Alternate heteroepitaxial growth of highly oriented organic multilayer films.

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Alternate Heteroepitaxial Growth of Highly Oriented Organic Multilayer Films Zi Wang, Hao Chang, Tong Wang, Haibo Wang, and Donghang Yan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp412310y • Publication Date (Web): 27 Mar 2014 Downloaded from http://pubs.acs.org on April 7, 2014

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The Journal of Physical Chemistry

Alternate Heteroepitaxial Growth of Highly Oriented Organic Multilayer Films Zi Wang,†, ‡ Hao Chang,†, ‡ Tong Wang,† Haibo Wang† and Donghang Yan*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022 (P. R. China) ‡

University of Chinese Academy of Sciences, Beijing, 100049 (P. R. China)

* Corresponding author. Changchun Institute of Applied Chemistry, Chinese Academy of Sciences Changchun, 130022 (P. R. China), Tel: +86-431-85262165, Email: [email protected].

1 ACS Paragon Plus Environment


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Abstract. The heterostructure, a basic active unit applied in the device level, plays an important role in traditional inorganic optoelectronics. In the organic field, although the requirement for the heterostructure is crucial, achievement and understanding on the growth and functionality of organic heterostructure are still finite, especially for ordered crystalline organic multilayers with smooth interfaces. Here a series of highly ordered crystalline heterostructures with molecule-level smoothness were obtained from single layer to alternate-multilayer with a phthalocyanine molecule and a perylene derivative. Well-defined epitaxy relationship and crystal alignment were evidenced from the atomic force microscopy (AFM), X-ray diffraction (XRD), and transmission electron microscope (TEM) results. The evolution of the films reveals that for organic-organic alternatemultilayer growth, along with the intrinsic properties of organic molecules such as the packing and preferred growth direction, the soft matter property of organic films contributes to well-defined heteroepitaxy in spite that the lattice mismatch between the two materials’ bulk phases is large. Thin film phases of the first few layers benefit the grain coalescence and thus the formation of smooth films. Potential application is implied from the heterojunctions’ good transport ability.

Keywords: organic heterostructure, epitaxy growth, organic electronics, ambipolar transistor.


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INTRODUCTION Organic crystalline heterostuctures are crucial for their widely application in the optoelectronic area, such as ambipolar transistors, photovoltaics or light emitting diodes15

. The heterostructure film’s crystal ordering and interfaces have been proved to be

critical factors determining the electronic structures and charge transport behavior of the devices. Although much process has been made to achieve efficient charge transport by improving the quality of organic films6-8, constructing oriented multi-layer crystalline organic heterostructures with molecular level smoothness is still a big challenge. A big obstacle for organic semiconductor system is the lack of valid methods to realize large area smooth crystalline heterostructure films, regardless from solution process or vacuum deposition. Many groups tried different approaches and delicately chose material system towards forming crystalline heterostructures. The early study imitated the inorganic heteroepitaxy using molecular beam epitaxy (MBE) technique. Typical examples are the Naphthalene tetracarboxylic dianhydride (NTCDA)/ perylene tetracarboxylic dianhydride (PTCDA) or PTCDA/copper phthalocyanine (CuPc) system9-15, in which the substrates usually are the inorganic single crystals such as alkali halide crystals, highly oriented pyrolytic graphite (HOPG) and so on. Although crystalline films with preferred orientation could be achieved, the crystal substrates and the limited thickness prevent them from further application on the large scale organic devices. Other groups tried to fabricate heterostructure films directly on the amorphous substrate such as the Si/SiO2 or ITO glass, from which organic devices can be directly fabricated16-22. Wakayama et al. built an organic super-lattice through finely tuning the growth condition using the perylene tetracarboxylic diimide (PTCDI) derivatives and quanterrylene, which showed a



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nice layer by layer growth fashion through precisely controlling the substrate temperature and thickness23. Another heterostructure system based on phthalocyanine materials was finely formed owing to the similarity of molecular structures and film properties between the planar phthalocyanines24. The pentacene and fully fluorinated pentacene heterostructure

similarly

showed

good

morphology,

but

alternate

multilayer

heteroepitaxy always gave rise to rough or non-uniform films25. So far, multi-layer organic heterostructures with both high quality and valid optoelectronic property are rare. The reasons are probably that 1) alternate heteroepitaxy with smooth interfaces is hard to achieve, which to much extent originates from the complexity of organic semiconductors and their unpredictable growth behaviors 2) the finite of high quality multi-layer film systems limited further property investigation, usually only molecules with similar chemical structure could possibly show promising film morphology. And growth mechanism related to organic-organic heterostructure is unclear. Here, we constructed a series of highly ordered crystalline multilayer heterostructures using a p-type material zinc phthalocyanine (ZnPc) and an n-type material N, N’-diphenyl perylene tetracarboxylic diimide (PTCDI-Ph) (Figure 1a) on Si/SiO2 substrate through a simple method named weak epitaxy growth (WEG), in which an ultrathin organic film was deposited first on the SiO2 to induce the following active layer growth26-27. Highly oriented organic-organic heterostructures with roughnesses on the molecular level were formed, which show great potential in exploring organic heterostructure devices. The uniformed crystal orientation and the well-defined epitaxy relationship confirmed good alternate heteroepitaxial growth. The evolution of the films’ morphology and structure suggested the intrinsic molecular interaction (packing), the interface template and the


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energy favorable thin film phases determined the growth and the final morphology of the heteroepitaxy structure. The soft matter property of organic materials was proved to be a crucial role benefitting this well-defined heteroepitaxial growth. Ambipolar charge transport property with fairly good performance was also observed.

p-6P O

O

N

N

O

O

PTCDI-Ph

ZnPc

a) PTCDI-Ph (5 nm)

3

ZnPc (5 nm)

ZnPc (5 nm)

2

PTCDI-Ph (5 nm)

PTCDI-Ph (5 nm)

1

ZnPc (5 nm)

p-6P

p-6P

Si/SiO2

Si/SiO2

Type A

Type B b)

Figure 1. a) Molecular structures of p-6P, PTCDI-Ph and ZnPc; b) structure configurations of two types of heterostructures.



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EXPERIMENTAL METHODS Materials and Film Fabrication. ZnPc was purchased from Aldrich, PTCDI-Ph and para-sexiphenyl (p-6P) was synthesized according to ref. 28 and ref. 29. All the materials were purified twice by gradient thermal sublimation prior to deposition. Highly doped ntype silicon wafers with thermally grown SiO2 (300 nm, capacitance per unit area (Ci):10 nFcm-2) were used as the substrate. p-6P was firstly deposited on the pre-cleaned substrate as the inducing layer through vacuum deposition, PTCDI-Ph or ZnPc deposited in sequence to form different heterostructures. During the deposition, the substrate temperature was kept at 150 °C, the deposition rate is 1nm min-1 and the pressure is 10-410-5 Pa. Film Characterization. The film morphologies were imaged by a SPI 3800/SPA 300HV (Seiko Instruments Inc., Japan) with tapping mode, in which the cantilevel is SiN4 with a spring constant of 3 N/m. The wide angle XRD pattern were taken from a D8 discovery thin film diffractometer with Cu Kα radiation (=1.54056 Å). The selected voltage and current were 40 kV and 35 mA, respectively. The selected area electron diffraction (SAED) was performed by JEOL JEM-1011 transmission electron microscope operated at 100 kV. Dark field was used for experiments to provide weaker-intensity beam and high contrast. The sample for SAED mensurement was fabricated using the following precedure: The normal film sample that an organic film deposited on Si/SiO2 substrate was directly used to fabricate SAED sample. After organic film deposition, an adding layer of carbon film (using thermal evaporation, normally 20 nm) was deposited on top of the organic film as supporting layer, and then slowly dipped the sample into a water solution with 10%HF. The organic film will separate from the Si/SiO2 substrate and float 6 ACS Paragon Plus Environment

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on the solution surface. The carbon films on top of the organic film keep the whole films from cleavage. After that we used the copper grid to catch the floating organic film to make a SAED sample. 2.3 Transistor fabrication and measurement. The gold contacts were thermally evaporated on the pre-fabricated film surface through shadow masks to form bottom-gate top-contact transistors with a channel width (W) and length (L) of 6000 µm and 200 µm, respectively. The current-voltage characteristics of the devices were measured with two Keithley 236 source-measurement units under ambient conditions at room temperature.

RESULTS AND DISCUSSION Morphology and Structure of the Organic Heterostructures. A bilayer p-6P film was directly deposited on the Si/SiO2 substrate as the inducing layer. The choosing of bilayer p-6P is because almost only one oriented crystal for both ZnPc and PTCDI-Ph could be achieved in each p-6P domain based on previous work.26-27 5 nm ZnPc (about 4 ML, according to the XRD result, each monolayer of ZnPc is about 1.3 nm) and 5 nm PTCDIPh (about 3 ML, according to the XRD result, each monolayer of PTCDI-Ph is about 1.7 nm) were deposited on the p-6P film subsequently as designed to form heterostructure films. Since ZnPc and PTCDI-Ph are notably different and their morphologies grown on the p-6P are not alike, the choice of ZnPc or PTCDI-Ph as the first deposition on bilayer of p-6P might give distinct results. Therefore, two types of the organic heterostructures were fabricated to fully clarify the growth behavior and mechanism. Figure 1b shows the configurations of the two types of organic heterostructures: in type A PTCDI-Ph was first deposited on the inducing layer while ZnPc was first deposited in type B. For both types,



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PTCDI-Ph/ZnPc heterostructure with 1 and 1.5 periods (here, 3 ML PTCDI-Ph combined with 4 ML ZnPc was defined as 1 period) were fabricated. In order to clearly track the structure and morphology evolution, the AFM, XRD and TEM were performed to characterize the films from the single layer to 1 and 1.5 periods organic heterostructures. Figure 2 shows the AFM height images of the series of films, for comparison, heterostructure films without p-6P were also provided.

Figure 2. AFM topographies of films a) p-6P/PTCDI-Ph, b) p-6P/ZnPc, c) p-6P/PTCDIPh/ZnPc,

d)

p-6P/ZnPc/PTCDI-Ph,

e)

p-6P/PTCDI-Ph/ZnPc/PTCDI-Ph,

f)

p-

6P/ZnPc/PTCDI-Ph/ZnPc, g) PTCDI-Ph/ZnPc, h) ZnPc/PTCDI-Ph, a),c),e) represent the morphology of type A from the first layer (0.5 period) to third layer (1.5 periods), b), d), f) represent the morphology of type B from the first layer (0.5 period) to third layer (1.5 periods). The thickness of PTCDI-Ph and ZnPc are about 3 ML and 4 ML, respectively.


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Figure 2b gives the morphology of ZnPc growing on the inducing layer: typical needlelike crystals oriented on each domain. While PTCDI-Ph exhibits oriented ribbon-like crystals on the bilayer p-6P, as the previous reported (Figure 2a). On the above films, keeping depositing PTCDI-Ph or ZnPc to form the 1 period heterostructure films, the morphologies are showed in Figure 2d and 2c. Both heterostructure films are consisted of highly oriented needle-like crystals, no matter whether needle-like ZnPc or the ribbonlike PTCDI-Ph on the top. Such morphology indicates that the PTCDI-Ph adopts the morphology of the underlying ZnPc film, while the ZnPc still grows to the needle-like crystal on the ribbon-like morphology. Although the film morphology is similar, the roughness of the two films is quite different, the p-6P/ZnPc/PTCDI-Ph presents a rougher surface (root-mean-square roughness (rms= 3.4 nm) than the p-6P/PTCDI-Ph/ZnPc film (rms= 1.1 nm). Apparently, the latter films reserve a smooth surface at molecular level as the single layer films. The heterostructure films without p-6P layer show random oriented morphologies, and some are quite rough. Keeping alternately depositing another layer to form 1.5 periods heterostuctures, both types remain the needle-like morphology in each domain, while the roughness increased slightly (Figure 2e and f ). The needle-like crystals maintain the oriented alignment on each domain of the inducing layer, giving rise to highly ordered oriented films. Organic heterostructure with one period is easy to fabricate in many materials systems, while alternate epitaxy growth and the formation of highly oriented multi-layer are rare. Here, in this system, the 1.5 periods film could still remain oriented and grow to highly ordered crystalline films, indicating high quality multi-layer organic heterostructures were formed, and potential application could be expected. The similar



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needle-like morphology for two types could make sense because that both molecules adopt herringbone packing and tend to grow along the π-π stacking direction combined with similar π-π distance. Hence, the similar anisotropic growth leads to similar orientation or crystal shape when the two grow alternately. Intrinsically, the PTCDI-Ph forms ribbon-like crystal that is much wider than needle crystal. But when growing on the surface of ZnPc, the PTCDI-Ph molecules turned to needle crystal because of limit of the more slim needle-like crystal of ZnPc. Such phenomenon on one hand indicates the template effect influences the film growth and on the other hand implies the flexibility or soft matter property of organic compounds.

Figure 3. out-of-plane XRD patterns of heterostructures from first layer to 1.5 periods

films. a) type A; b) type B The out-of-plane film structure was characterized by the XRD, of which the diffractions from (100) of PTCDI-Ph and (200) of ZnPc were clearly shown in the pattern for both types (Figure 3). For type B, the corresponding lattice spacing of (200)ZnPc in the 0.5 period sample is 12.9 Å, consist well with its α-phase30-31. While the subsequent PTCDI-Ph layer in the 1 period sample presents diffraction at 5.3o, corresponding to a 10 ACS Paragon Plus Environment

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spacing of 16.7 Å, which shows almost the same as d-spacing of (002) plane (16.6 Å) of its single crystal structure32. In the 1.5 periods sample, the ZnPc shows lattice spacing the same as α-phase. For type A, the corresponding lattice spacing of (100)PTCDI-Ph of the 0.5 period sample is 16.9 Å, which is also slightly different from both the single crystal structure and type B layer structure of PTCDI-Ph. In 1 period film, the ZnPc deposited on the PTCDI-Ph presents diffraction at 7.1o, corresponding to lattice spacing of 12.4 Å, possibly indicated a different phase from α-phase. XRD of 1.5 period sample shows that PTCDI-Ph shows lattice spacing about 16.5 Å, closer to its single crystal structure and also the PTCDI-Ph in type B. In both types, slight change of the lattice spacing and structure parameters different from bulk phase were observed. The new d-spacing values of PTCDI-Ph and ZnPc in the both types indicate possible new film phases forming and evolving in the heterostructures, which are demonstrated and confirmed in details in the SAED part below. Generally, the lattice spacing of the (200) of PTCDI-Ph is close to the molecular long axis, indicating the up-right orientation of the PTCDI-Ph relative to the substrate, so is ZnPc. The molecular orientations of the ZnPc and PTCDI-Ph films in all the films adopt an edge-on fashion, in which that the molecular long axis is stand-up on the substrate while the π-π stacking is parallel to the substrate. Such orientation is beneficial to the application like transistors. SAED was used to measure the in-plane structure of the heterostructure films, which could show not only the in-plane structure but also the related orientation and epitaxy relationship between the heterostructure interfaces. These results will be beneficial to understand the epitaxy growth behavior. The SAED samples were fabricated through



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directly transferring the film grown on the Si/SiO2 to the copper grid, detailed information see the experimental section. Figure 4 shows the series of SAED patterns of a)

b)

c)

d)

(002)’ZnPc

e)

f)

Figure 4. SAED patterns of a) p-6P/PTCDI-Ph, b) p-6P/ZnPc, c) p-6P/PTCDI-Ph/ZnPc, d) p-6P/ZnPc/PTCDI-Ph, e) p-6P/PTCDI-Ph/ZnPc/PTCDI-Ph, f) p-6P/ZnPc/PTCDI-


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Ph/ZnPc. Direction of b* axis of the p-6P, PTCDI-Ph, ZnPc was the same, and marked with the dash arrow line. The coincidence points are marked with circle. film from the first layer to 1.5 periods heterostructure for both types. The first layer of both PTCDI-Ph and ZnPc growing on p-6P show the similar patterns as previous reported, nearly one set of strong diffraction was observed of PTCDI-Ph and ZnPc, respectively (Figure 4a and b). Both molecules adopt monoclinic system with (h00) exposure on the surface as indexed according to their single crystal structure30-32, in which the angle of two primitive axes (b*, c*) in reciprocal space is 90o just as the SAED image indicated. For the PTCDI-Ph films, the c* axis of PTCDI-Ph is parallel to the a* axis of p-6P and the b* axis of PTCDI-Ph is parallel to the b* axis of p-6P with a lattice coincidence along the a* of p-6P, with a lattice match of 4c*PTCDI-Ph = a*p-6P or d(002)PTCDI-Ph = 2d(200)p-6P according to the diffraction pattern (The coincidence spots were marked in blue circle in Figure 4a). In addition, the in-plane structure parameters of PTCDI-Ph film calculated from diffraction pattern are: d(002)PTCDI-Ph(I) = 8.0±0.1 Å, d(010) PTCDI-Ph(I) = 5.0±0.1 Å, α=90o, different from its single crystal structure or bulk phase (d(002) PTCDI-Ph(bulk)= 9.07 Å, d(010) PTCDI-Ph(bulk) = 3.87 Å, α=90o)32. The PTCDI-Ph crystal shrinks about 10% in c axis from its single crystal to fit the unit cell of p-6P, while expands more than 10% in the b axis. With the new value of d(200) PTCDI-Ph from XRD, it can be concluded that this PTCDI-Ph layer exhibits a new thin film phase. In order to clarify the structure, we named this PTCDI-Ph film structure here as PTCDI-Ph (I), corresponding structure parameters were summarized in Table 1. Consist with the out-of-plane result, the ZnPc films growing on p-6P also present the α-phase of ZnPc and the in-plane unit cell is ascribed to (bc) plane, in which the strongest diffraction spot was indexed to (002) plane



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with a lattice spacing of 12.7 Å. Similarly, the c* axis of ZnPc is parallel to the a* axis of p-6P with a lattice match of 3c*ZnPc = a*p-6P or d(002)ZnPc = 3d(200)p-6P (Possible coincidence spots were marked in blue circle in Figure 4b). The epitaxial relationships of the both types, which exhibit coincidences of reciprocal lattice vectors (such as 4c*PTCDIPh

= a*p-6P) and thus the coincidences of lattice lines between the substrate layer and

absorbate layer in real space ([001]PTCDI-Ph // [100]p-6P), are greatly consistent with the feature of line-on-line epitaxy11,15,34, which has deeply investigated organic-organic heteroepitaxy on single crystal substrate with molecules flat-lying on substrates.

Table 1. Structure Parameters of ZnPc and PTCDI-Ph Measured from the SAED

In-plane Out-ofplane

ZnPc

ZnPc (α) /Å a

ZnPc (I)/Å

PTCDIPh

PTCDIPh (I)/Å

PTCDIPh (II)/Å

PTCDIPh (bulk) /Å b

(002)

12.71

10.50

(002)

8.0

8.5

9.07

(010)

3.82

4.20

(010)

5.0

4.2

3.87

(200)

12.90

12.4

(200)

16.9

16.7

16.6

a

single crystal structure of α-ZnPc30-31: a=25.98 Å, b=3.78 Å, c=24.21 Å, β=90.6º b single crystal structure of PTCDI-Ph32: P21/c, a=16.80 Å, b=3.87 Å, c=18.38 Å, β=98.9o When PTCDI-Ph and ZnPc were deposited separately on the above first layer to form the 1 and 1.5 periods p-n heterostructures, their in-plane structures changed. Figure 4c-f show the SAED patterns of the two kinds of heterostructures which only differ in the deposition order. Interestingly, for both types, their orientation maintained the same according to the diffraction pattern that the c* of ZnPc is parallel to the c* of PTCDI-Ph. This is clearly consistent with their morphologies. Coincidences of diffraction spots (marked in red circle in Figure 4c and d)) were observed in both types, that is 4c*ZnPc =


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3c*PTCDI-Ph in type A and 3c*ZnPc = 2c*PTCDI-Ph in type B, which also fit line-on-line epitaxy. However, their in-plane structures varied with the deposition order. For the type A heterostructure, the ZnPc films growing on the PTCDI-Ph present a strong new diffraction spot indexed to (002)’ plane with a lattice spacing of 10.7 Å, which shrinks about 19% comparing to the spacing of (002) in α-ZnPc (Figure 4c). Combined with the changing of out-of-plane lattice parameter from XRD, the new film structure formation of ZnPc can be comfirmed. Here in order to clarify the new structure, we use (002)’ to index the new diffraction of ZnPc, and named the new structure of ZnPc as ZnPc(I). Such a structure change results from the coincidence of (004)’ZnPc with the underlying (003)PTCDI-Ph. Similarly, the c axis of ZnPc shrinks, just-like the underlying PTCDI-Ph, indicating the strain originating from heteroepitaxy. With further deposition of PTCDI-Ph on the ZnPc, the 1.5 periods heterostructure displays almost the same SAED pattern to the 1 period except a slight difference of PTCDI-Ph diffraction (Figure 4e). The zoom in image of the SAED along the (002)PTCDI-Ph clearly presents two diffraction spots. One is the (002) of PTCDI-Ph(I) from the first layer of PTCDI-Ph. The other diffraction spot which is very close to (002)PTCDI-Ph gives a lattice parameter of d(002)’= 8.5±0.1 Å and d(010)’=4.2±0.1 Å. Although the out-of-plane lattice parameters didn’t show much difference from the single crystal, the in-plane parameters change a lot. Hence we named this new structure of PTCDI-Ph film as PTCDI-Ph(II) to distinct from the PTCDI-Ph(I) and bulk phase. Compared with PTCDI-Ph(I), this structure is closer to the single crystal structure, indicative of a more relaxed structure. Since the first layer of PTCDI-Ph shows obvious shrinkage in c axis, the lattice strain brings stress in the film. The following ZnPc film still forms a strained structure. The stress releases gradually with the adding of the



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films’ thickness. Hence as the third layer deposited, the stress release drives the formation of a more relaxed structure that is close to the single crystal phase. On the other hand, in type B the solved in-plane lattice parameters of PTCDI-Ph growing on top of the ZnPc film directly show the relaxed structure of PTCDI-Ph (II) as the α-phase of ZnPc growing on the p-6P bilayer without obvious stress. For the 1.5 periods structure, the third layer of ZnPc in type B reserves the same α-phase structure with its first layer, evidencing the alternate heteroepitaxy growth between each other with a stable structure. Obviously, these results are consistent with the XRD results.

Figure 5. Schematic diagram illustrating lattice vectors in reciprocal and real space of

each layer and the epitaxy relationship of type A, a) and type B, b).


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According to the SAED, XRD and the coincidence relationship, the epitaxy relationships of the two type heterostructures were summarized in Figure 5, lattice vectors in reciprocal space and real space was illustrated. The lattice mismatches along the c direction were calculated, as Table 2 shows. It is very interesting that, although the structures of PTCDI-Ph (I) and α-ZnPc exhibit a mismatch larger than 15%, the two materials do form lattice-match crystalline films during the epitaxy growth through forming a new ZnPc phase ZnPc(I). Such growth behavior indicated that the organic materials are much flexible and possess soft matter property which does favor to the welldefined epitaxy growth. Although the thin film possesses large strain compared to the single crystal structure, the ordered film without any cracks after cooling from high

Table 2. Epitaxy Relationship and Lattice Mismatch of the Two Types of Heterostructure Epitaxy relationship

Lattice mismatch

Type A

(001)PTCDI-Ph // (002)ZnPc (010)PTCDI-Ph // (010)ZnPc d(003)PTCDI-Ph= d(004)ZnPc

(d(003)PTCDI-Ph-d(004)α-ZnPc) /d(003)PTCDI-Ph = -19% a

Type B

(001)PTCDI-Ph // (002)ZnPc (010)PTCDI-Ph // (010)ZnPc 2d(002)ZnPc= 3d(002)PTCDI-Ph

(2/3d(002)α-ZnPc-d(002)PTCDI-Ph) /(2/3d(003)PTCDI-Ph)= -7% b

a

The mismatch calculated using the epitaxy PTCDI-Ph and ZnPc bulk phase The mismatch calculated using the epitaxy ZnPc films (α-phase) and PTCDI-Ph bulk phase b

temperature indicates its stable feature. Such stable film corresponding to the thin film phase was common in organic systems, which should be contributed to the weak Van der Waals force between organic materials. In the deeper energetic view from previous analysis of line-on-line epitaxy15, 33, such reciprocal lattice vectors or real space lattice 17 ACS Paragon Plus Environment


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vectors coincidence perimits the system the minimum energy: the coincidence lines in real space acctually reflect the minimum interlayer interaction energy distribution of the sublayer, so the adopted arrangement of the overlayer lattice along such lines would bring the heterostructrue the minimum potential energy and thus a stable film structure. Based on the AFM, XRD and SAED results and discussions, the growth behavior of the heterostructure can be inferred. The schematic diagram for structure evolution of each layer was shown in Figure 6. For type A where the PTCDI-Ph was deposited on top of the p-6P, the first PTCDI-Ph layer forms a thin film phase that is different from its bulk phase, which induce forming another thin film phase of ZnPc film; the third layer of PTCDI-Ph growing on top of the ZnPc tends to form a stable film structure close to the bulk phase, and the thin film phase might be accounted for the smoother surface of this type heterostructure. For type B where ZnPc was first deposited on the p-6P surface, an α-phase of ZnPc (which is a meta-stable bulk phase of ZnPc) directly appears, and induces the formation of the stable film structure of PTCDI-Ph, which in turn epitaxially induces the growth of α-phase of ZnPc. Apparently, the film morphology and structure evolution confirmed highly oriented films with successful alternate heteroepitaxy growth were achieved. On the other hand, structure evolution indicates deep understanding on the growth of organic heterostructure. First, the upper layer morphology such as grain size and orientation is not only controlled by the intrinsic growth behavior but also critically influenced by the underlying layer, particularly the crystal shape, orientation. Second, the film structure is similarly influenced by the underlying film structure, meanwhile, it is also dependent on the thickness which influences strain and results in


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stress release. And third, the soft matter property of organic materials plays a key role in the organic-organic heteroepitaxial growth.

Figure 6. Schmetic diagram of the structure evolutions of type A (above) and B (below) Ambipolar Transistors Based on the Organic Heterostructure. Through directly evaporating Au through a shadow mask on the organic heterostructure films, a series of top contact bottom gate transistors were fabricated. The organic heterostructures, no matter whether ZnPc or PTCDI-Ph as the bottom layer, present obvious ambipolar charge transport behavior. Figure 7 shows the family curves of ZnPc/PTCDI-Ph heterostructures. The type A with PTCDI-Ph as the bottom layer present electron and hole mobilities of 0.12 and 0.08 cm2/Vs respectively. Type B gives a more balanced ambipolar transport with electron and hole mobilities of 0.08 and 0.07 cm2/Vs. The slight higher electron mobility in type A might come from the higher quality film and the additional p-n charge accumulation type heterojunction effect between p-6P and PTCDI-Ph27,34. The typical



family curves clearly evidenced the ambipolar transport behavior. The good charge transport behaviors of the series of heterostructures open up an access to plenty potential applications in the future.

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Figure 7. Family curves of ambipolar transistors for the type A, a); type B, b). The legends in each figure represent the gate bias for every curve.

CONCLUSIONS In summary, we constructed a series of highly ordered organic heterostructure films based on ZnPc and PTCDI-Ph with well-defined alternate heteroepitaxial growth. Crystalline films with roughnesses on the molecular level and highly oriented crystal


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grains were formed through an easy vacuum deposition process. What’s more important is that the crystalline films could not only achieve one period heterostructure with preferred orientation but also realize highly ordered multi-heterostructure through alternate heteroepitaxial growth between each other, which shows great potential in exploring organic heterostructure devices. The heritable orientation evidenced by the AFM, XRD and SAED demonstrates the high quality of the organic heterostructure films, while the structure evolution from layer by layer reveals the main growth behaviors in the heterostructure film formation. It is clear that the morphology is controlled by the intrinsic growth preferred direction and packing, but substrate template effect is another crucial factor. The structure evolution displays the importance of soft matter property on the organic-organic heteroepitaxy besides the lattice mismatch. And the epitaxy relationship also fits the minimum energy principle. In addition, the crystalline heterostructures show high performance ambipolar transport behaviors owing to the high quality film and balanced transport abilities of ZnPc and PTCDI-Ph, laying a foundation for future optoelectronic application.

Acknowledgement This work was financially supported by the National Natural Science Foundation of China (51133007 and 51303171)

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For Table of Contents use only

Alternate Heteroepitaxial Growth of Highly Oriented Organic Multilayer Films † ‡

† ‡

†

†

Zi Wang , , Hao Chang , , Tong Wang , Haibo Wang and Donghang Yan*, †

†

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022 (P. R. China) ‡

University of Chinese Academy of Sciences, Beijing, 100049 (P. R. China)


Low-temperature growth of bismuth thin films with (111) facet on highly oriented pyrolytic graphite.

Highly oriented polymer semiconductor films compressed at the surface of ionic liquids for high-performance polymeric organic field-effect transistors.

Si(111) Virtual Substrates.

metal interfaces.

Growth of c-axis-oriented superconducting KFe₂As₂ thin films.

Al Ratios.

Boosting Photon Harvesting in Organic Solar Cells with Highly Oriented Molecular Crystals via Graphene-Organic Heterointerface.

Bimodal island size distribution in heteroepitaxial growth.

Highly size-selective ionically crosslinked multilayer polymer films for light gas separation.

Programmed degradation of DNA multilayer films.

Anisotropic Effects on the Thermoelectric Properties of Highly Oriented Electrodeposited Bi2Te3 Films.

Chitin nanocrystal-xyloglucan multilayer thin films.

(Ga1-xFex)2O3 multilayer thin films.

nanoclay-polymer nanocomposites for flexible gas barrier films.

Highly conductive transparent organic electrodes with multilayer structures for rigid and flexible optoelectronics.

Organic contamination of highly oriented pyrolytic graphite as studied by scanning electrochemical microscopy.

Epidermal growth factor-functionalized polymeric multilayer films: interplay between spatial location and bioavailability of EGF.

Controlled growth of c-axis oriented ZnO nanorod array films by electrodeposition method and characterization.

Anatomy of vertical heteroepitaxial interfaces reveals the memristive mechanism in Nb₂O₅-NaNbO₃ thin films.

Ag multilayer films.

Characterization of casein and poly-L-arginine multilayer films.

Morphological and Functional Properties.

Luminescent metal-organic framework films as highly sensitive and fast-response oxygen sensors.

highly oriented nanorod arrays on copper.