766 – 71.
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polyimide thin films, Jap. J. Appl. Phys. 47, 1651-56 (2008).
[25] W. Zheng, C.-C. Wang, S.-P. Wang, and C.-L. Ko, Surface Wettability of Rubbed Polyimide
Appendix
Publications sponsored by or relevant to the current project
[1] W. Zheng, C.-Y. Chiang, and C.-W. Ong, Effect of surface free energy on stacking of discotic
molecules in the columnar phase, Proc. Asia Dis. 2007, 2, 1766 – 71.
[2] W. Zheng, C.-Y, Chiang, C. W. Ong, S.-C. Liao, and J.-Y. Huang, Temperature control
molecular stacking of discotic liquid crystal in columnar mesophase, Proc. of SPIE, Vol. 6587,
658719-1-8 (2007).
[3] W. Zheng, C.-H. Lu, and Y.-C. Ye, Effects of mechanical rubbing on surface tension of polyimide thin films, Jap. J. Appl. Phys. 47, 1651-56 (2008).
[4] W. Zheng, Y.-T. Hu, C.-Y. Chiang, and C.-W. Ong, Moelcular stacking of discotic liquid
crystals on the surfaces treated by O2 plasma, accepted to be published in Proc. of SPIE,
2008.
[5] W. Zheng, C.-C. Wang, S.-P. Wang, and C.-L. Ko, Surface Wettability of Rubbed Polyimide
Thin Films, submitted.
[6] W Zheng, and Y.-Z. Su, Rubbed Polyimide Thin Films for Zigzag Free Surface Stabilized
