Nanoseed Assisted PVT Growth of Ultrathin 2D Pentacene Molecular

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Nanoseed Assisted PVT Growth of Ultrathin 2D Pentacene Molecular Crystal Directly onto SiO2 Substrate S. Atika Arabi,†,‡,# Ji Dong,†,# Misbah Mirza,† Peng Yu,† Liang Wang,† Jun He,§ and Chao Jiang*,† †

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, & CAS Center for Excellence in Nanoscience and CAS Key laboratory of Nanosystem and Hierarchical Fabrication, National Centre for Nanoscience and Technology, No. 11 Zhongguancun Beiyitiao, Beijing 100190, China ‡ University of Chinese Academy of Science, No. 19A Yuquan Road, Beijing 100049, China §

S Supporting Information *

ABSTRACT: High order of molecular packing and perfect semiconductor/dielectric interface are two key factors to achieve high performance for organic field-effect transistors (OFET). Moreover, the thin crystal offers an improved efficiency of carrier injection for OFETs. To this aim, formation of thin and large single crystal directly on dielectrics is the basis to obtain the ideal crystal OFETs. Herein, we report the controlled growth of ultrathin 2D Pentacene (Pn) crystal via nanoseed assisted physical vapor transport (PVT) method grown directly on SiO2. The size, thickness, and density of Pn crystals are systematically studied. Potentially effective parameters such as initially lowered Pn coverage and decreased supersaturation with the aid of carrier gas flow were optimized to grow large, ultrathin 2D Pn crystalline flakes efficient for the fabrication of crystal OFETs. The typical size and thickness of as-grown Pn crystalline flakes can be controlled to be large and thin enough. Device of ultrathin crystal with bottom gate and top contact configuration showed mobility as high as 5.6 cm2 V−1 s−1, indicating that the proposed novel architecture of organic molecular crystals may pave the way toward the application of large-sized single crystals of Pn in organic electronics.

1. INTRODUCTION Over past few years, single crystal field effect transistors (SCFET) have become a subject of interest for understanding the mechanism of charge transport in organic semiconductors. Single crystals exhibit incomparable advantages over thin films such as high purity, long-range-order molecular packing, and no grain boundaries with minimized structural defects, establishing the structure property relationship and ensuring higher device performance.1,2 To date, it is generally acknowledged that the SCFETs performance is highly sensitive to semiconductor/ dielectric interface. Many groups have reported the growth of organic single crystal and their device fabrication techniques to attain high mobility. By introducing a new technique of air flow navigation, Zhengran fabricated a crystal of TIPS Pentacene.3 Onojima et al. used a simple method of electrostatic spray deposition for organic crystal growth.4 Horowitz,5 Kloc,6 and Laudise et al.7 are among the pioneers who developed a PVT technique for the growth of several organic semiconductors by © 2016 American Chemical Society

improving the design of high quality organic semiconductor crystal devices. With emphasis on these PVT-based techniques, Alborghetti et al. fabricated nanogap single-crystal organic transistors8 while anisotropic measurements of fused-ring thienoacene9 and Pn10 have also been reported. The high performance single-crystal organic semiconductors are endowed with clarification of their intrinsic electronic properties and significance for potential applications such as displays,11,12 large area electronics,11 and sensors.13 In most previously reported SCFET devices, grown organic single crystals were usually thick and transferred onto other substrates for the fabrication of SCFET. While in the solution method, the solvent and impurities can inevitably deteriorate the quality of crystal and the interfaces. The contaminated Received: December 7, 2015 Revised: March 19, 2016 Published: March 28, 2016 2624

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Figure 1. (a) Scheme of experiments during PVT growth. (b,c) AFM image (30 × 30 μm2) of ultrathin pentacene crystal with its height profile and optical image, respectively. (d) SEM image of large-sized crystal obtained after PVT. (e) XRD of the corresponding crystal. (f,g) TEM image and relative SAED pattern, respectively.

from several hundreds of nanometers to about 50 nm was successfully synthesized. To the best of our knowledge, this is the first report to come up with ultrathin Pn crystals by PVT method. Furthermore, growth mechanism of different crystal shapes manipulated by partly tailoring the deposition was proposed. Significantly, the competing relations of surface energy between the (110), (010), and (11̅0) surfaces lead perfect hexagonal to rhombic shape. Moreover, by investigating field effect behavior, the maximum mobility reaches up to 5.6 cm2 V−1 s−1 with average mobility 2.0 (±1.1) cm2 V−1 s−1. The huge variation may be somewhat attributed to the anisotropy of charge transport. Thus, our method provides an opportunity to reveal the growth mechanism of Pn crystals and realize their practical applications in future organic electronic technologies.

semiconductor/dielectric interface would result in poor device performance and make it difficult to investigate the intrinsic transport mechanism. Therefore, to achieve high device performance, the best way to keep the entire interface is to grow a thin organic single crystal in situ on a substrate with the PVT method. However, the reduced thickness and interfacial defects still exist as major challenges.14,15 The control of supersaturation during reaction time and in situ growth of crystal may solve the problems addressed above. In our previous work, we reported the in situ growth of organic crystal using the seed growth method which promotes the growth of crystal by reducing the Gibbs free energy of the system with nanoseeds.16 This work explores the basis of materials for the experimental research in obtaining large and ultrathin 2D crystal and seeks the device applications for Pn crystals. The main aim of this study is to get a better understanding of the underlying mechanism of the formation of ultrathin 2D Pn crystal via PVT process. Here, the growth of large and ultrathin 2D Pn crystals with specific facets is demonstrated via nanoseed assisted growth method stimulated by supersaturation. The obtained Pn crystals have been carefully characterized and a detailed study on the growth of the crystallization mechanism is also provided. Pn single crystal of size as large as 50 μm with thickness ranging

2. EXPERIMENTAL SECTION 2.1. Formation of Nanoseeds. Heavily doped Si wafers with thermally grown SiO2 of 300 nm were chosen as substrates. The substrates were cleaned consecutively with a solution of ethanol, acetone, ammonia, and deionized water with a 1:6 volume ratio and in the end with deionized water by using ultrasonic cleaner. Finally substrates were blown with high purity dry nitrogen. Pn (SigmaAldrich used without further purification) monolayer (0.7 ML) was deposited on substrates by thermal evaporator (AUTO 306, BOC Edward Co.) at ambient substrate temperature under a vacuum 2625

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pressure of 7 × 10−5 Pa. After deposition of thin film, substrates were transferred to the glovebox having argon environment to protect them from being oxidized in the air. Substrates with MLs were annealed at 120 °C to get seed crystal using a heating plate. 2.2. PVT Growth. A conventional horizontal furnace setup has been used. For crystal enlargement, the annealed samples were shifted to the furnace for subsequent PVT growth. Pn powder (97%, from Aldrich Chemical Co.) was used as a source material while SiO2 substrates having nanocrystals (from annealing 0.7 ML) were placed downstream in the furnace. The following procedure was used to grow the Pn crystal as a reference throughout this report: the source was heated at 285 °C, while substrate temperature maintained to 180 °C at a reactor total pressure of 1 atm and 300 sccm flow. The growth time was fixed to 30 min. This procedure was modified in four series to determine the growth conditions for nucleating the Pn crystal. Series A consists of substrates with four Pn coverages of 0.3, 0.5, 0.7, and 1 ML. In the second series (series B) the growth temperature of the Pn crystal was modified to Tsub = 210, 180, 150, and 120 °C while otherwise following the reference procedure. Series C was modified by using four different gas flows as 100, 200, 300, and 400 sccm while maintaining other conditions the same as the reference. In the fourth series (series D) the growth time was varied to 10, 30, and 60 min while otherwise following the reference procedure. 2.3. Device Fabrication and Characterization. Copper grid was used as shadow mask for the device fabrication and a configuration with top contact and bottom gate was adopted. Copper/gold electrodes with 100 and 40 nm thickness were deposited respectively directly on to the as grown crystal without disturbing its interface. The channel length is 10 μm while the width is 20 μm. The electrical property of the devices was carried out via a Keithley 4200 instrument in air at room temperature. The morphology of the monolayer was monitored by AFM,16 while Pn structure after PVT was characterized by field emission scanning electron microscopy (FESEM) S4800 (Hitachi).

Their roles in the formation of Pn crystals are discussed in the following section. 3.1. Coverage Effect. Nanoseed density provides control over the nucleation of vapor-grown organic single crystals onto the SiO2 surface Figure 2 displays the role of nanoseed density

Figure 2. (a1, b1, c1, d1) AFM images (5 × 5 μm2) of pentacene 1, 0.7, 0.5, 0.3 ML, respectively. (a2, b2, c2, d2) SEM images after annealing of Pn ML with calculated nanoseed density. (a3, b3, c3, d3) SEM images of single crystal after PVT growth.

3. RESULTS AND DISCUSSION Schematic diagram based on experimental setup is given in Figure 1a to depict the important stages of the molecular transport process during PVT growth. The morphology of in situ grown large Pn crystal was characterized by AFM as shown in Figure 1b. The height of the Pn crystal indicates 48.5 nm thicknesses while Figure 1c shows the optical image of the corresponding large crystal. The SEM image of the ultrathin Pn crystal fabricated using the reference conditions (see Experimental section) is shown in Figure 1d. TEM observations have shed light on the crystal upper surface while selected area diffraction (SAED) patterns of Pn crystal in Figure 1f,g unveil the single crystalline quality and growth direction. The upper surface of the crystal is indexed to the (001) plane. X-ray diffraction studies verify the high quality of Pn crystals. Figure 1e is the XRD pattern of in situ grown large Pn crystals; the strong diffraction peak at 6.24° corresponds to (001) crystal plane. The calculated full width at half maxima of the (001) peak is 0.18°. The morphology of the final crystal structures depends upon the nucleation, orientation, and reaction conditions. Although there are reports on the PVT growth of Pn crystals, but no systematic studies about the effect of carrier gas, growth time measurements on the morphology have been carried out. The following study shows that the density and thickness of the Pn crystal can be systematically controlled. The determining factors for the formation of large crystals are found to be monolayer pentacene coverage and substrate temperature, while for ultrathin crystal they are gas flow and growth time.

on the growth (series A). AFM images of ∼1, 0.7, 0.5, and 0.3 ML are shown in Figure 2 (a1, b1, c1, d1), respectively, for all cases the morphology of Pn is disc-like comparable with the reported work.17 Nanoseeds obtained after annealing of respective coverages are presented in Figure 2 (a2, b2, c2, d2). It is clear that density of nanoseeds increases with the increase in coverage or vice versa. However, the phenomena of this molecular redistribution vanished up to a certain thickness, i.e., when coverage exceeds 5 ML. SEM images of large crystal after subsequent PVT growth are shown in Figure 2 (a3, b3, c3, d3). On the basis of the experimental results, as density of nanoseeds increases (up to 0.3/μm2), after subsequent PVT growth, density of large crystals also increases. This shows that nanoseeds promote the growth of crystals, as they provide specific sites for the incoming molecules to accommodate. The selectivity achieved in our system is more likely due to the difference in thermodynamics on substrates having nanoseeds or without nanoseeds. This view is supported by the following explanation. For Pn, the critical island size was assumed to be 3 < i < 4 at substrate temperature between 25 and 70 °C.18 In the case of bare SiO2 substrate, without nanoseeds, when Pn molecule arrives, it is not easy for it to diffuse with another molecule to become a stable nucleus, as its island size is below the critical island size. Most of the rare Pn molecules lying beyond the mean diffusion length will be desorbed from the substrate before they collide with each other to form stable 2626

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⎛ p⎞ ⎛ B⎞ ln⎜⎜ ⎟⎟ = ⎜A − ⎟ T⎠ ⎝ po ⎠ ⎝

nuclei. As a result, crystals with very low density can be observed. However, when identical Pn molecules arrived at the substrate having nanoseeds, it is easy for them to diffuse to the neighboring nanoseeds because the distance between these species is limited by the mean diffusion length. This proposes that there should be formation of a critical nucleus which promotes nucleation. SEM images in Figure S1 show the comparison of samples without nanoseeds and with nanoseeds. Furthermore, thermodynamic analysis implies that the number of molecules required for a stable island increases with increasing substrate temperature19 which suggests that, for our case, the value of critical island size may exceeds 4. So, in this case nanoseed assisted growth helps incoming molecules make a stable island. From the images in Figure 2, it can be predicted that the density of large crystals depends on the density of nanoseeds. When monolayer coverage is beyond 0.7 ML, the nanoseed density is so high that incoming molecules just take part in increasing the density and thickness because they have less space to enlarge the crystal size. Our experimental result suggests that the low density of nanoseeds is indeed helpful for achieving the large crystals. 3.2. Substrate Temperature. Temperature at the substrate has a key effect on the crystal growth. For better understanding of the formation of Pn crystal, we systematically studied different growth temperature (series B) as illustrated by SEM images in Figure 3. It is observed that growth is different

where p and po is equilibrium and ambient vapor pressure, respectively, of Pn, while A and B are constants, and it is recognized that B = ΔHsub/R, where “ΔHsub” defines enthalpy of sublimation while “R” is the gas constant. By taking values of A and B21 we can calculate the vapor pressure at source (285 °C) and different substrate temperatures. Graph (a) in Figure S2 shows linear behavior of vapor pressure with respect to temperature. It demonstrates that the vapor pressure of Pn also decreases as temperature decreases from 210 to 120 °C. The calculated vapor pressure at 120 °C is exp (−12) Pa, which suggests that most of the Pn molecules will not desorb and help in increasing the density of crystals. At 210 and 180 °C, because of high substrate temperature, desorption of Pn molecules occurs along with the absorption and diffusion processes. At these temperatures supersaturation is low, which results in lower nucleation probability. Due to this, the density of crystals is less as shown by the SEM images in Figure 3, while at 150 and 120 °C, because of lower vapor pressure value ∼Psub = exp (−12) Pa desorption nearly ceases and incoming molecules from the source will be immobile. The increased supersaturation results in higher nucleation probability, and growth occurs only because of adsorption and diffusion. This leads to high density crystals. Hence, there are two important observations: (i) at higher substrate temperatures low density crystal is obtained because of lower supersaturation; (ii) at lower substrate temperatures supersaturation is higher which results in high density crystals. Calculated values of nucleation probability with varying Pn vapor concentrations at different substrate temperatures is illustrated in Figure S2(b). The observed trend of growth is in consistent with the calculated values. 3.3. Gas Flow Rate Effect. Table 1 shows a series of SEM images to demonstrate the detailed growth process of Pn crystal with various gas flow rates from 100 to 400 sccm while keeping other parameters as reference (series C). Corresponding AFM images shown in Table 1 illustrated the thickness of Pn crystals ranging from 700 nm down to 48 nm. At 100 sccm, we obtained thick and large-sized crystals. In horizontal PVT, only a small amount of vapor reverses flow either descending or ascending in close proximity of the crystal and the heat source, respectively.22 This results in less condensation of vapor on the substrate through the boundary layer of gas. Thus, at lower gas flow rate the crystals are large but with very low density. Lower flow rates made the Pn vapors have sufficient residence time to interact with the nanoseeds, which results in thick crystals. The forced diffusion of vapor using carrier gas has more impact than the molecular diffusion driven by concentration gradient as proposed by Nam et al.23 Under this advection state, there generally exists a maximum Pn vapor concentration and supersaturation downstream from the source. As the flow rate of N2 carrier gas increases, the supersaturation shifts further downstream, as flow gives direction to the vapor, which means by adjusting gas flow we can change supersaturation at a point. At 300 sccm flow we got the best results. The typical thickness of as-grown Pn crystalline flakes can be narrowed in a range from 200 nm to around 50 nm. The upper surface of the crystal is indexed to the (001) plane which has the lowest surface energy.24 This results in offering the molecules to diffuse to the side surfaces ((11̅0), (010), (110)), where the surface energies

Figure 3. SEM images of obtained single crystal at different substrate temperatures (series A).

at different substrate temperatures. In our case, substrate temperature of 180 °C shows the best results. These results can be explained in terms of supersaturation and nucleation probability of grown adatoms (here Pn molecules) given by Sears20 ⎡ ⎛ σ 2π ⎞⎤ PN ∝ exp⎢ −⎜ 2 2 ⎟⎥ ⎣ ⎝ k T ln α ⎠⎦

(2)

(1)

where σ is surface energy, T is substrate temperature, and α is supersaturation. Assume that vapor pressure of Pn coming from the source is steady over the entire tube with background pressure of 1 atm. Vapor pressure can be calculated from the integrated form of the Clausius−Clapeyron equation. 2627

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Table 1. SEM and AFM Images of Obtained Pn Crystals with Different Gas Flow (Series B)

are higher; thus, the molecules diffuse much more easily into the lattice planes on these surfaces. The faster the growth rates of the side surfaces, the larger the area for the top surface. This competitive behavior among different planes leads to the extension of crystal size along the (010) plane. At the same time accelerated speed of gas flow decreases the deposition rate, which helps to obtain ultrathin crystals down to 48 nm. Our PVT approach, therefore, resulted in very thin Pn crystals endowing immense application in organic electronic. Exceeding 300 sccm, flakes of Pn appeared which stand on their edge onto the substrate. Because of high flow speed, most of the volatilized molecules travel through the nanoseeds without helping them with maturation. These results suggest that to obtain ultrathin crystal, it is necessary to control supersaturation which can be adjusted by the aid of gas flow rate. 3.4. Time Effect. To further understand the formation of Pn crystals, we also studied the time effect (series D). From the images in Figure 4a, it is clear that size and thickness increase as time extends. Different Pn shapes have also been observed. The prominent shape of the crystal obtained is pentagonal and rhombic, while the lifetime for the hexagonal crystal is very small. The result indicates that the shape of Pn crystals with distinct facets can be partially tailored by the deposition time. In essence, the formation of different crystal shapes of Pn crystals at different deposition times is governed by minimizing the surface energy of the crystal system. An overall growth mechanism has been proposed. Normally, during the nucleation and growth, the vapor atoms prefer to fuse with the high energy crystallographic planes in the crystal lattice system. The high-energy planes preferentially disappear after growth, and then the final crystals are terminated by low energy planes.25 We have calculated the angles among the side faces for large-sized crystals. The corresponding plane indexes of side faces and the calculated values of angles which are consistent with the reported work26 are presented in Figure 4a. Based on the above analysis for Pn shapes, the evolution process of their edge planes could be described as follows. For hexagonal shape, large-sized crystal are confined by (110), (010), and (110̅ ) surface energy planes. Due to the lower surface energy, the growth rate of the (001) plane could be very slow. Then, as time goes on, since the growth rate of the (010) plane is high, so are those of (110) and (11̅0) planes. In our

Figure 4. (a) Crystal shape with corresponding angles between different planes. (b) Schematic illustration of shape evolution with respect to supersaturation and surface energy.

experiments, up to 60 min of growth, only (110) planes will be left behind to become the final edge planes of Pn crystal and to maintain the whole crystal with a minimum surface energy. Above 60 min growth time, because of high supersaturation, growth of the crystal along the (001) direction is high and slower along other crystal planes. This is the limiting step where the enhancement in size is diminished while thickness increases. Over a growth period of 120 min it is observed that the thickness of the crystal increases from a few tenths of nanometers up to several micrometers. This infers that crystal size and thickness are influenced by evolution of crystalline planes. Growth mechanism of Pn crystal with increasing supersaturation and surface energy is schematically illustrated in Figure 4b. 2628

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Figure 5. (a) Schematic of crystal device (inset: optical image of device). (b,c) Transfer and output curve of device, respectively. (d) Differential output conductance of different thickness OSCFET.

(3)

gate voltage, the distribution of the gate-induced charges along the channel becomes narrower as the VSD increases and at the saturation regime a pinched off point occurs. This means the channel resistance would increase with increasing VSD. However, the differential conductance vertical to the substrate in the source region has a positive correlation with the applied voltage. It should increase with increasing the VSD from 0 in the vicinity of VSD = 0 V. The total differential conductance is dominated by the smaller differential conductance. When two differential conductances are nearly the same, there will be a turning point as shown in the Figure 5d. Compared to the thin crystal (95-nm-thick) device, the turning point in the thick crystal (360-nm-thick) device has a shift to higher VSD as shown in Figure 5d. This indicates that the injection resistance leads to much deterioration in the thick crystal device performance. We can conclude that ON current increases with reduction in crystal thickness. This suggests that it is meaningful to get an ultrathin single crystal.

where IDS is the drain current, μ is the mobility, Ci is the dielectric capacitance, W and L are the channel width and length, respectively, VGS is the gate voltage, and Vth is the threshold voltage. Our devices have good device performance, and the highest hole mobility reaches up to 5.6 cm2/(V s). We found that the device performance is influenced by the crystal thickness because different crystal thickness results in different injection barrier (hereafter called injection resistance) from the top contact to the semiconductor/dielectric interface. The effect of injection resistance (Ri) on the device performance can be directly shown from the differential conductance of the device output curves as displayed in Figure 5d. If we neglect Ri, the differential conductance in the channel should monotonically decrease to 0 when increasing the sourcedrain voltage (VSD) from VSD = 0 V. This is because, at fixed

4. CONCLUSION Our experiments have demonstrated the controlled growth of large and ultrathin 2D Pn crystal via nanoseed assisted growth through PVT method stimulated by supersaturation. By optimizing the deposition parameters, we successfully illustrate the synthesis of high quality, Pn single crystal with tens of micrometers in size and around 50 nm in thickness. We systematically monitor the effect of coverage and substrate temperature on crystal size. We also demonstrated that use of high gas flow rates decreases the deposition rate while at the same time lowering the supersaturation, thus helping the growth of ultrathin 2D crystal. Moreover, growth mechanism of the shape evolution of as-grown Pn crystal has also been explained with respect to different surface energies of various

Large scale optical images of crystals over a broad range of time are shown in Figure S3. However, it is important to note that if the heating of the furnace stops, the growth of the crystal will also be terminated. Thus, besides the majority of crystals in the final stage, there should some products staying in the intermediate stage probably due to the late nucleation. Furthermore, the growth condition for each nanoseed cannot be precisely the same. 3.5. Device Characterization. In this study, the device structure is shown schematically in Figure 5a; the interface between single crystal and substrate is preserved during the whole device fabrication. Figure 5b,c shows the transfer and output curves of an organic single crystal field effect transistor. The mobility of the device in the saturation region is derived from the following equation IDS =

μCiW (VGS − Vth)2 2L

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(12) Huitema, H. E. A.; Gelinck, G. H.; van der Putten, J. B. P. H.; Kuijk, K. E.; Hart, C. M.; Cantatore, E.; de Leeuw, D. M. Adv. Mater. 2002, 14, 1201−1204. (13) Zhang, Y.; Hu, W. P. Sci. China, Ser. B: Chem. 2009, 52, 751− 754. (14) Menard, E.; Podzorov, V.; Hur, S. H.; Gaur, A.; Gershenson, M. E.; Rogers, J. A. Adv. Mater. 2004, 16, 2097−2101. (15) Sundar, V. C.; et al. Science 2004, 303, 1644−1646. (16) Jin, Q.; Li, D.; Qi, Q.; Zhang, Y.; He, J.; Jiang, C. Cryst. Growth Des. 2012, 12, 5432−5438. (17) Fritz, S. E.; Martin, S. M.; Frisbie, C. D.; Ward, M. D.; Toney, M. F. J. Am. Chem. Soc. 2004, 126, 4084−4085. (18) Stadlober, B.; Haas, U.; Maresch, H.; Haase, A. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 165302. (19) Stefan, C. B. M.; Alejandro, L. B.; Shuhong, L.; Colin, R.; Mark, E. R.; Zhenan, B. Adv. Funct. Mater. 2001, 17, 3545−3553. (20) Sears, G. W. Acta Metall. 1955, 3, 361−366. (21) Oja, V.; Suuberg, E. M. J. Chem. Eng. Data 1998, 43, 486−492. (22) Markham, B. L.; Rosenberger, F. J. Cryst. Growth 1984, 67, 241−254. (23) Nam, C. Y.; Tham, D.; Fischer, J. E. Appl. Phys. Lett. 2004, 85 (23), 5676−5678. (24) Northrup, J. E.; Tiago, M. L.; Louie, S. G. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 121404. (25) Yan, Y.; Zhou, L.; Zhang, Y.; Zhang, J.; Hu, S. Cryst. Growth Des. 2008, 8, 3285−3289. (26) Mattheus, C. C.; Dros, A. B.; Baas, J.; Oostergetel, G. T.; Meetsma, A.; de boer, J. L.; Palstra, T. M. Synth. Met. 2003, 138, 475− 481.

planes. The interface between the crystal and the SiO2 dielectric has been preserved during all processes of device fabrication, which is important for the electrical transport study. Electrical study based on crystals showed mobility as high as 5.6 cm2 V−1 s−1 revealed that the in situ growth of ultrathin crystals has a significant result in transport performance. We believe that the synthesis technique in this work manifests a new strategy for the growth of large and ultrathin Pn crystals.

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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.cgd.5b01726. SEM images for the comparison of growth on substrates without nanoseeds and with nanoseeds, graphs showing Pn vapor pressure in furnace and nucleation probability of Pn at different substrate temperatures depending on local supersaturation, and optical images of Pn crystal after PVT at different times (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-10-82545563. Fax: 86-10-62656765. E-mail: jiangch@ nanoctr.cn. Author Contributions #

S. Atika Arabi and Ji Dong contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 21432005, 11374070, 61327009), and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA 09040201).

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REFERENCES

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