Precise, Self-Limited Epitaxy of Ultrathin Organic Semiconductors and

Letter pubs.acs.org/NanoLett

Precise, Self-Limited Epitaxy of Ultrathin Organic Semiconductors and Heterojunctions Tailored by van der Waals Interactions Bing Wu,† Yinghe Zhao,‡ Haiyan Nan,‡ Ziyi Yang,† Yuhan Zhang,† Huijuan Zhao,† Daowei He,† Zonglin Jiang,† Xiaolong Liu,† Yun Li,† Yi Shi,*,† Zhenhua Ni,‡ Jinlan Wang,*,‡,§ Jian-Bin Xu,∥ and Xinran Wang*,† †

National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China ‡ Department of Physics, Southeast University, Nanjing 211189, People’s Republic of China § Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal University, Changsha 410081, China ∥ Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Hong Kong SAR, People’s Republic of China S Supporting Information *

ABSTRACT: Precise assembly of semiconductor heterojunctions is the key to realize many optoelectronic devices. By exploiting the strong and tunable van der Waals (vdW) forces between graphene and organic small molecules, we demonstrate layer-by-layer epitaxy of ultrathin organic semiconductors and heterostructures with unprecedented precision with well-defined number of layers and self-limited characteristics. We further demonstrate organic p−n heterojunctions with molecularly flat interface, which exhibit excellent rectifying behavior and photovoltaic responses. The self-limited organic molecular beam epitaxy (SLOMBE) is generically applicable for many layered small-molecule semiconductors and may lead to advanced organic optoelectronic devices beyond bulk heterojunctions. KEYWORDS: Two-dimensional, organic crystals, van der Waals epitaxy, heterojunctions

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limited to bulk heterojunctions,17−19 where precise control at the molecular scale remains difficult. Recently, several groups have shown that two-dimensional (2D) ultrathin organic crystals can be grown on many 2D atomic crystals (including graphene, BN, and MoS2) for investigation of structure−property relationships20 and for a myriad of device applications.21−24 We have found that the molecule−substrate interaction is particularly strong and tunable on graphene, leading to the distinct molecular packing near the interface.21,25 Here, we exploit the interfacial vdW forces to realize unprecedented precision in the epitaxy of organic semiconductors on graphene. We find that by controlling the substrate temperature, it is possible to grow monocrystalline organic semiconductors with well-defined number of layers and complete coverage in a self-limited (SL) manner. Furthermore, we can create organic heterojunctions by consecutive growth of different molecules. Evidence of interfacial dipole and charge transfer is observed by Kelvin probe force microscopy (KPFM) and photoluminescence (PL) measurements. Finally, to demonstrate possible device applications, we fabricate a vertical p−n

eterojunctions are widely used in today’s semiconductor devices.1,2 The ability to precisely control the film thickness and morphology down to monolayer by molecular beam epitaxy (MBE)3 and other technologies has led to important device discoveries such as laser diodes4 and bipolar transistors.5 Because of the formation of covalent bonds at the interface, MBE requires the lattice match between different layers or substrates, which are isonly suitable for limited material combinations. In addition, the requirement of ultrahigh vacuum (UHV) dramatically increases the complexity and cost of the equipment and operation. These constraints can be significantly relaxed for organic materials. The virtually infinite variety of molecular compounds also provides the design prospect of functional heterostructures at will. However, despite early research efforts on organic epitaxy,6 realizing material quality and level of precision on par with that of inorganic MBE is still daunting challenge,7−12 mainly because organic semiconductors are bound by much weaker vdW forces. Oftentimes, the precise molecular self-assembly requires solid−liquid interfaces.13−15 Although self-assembled monolayer (SAM) technique can be used to deposit monolayer organic thin-film on many surfaces,16 it is challenging to realize layer-by-layer heterostructures for advanced device applications. Therefore, most organic optoelectronic devices to date are © XXXX American Chemical Society

Received: March 14, 2016 Revised: April 23, 2016

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DOI: 10.1021/acs.nanolett.6b01108 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters

Figure 1. SLOMBE of C8-BTBT. (a) Top panel: schematic illustration of the growth furnace. Bottom panel: temperature inside the furnace as a function of distance to the center (black symbols, measured; red line, guide to the eye). The furnace can be divided into Zones I, II, and III with distinct growth behaviors. (b) Blue dots: calculated binding energies of a single C8-BTBT molecule on graphene, IL/graphene, 1L/IL/graphene, and 2L/1L/IL/graphene. Red dash line: C8-BTBT−C8-BTBT interaction. Inset shows the molecular structure of C8-BTBT and molecular packing of different C8-BTBT layers on graphene. (c−e) AFM images of typical samples after C8-BTBT growth in Zone I (c), Zone II (d), and Zone III (e), respectively. Scale bars: 2 μm.

Figure 2. Characterization of C8-BTBT crystals on graphene. (a) AFM images of a graphene before (left) and after (right) SLOMBE of monolayer C8-BTBT. Scale bars: 2 μm. (b) Raman spectrum of the sample in (a), clearly showing the signal from both graphene and C8-BTBT. Inset is the Raman mapping of the C8-BTBT signal. Scale bar: 2 μm. (c) AFM images of a graphene before (left) and after (right) SLOMBE of bilayer C8BTBT. The inset of the right panel is a 2.3 nm × 2.3 nm high-resolution AFM image, showing the expected molecular packing. Scale bars: 2 μm. (d) Polarization-dependent absorption microscopy images of the sample in (c). The sample was rotated by 90° between the two images. Scale bars: 2 μm. (e) MD simulation of a C8-BTBT molecule adsorbed on graphene (top) and IL C8-BTBT (bottom) when an upward velocity of 0.95 nm/ps assumed. (f) MD simulation of a C8-BTBT molecule adsorbed on IL (top) and 1L C8-BTBT (bottom) when an upward velocity of 0.65 nm/ps given.

(PVT) method (Figure 1a). We placed the C8-BTBT source powder in the center of the heating zone and the mechanically exfoliated graphene on SiO2/Si downstream. We found that after heating to the target temperature of 110−120 °C, the temperature inside the tube was not a constant, but rather monotonically decreased away from the center (Figure 1a). The temperature gradient created three zones with distinct growth behavior. In Zone I (within ∼10 cm from the center, T ≥ 85 °C), the graphene substrate was completely covered by monolayer of C8-BTBT (interfacial layer, or IL) in less than 5

junction photodetector with near-unity ideality factor and photoresponsivity of 0.37 mA/W. The SLOMBE developed in this study is generically applicable for many layered smallmolecule semiconductors and may lead to advanced optoelectronic devices beyond bulk heterojunctions. We first discuss the concept of SLOMBE using the combination of dioctylbenzothienobenzothiophene (C 8BTBT), a p-type small-molecule semiconductor, and graphene substrate. The epitaxial growth was carried out in a home-built single-zone vacuum tube furnace by physical vapor transport B

DOI: 10.1021/acs.nanolett.6b01108 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 3. Reverse growth of C8-BTBT layers. (a−d) AFM images of the same C8-BTBT/graphene sample at increasing annealing temperatures (see text for details). Scale bars: 2 μm. (e) MD simulation of the reverse growth process. The substrate temperature was 700 K for the first two steps and 800 K for the third step.

result in additional layers (Figures S5 and S6). These experiments proved that the SLOMBE was highly precise, controllable, and robust to experimental variations and ambient exposure. Furthermore, the process was facile to implement without expensive UHV equipment. These attributes make SLOMBE a potentially affordable technology for achieving well-defined molecular nanostructures. In order to understand the mechanism of SLOMBE, we performed molecular dynamics (MD) simulations of the growth process, which have been proven to be a powerful tool for exploring 2D supramolecular self-assembly.25,28−32 The key element making SLOMBE possible is that the C8-BTBTsubstrate binding energy (BE) depends highly on the underlying substrate. As shown in Figure 1b, the BE is highest on graphene, but rapidly decreases on IL and 1L. Such BE gradient creates a temperature window where the adsorbed C8BTBT molecule is thermodynamically stable on graphene but not stable on IL and therefore the self-assembly into 1L will not occur. Within this window, SL epitaxy of IL can be realized. This hypothesis is supported by MD simulations. Figure 2e shows the MD simulation of a C8-BTBT molecule on graphene and on IL. We find that with the same initial velocity of 0.95 nm/ps, the C8-BTBT molecule could be captured by graphene, but not by IL. Similarly, SL epitaxy of 1L is possible but with lower initial thermal energy (corresponding to lower substrate temperature experimentally) (Figure 2f). We note that in the case of C8-BTBT, the BE becomes approximately constant beyond 1L, as supported by the constant height of ∼3 nm (Figure S7). Therefore, it is difficult to achieve SL growth of thicker layers by simply adjusting the substrate temperature. Nevertheless, uniform epitaxy of thicker films may be achieved by controlling other growth parameters (Figure S8). The different BE is also responsible for the distinct molecular packing in IL and 1L C8-BTBT. Initially in the growth process, the C8-BTBT molecules are adsorbed onto graphene with little intermolecular interactions. So individual adsorbed molecules most likely flat lie so as to maximize molecule−graphene interactions.21 When the molecule coverage is increased to a critical value, the adsorbed molecules can flip up from a parallel to upright direction to the substrate.32−34 The condition of the orientation transformation is the molecule−substrate interaction less than the intermolecular interaction. In our system, the calculated C8-BTBT intermolecular interaction of 205 kJ/

min. No additional layers were grown atop even after a long time, suggesting that the growth was SL (Figure 1c). Similarly, in Zone II (∼10−13.5 cm from the center, 65 °C < T < 85 °C), the graphene substrate was completely covered by bilayer C8BTBT (IL and 1L) (Figure 1d). In Zone III (further than ∼13.5 cm from the center, T < 65 °C), however, the epitaxy was no longer SL with multiple layers gradually accumulating as the growth continued (Figure 1e). We carefully characterized the epitaxial growth process and C8-BTBT ultrathin films by atomic force microscopy (AFM), Raman spectroscopy, and polarization-dependent optical absorption. Figure 2a (2c) shows the same graphene substrate before and after SLOMBE of monolayer (bilayer) C8-BTBT. After growth, the height was uniformly increased by ∼0.9 and ∼2.4 nm respectively, consistent with the height of IL and 1L C8-BTBT21 (Figure S1). Raman spectroscopy and mapping further confirmed that C8-BTBT molecules were only grown on graphene26 (Figure 2b). Remarkably, the C8-BTBT films showed atomic flatness without any visible defects or adlayers. Both molecular-resolution AFM (Figure 2c inset) and polarization-dependent absorption images (Figure 2d and Figure S2) revealed the monocrystalline nature of 1L C8-BTBT films. The latter showed uniform modulation with a period of ∼180° (Figure 2d and Figure S2), as expected from the crystal symmetry of C8-BTBT. The monocrystalline nature of 1L can be attributed to the large surface diffusion coefficient of C8BTBT molecules due to the substrate flatness and weak molecule−substrate vdW forces. This is evidenced by the nonfractal, island-shaped 1L domains27 (Figure S1). In addition, we found that 1L most often grew from one nucleation site (usually from the edge of graphene) and expanded to cover the substrate completely. The SL nature of the monolayer and bilayer epitaxy was unambiguously verified by two series of experiments. First, we placed different samples in Zones I or II and varied the growth time from 5 to 20 min. Complete coverage of monolayer or bilayer C8-BTBT was observed regardless of the growth time (Figures S3 and S4), indicating that the morphology of the film did not further evolve after the target layer was completed. Second, we performed repeated growths on one sample under the same experimental condition, each time followed by ambient AFM characterization. The epitaxy was completed in 5 min for all samples, and further repeated growth did not C

DOI: 10.1021/acs.nanolett.6b01108 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 4. SLOMBE of PTCDA and heterojunction. (a−c) Optical microscopic images of a graphene sample before growth (a), after SLOMBE of monolayer PTCDA (b), and (c) after SLOMBE of bilayer C8-BTBT on PTCDA, respectively. The insets of (b,c) show the schematic illustrations of the structure. (d−f) AFM images of the same sample before growth (a), after SLOMBE of monolayer PTCDA (b), and (c) after SLOMBE of bilayer C8-BTBT on PTCDA, respectively. Scale bars: 5 μm. (g) Raman spectrum of the sample in (b), showing clear Raman fingerprints of PTCDA. Inset shows the molecular structure of PTCDA. (h) PL spectra of PTCDA before and after C8-BTBT growth on the top. Inset shows the energy-band diagram and charge transfer process of the heterojunction. (i) AFM topography and (j) KPFM images of the same area with few-layer C8-BTBT on PTCDA. Scale bars: 200 nm. (k) Relative surface potential as a function of C8-BTBT thickness (symbols). The red line is the quadratic fitting curve using an abrupt junction model.

make SL growth easier to implement. Second, the molecules have to favor 2D layered growth, instead of vertical growth. Many small molecules including C8-BTBT adopt layered crystal structures in their bulk form. To show such versatility, we also studied the growth of an n-type small molecule semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), and heterojunctions of PTCDA/C8-BTBT (Figure 4 and Figures S10 and S11). PTCDA is a planar molecule favoring the faceon packing on graphene35 (Figure S10). Although the structure, properties and evaporation temperature of PTCDA are very different from C8-BTBT, we are able to achieve SLOMBE of monolayer PTCDA on graphene (Figure S11). After growth, the thickness of the sample was increased by ∼0.5 nm (Figure 4d,e) as expected for monolayer PTCDA. The growth of PTCDA was confirmed by color contrast of optical microscope images (Figure 4b) and Raman spectroscopy (Figure 4g). We further calculated the BE of PTCDA on graphene and on monolayer PTCDA/graphene to be 200 and 180 kJ/mol (Figure S12). Therefore, PTCDA is indeed expected to show the SL growth. To fabricate PTCDA/C8-BTBT heterojunctions, we placed the monolayer PTCDA sample in Zone II and grew C8-BTBT. Remarkably, the C8-BTBT was uniformly grown on PTCDA in a SL manner, similar to graphene substrate (Figure 4c,f). The height increase of ∼2.5 nm was consistent with bilayer C8BTBT. If we take monolayer PTCDA on graphene as the substrate, the calculated BE between a C8-BTBT molecule and PTCDA/graphene, IL C8-BTBT, and 1L C8-BTBT are 187, 160, and 86 kJ/mol, respectively. The trend is similar to that of graphene substrate (Figure 1b), giving rise to the SL growth.

mol is indeed between that of C8-BTBT/graphene and C8BTBT-IL/graphene (Figure 1b). This explains why IL lies flat on graphene but 1L stands up. The different molecular packing is also reproduced by MD simulations (Figure S9). The different BE can also be exploited to remove defects and other imperfections in the epitaxial films. Figure 3a−d demonstrates an interesting example of precisely controlled reverse growth of C8-BTBT. To this end, we deliberately prepared a sample with a small portion of 2L C8-BTBT (Figure 3a, note that 1L had a small void). We found that annealing the sample in vacuum at low temperature (75 °C) could simultaneously remove the 2L and fill the voids in the 1L, so as to restore the highly uniform 1L across the entire sample (Figure 3b). The annealing temperature of 75 °C was consistent with the SL growth temperature of 1L. Further annealing at 85 °C could selectively remove the 1L, while keeping the IL intact (Figure 3c). Finally, the annealing at 110 °C removed the IL C8-BTBT molecules completely and restored the original graphene substrate (Figure 3d). The process was qualitatively reproduced by MD simulations (Figure 3e and Movie S1), which showed that higher substrate temperature was necessary to remove C8-BTBT molecules closer to the graphene substrate. The reverse-engineering capability provides an extra means to precisely control the morphology of epitaxial films. SLOMBE is also applicable to other layered organic semiconductors and is capable to build complex heterostructures. We believe the requirements for SLOMBE are two-fold. First, a relatively large molecule−substrate BE gradient exists near the interface. Larger gradient tends to D

DOI: 10.1021/acs.nanolett.6b01108 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 5. Optoelectronic device of PTCDA/C8-BTBT p−n heterojunction. (a) Schematic layout of the device. (b) Energy-band diagrams of the device under reverse bias (left) and forward bias (right), respectively. (c) Output characteristics of the p−n junction under linear scale (black) and log scale (blue). Red line is the fitting curve by standard diode model with a series resistance. (d) Output characteristics of the p−n junction under the dark conditions (black) and under the 0.67 μW laser illumination (red).

The excellent flatness indicated that the interface between PTCDA and C8-BTBT was pristine without mixing phase and clustering as in bulk heterojunctions. The PTCDA/C8-BTBT heterojunction is expected to be type II with a staggered gap (Figure 4h inset). We used SKPM to characterize the interfacial properties. Figure 4i,j shows, respectively, the topological and surface potential image of fewlayer C8-BTBT on PTCDA. The surface potential contrast between C8-BTBT and PTCDA clearly shows the formation of interfacial dipoles. The built-in electric field is strongest at the interface but gradually vanishes in thicker C8-BTBT layers (Figure 4k). This indicates that the charge transfer is most effective in the first few molecular layers, reinforcing the importance of ultrathin organic heterostructures in optoelectronic devices. The scaling of surface potential can be quantitatively described by an abrupt junction model36 from which the width of the space-charge region ∼18 nm is extracted. The charge transfer between C8-BTBT and PTCDA is further corroborated by PL measurements, where the PL intensity from PTCDA is reduced after deposition of C8BTBT37 (Figure 4h). The realization of high-quality layered organic heterojunctions opens many new possibilities for optoelectronic devices. Here we demonstrate a simple p−n junction based on graphene/PTCDA/C8-BTBT/Au vertical stacks (Figure 5). In this device, we used multilayer organic films (∼15 nm) to minimize the direct tunneling effects. Under forward bias, electrons and holes can be injected from graphene/PTCDA and Au/C8-BTBT interface, respectively, via thermionic emission and recombine at the PTCDA/C8-BTBT interface (Figure 5b, right panel). Under reverse bias, however, the injection is limited by the increasing Schottky barrier width, and current conduction mechanism is dominated by tunneling (Figure 5b, left panel). At room temperature, the device shows excellent rectifying I−V characteristics with rectifying ratio over 1000. The I−V characteristics are well described by standard

diode model with a series resistance38 (Figure 5c). The derived ideality factor of 1.27 confirms the nearly ideal heterointerface. The p−n junction was further exploited for photodetector applications. We used 514 nm laser to illuminate the exposed area of PTCDA/C8-BTBT and observed prominent photovoltaic responses with open-circuit voltage of ∼0.5 V and photoresponsivity of ∼0.37 mA/W (Figure 5d). In this device, PTCDA acts as the photoactive layer and C8-BTBT is the holetransporting-and-electron-blocking layer. The built-in electric field dissociates the excitons in PTCDA with holes transporting through C8-BTBT layers to Au and electrons to graphene. We believe further performance improvement is possible by optimizing the materials combination. In conclusion, by harnessing the vdW interactions at the interfaces, we demonstrate highly controllable, SL epitaxy of layered organic semiconductors and heterojunctions. The technique is facile, robust, and applicable to a wide range of organic small molecules. The concept developed here may lead to more advanced organic nanostructures such as quantum wells and superlattices, which are expected to significantly expand the applications of organic bulk heterostructures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b01108. Experiment and calculation details and additional information on growth characterization. (PDF) Movie illustrating the process in Figure 3e. (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (X.W.) [email protected]. *E-mail: (Y. S.) [email protected]. *E-mail: (J.W.) [email protected]. E

DOI: 10.1021/acs.nanolett.6b01108 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters Author Contributions

(26) Li, Y.; Liu, C.; Lee, M. V.; Xu, Y.; Shi, Y.; Tsukagoshi, K. J. Mater. Chem. C 2013, 1, 1352−1358. (27) Zhang, Z. Y.; Lagally, M. G. Science 1997, 276, 377−383. (28) Palma, C.-A.; Samorì, P.; Cecchini, M. J. Am. Chem. Soc. 2010, 132, 17880−17885. (29) Ciesielski, A.; Cadeddu, A.; Palma, C.-A.; Gorczyński, A.; Patroniak, V.; Cecchini, M.; Samorì, P. Nanoscale 2011, 3, 4125−4129. (30) Palma, C.-A.; Cecchini, M.; Samorì, P. Chem. Soc. Rev. 2012, 41, 3713−3730. (31) Martsinovich, N.; Troisi, A. J. Phys. Chem. C 2010, 114, 4376− 4388. (32) Wang, T. H.; Zhu, Y. F.; Jiang, Q. Chem. Sci. 2012, 3, 528−536. (33) Loi, M. A.; da Como, E. D.; Dinelli, F.; Murgia, M.; Zamboni, R.; Biscarini, F.; Muccini, M. Nat. Mater. 2004, 4, 81−85. (34) Muccioli, L.; D’Avino, G.; Zannoni, C. Adv. Mater. 2011, 23, 4532−4536. (35) Kendrick, C.; Kahn, A.; Forrest, S. R. Appl. Surf. Sci. 1996, 104105, 586−594. (36) Xie, W. G.; Xu, J. B.; An, J.; Xue, K. J. Phys. Chem. C 2010, 114, 19044−19047. (37) Lee, C. H.; et al. Nat. Nanotechnol. 2014, 9, 676−681. (38) Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y. F.; JarilloHerrero, P. Nat. Nanotechnol. 2014, 9, 262−267.

X.W. and Y.S. conceived and supervised the project. B.W., H.N., Z.Y., Y. Zhang, H.Z., D.H., Z.J., and X.L. performed experiments and data analysis. Y. Zhao and J.W. performed MD simulations. Z.N. and J.-B. X. contributed to data analysis. X.W., J.W., and B.W. cowrote the paper. All authors discussed the results and commented on the manuscript. Correspondence and requests for materials should be addressed to X.W. or Y.S. for general aspects of the paper and to J.W. for theoretical aspects. B.W., Y. Zhao, and H.N. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank NIPPON KAYAKU co., Ltd. Japan for providing C8-BTBT materials. This work was supported in part by National Key Basic Research Program of China 2013CBA01604 and 2015CB921600; National Natural Science Foundation of China 61325020, 61261160499, 11274154, 61521001, 21525311, and 21373045, and 11574136; NSF of Jiangsu (BK20130016) and SRFDP (20130092110029) in China; Research Grant Council of Hong Kong SAR N_CUHK405/12; MICM Laboratory Found ation 9140C140105140C14070, “Jiangsu Shuangchuang” program and “Jiangsu Shuangchuang Team” Program.

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DOI: 10.1021/acs.nanolett.6b01108 Nano Lett. XXXX, XXX, XXX−XXX