Electric-Field-Tunable Growth of Organic Semiconductor Crystals on

Subscriber access provided by Karolinska Institutet, University Library

Communication

Electric-Field-Tunable Growth of Organic Semiconductor Crystals on Graphene Nguyen Ngan Nguyen, Hyo Chan Lee, Boseok Kang, Mankyu Jo, and Kilwon Cho Nano Lett., Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Electric-Field-Tunable Growth of Organic Semiconductor Crystals on Graphene Nguyen Ngan Nguyen‡, Hyo Chan Lee‡, Boseok Kang, Mankyu Jo and Kilwon Cho*

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 37673, Korea

ABSTRACT. Growth of organic semiconductor thin films on a two-dimensional template is affected by its properties, and is not well understood. This growth process dictates a thin film’s final morphology and crystal structure, and is controlled by the interactions between admolecules and the template. Here, we report that the template’s charge density determines the tuning of such interactions. We observe the dependence of pentacene nucleation on charge carrier density ng in graphene under an applied electric field and contact-doping, then deduce that the interaction energy EA between the ad-molecule and the graphene is related linearly to ng. This tunability of EA allows control of the pentacene crystals growth. We exploit these findings to demonstrate that graphene in which ng is controlled can be used to template pentacene thin films for improved optoelectronic properties, such as electrical conductivity and exciton diffusion length. KEYWORDS. graphene, charge carrier density, growth template, organic semiconductors

1

ACS Paragon Plus Environment

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Graphene has an aromatic sp2 network, indicating that it can function as an excellent template for the growth of novel nano-microstructures, especially organic semiconductor materials.1-9 Graphene’s extraordinary thinness enables “wetting transparency”, which means that the wettability of graphene closely resembles that of the underlying substrate.10-11 Similarly, the assembly of the organic semiconductor ad-molecules on top of the graphene complies with the substrate’s hydrophobicity.12 Given that single-layer graphene is doped by alien factors,13-15 the observed wetting transparency is related to the doping effects that the substrate induces in graphene.16-17 In this sense, even modest changes in the graphene’s electronic state can promote a great modulation in the ad-molecules’ assembly. Hence, it is possible that a clean graphene sheet with a controlled charge carrier density can serve as a 2D template for the controlled growth of organic semiconductor crystals. However, practical template applications require high-quality graphene with a large and uniform area, which can be provided only by chemical vapor deposition (CVD).18-20 Unfortunately, CVD graphene is inevitably subject to a transfer process that contaminates its surface with polymeric residues,13-15 causing unintentional doping and interfering with the ad-molecules’ assembly, diminishing the graphene’s template function and inducing undesirable crystalline growth forms.15, 21-22 Therefore, to develop 2D growth templates based on graphene with a tuned charge carrier density, the doping of the graphene must be controlled while its surface is kept perfectly clean to facilitate the adsorption of the organic semiconductor material. In this study, we show how to fabricate these 2D templates and present their abilities in terms of tailoring the growth, morphology and crystalline ordering of pentacene, a well-known organic semiconductor. The underlying molecular dynamics controlling the pentacene assembly are then quantified. Based on this new understanding, we developed new approaches to design graphene templates to impart desirable optoelectronic characteristics to the pentacene thin films. Because the optoelectronic properties of organic semiconductor materials are tightly coupled to their topological and crystalline features,23-27 pentacene layers grown on an 2


Page 2 of 30

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

appropriately designed template exhibit significantly increased exciton diffusion length and electrical conductivity, advancing the efficiency of organic electronic devices. The graphene templates used in this work were engineered with controllable doping effects induced by the electric field-effect (Figure 1a), the polymer contact (Figure 1b and Figure S1 in Supporting Information), or both. By coupling with various vinyl polymers bearing electrondonating or electron-withdrawing moieties, graphene exhibited a wide range of doping states, from p-doping to pristine, and to n-doping. The doping characteristics of these graphene coupled with polymers were revealed in the Raman average spectra (Figure 1c) with primary peaks at ~1600 cm-1 (G peak) and ~2700 cm-1 (2D peak). The positions and intensities of these two peaks varied as the graphene was in contact with different polymers. These changes imply that the graphene’s doping level was affected by its substrate. We then statistically quantified such dependencies by using data from the 2D-peak and G-peak positions (2D, G), the intensity ratios of the 2D/G peaks (I2D/IG) and the G-peak full-width at half-maximum (G) extracted from the 500-spectra Raman mappings (Figure 1d). The 2D band of the graphene was blueshifted as the electron density in the graphene increased and was redshifted as the electron density decreased. Both doping types redshifted the G band and decreased the I2D/IG ratio.28-30 In brief, our Raman results indicated that (i) the graphene on polystyrene (PS) remained at a pristine doping level, (ii) the graphene was increasingly n-doped with poly(4-vinylphenol) (P4VPh) and poly(4-vinylpyridine) (P4VP), and (iii) was p-doped with poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC). On the other hand, all these graphene templates showed flat surfaces with similar roughness (Figure S1). Ultraviolet photoelectron spectroscopy (UPS) measurements estimated that the work function for the polymer-doped graphene was widely tuned from 4.22 eV for P4VP, to 4.46 eV for PS, and to 4.87 eV for PVC (Figure 1e). Given that the pristine graphene has a work function in the range of 4.5-4.6 eV,30-32 the graphene with PS was negligibly subject to the doping effect, whereas graphene was increasingly p-doped by contact with PMMA and PVC and n-doped by 3


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

coupling with P4VPh and P4VP. These results correspond to how graphene field-effect transistors (GFETs) performed as the graphene channel was in contact with the polymers. We focused on the Dirac voltage (VDirac) of GFETs, i.e., the gate voltage at which the electrical current is minimized, because its position strongly correlates with the graphene’s doping level (Figure 1f).15, 22 When the graphene channel was in contact with PS, the VDirac of the device was ~0 V, indicating a trivial doping effect for this graphene sample. Negative VDirac values of ~-15 V and ~-33 V were observed when the graphene was in contact with P4VPh and P4VP, respectively, implying electron doping in these graphene samples. Positive shifts in the VDirac to ~20 V for PMMA and to ~53 V for PVC imply hole doping in these graphene channels. With these doping methods, we could in situ dope graphene as pentacene ad-molecules assemble on graphene’s surface, and investigate how this doping state affects the nucleation and growth of the corresponding pentacene thin film. We monitored the same area on the surface of undoped graphene and the n-doped graphene during the pentacene growth through a nominal thickness of 20 Å by using atomic force microscopy (AFM) (Figure 2a). In these areas, the height hi and surface coverage  of numerous pentacene islands were analyzed statistically, revealing that they were clearly influenced by the doping state of the graphene templates (Figure 2b). On the surface of the undoped graphene, the islands expanded laterally to achieve large grain sizes, causing hi to be almost constant and  to increase. However, the crystallites merely increased in height without spreading on the surface of the n-doped graphene to produce small grains with more neighboring nuclei. At 2 nm nominal thickness, a great number of small rodlike nuclei formed on doped graphene, whereas smooth large crystallite grains formed on the undoped graphene. As the growth continued, the pentacene islands extended in both lateral and vertical dimensions but grew preferentially laterally on the undoped graphene and preferentially vertically on the doped graphene (Figure S2, Supporting Information). We also noticed that the actual volumes of pentacene thin films grown on undoped graphene were substantially smaller than that of those films grown on the n-doped graphene along the initial growth process (Figure 4


Page 4 of 30

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

2c). Such difference indicates that at the same deposition dose, there was a larger number of pentacene ad-molecules desorbed from the surface of undoped graphene than from the surface of n-doped graphene. These observations clearly demonstrate that the doping state of graphene template determined the growth of pentacene crystallites by controlling interaction between pentacene ad-molecules and the graphene surface. In addition to the morphology and growth kinetics, the crystal structure and molecular order of the pentacene thin films were also affected by the electronic states of graphene (Figure 3). To investigate such effects, we used grazing-incident X-ray diffraction (GIXD) to analyze the pentacene layers with different thicknesses grown on graphene templates of which the doping level was controlled (Figure 3a). For the initial growth (2 nm nominal thickness) on the undoped, p-doped and n-doped graphene samples, the pentacene molecules were similarly in the “lying-down” orientation in which the molecular long axis is parallel to the substrate. This structure can be visualized by the corresponding bulk-phase-features—specifically, the set of patterns (00l)B and the reflection of the (020)B crystal planes, which are tilted 18 ° and 78 ° from qxy, respectively. Among the three samples, the pentacene layer grown on the undoped graphene showed the sharpest reflections due to its higher quality and larger pentacene grain size, as seen in the AFM images. As the deposition proceeded, more pentacene molecules arrived at the graphene surfaces and arranged themselves in the lying-down orientation. Thus, the correlated reflections intensified, as seen in the 2D-GIXD patterns of the 20 nm-thick pentacene layers. Furthermore, pentacene ad-molecules also landed on existing islands, where the effects of the graphene surface were screened. In this case, these pentacene molecules adopted the standing-up orientation, which produces the set of thin-film-phase patterns along qz. For a more quantitative analysis, we quantified the coherent domain size (Rc) in the pentacene thin films by using the Scherrer equation Rc 

5

K , where K is the dimensionless  cos( Bragg )


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scherrer constant,  is the incident X-ray wavelength,  and Bragg are the full-width at halfmaximum and Bragg angle of the corresponding reflection, respectively. During the initial growth stage (2 nm thickness), the coherent domain size of the lying-down pentacene crystals Rc-lying, which was analyzed from the (00l)B peaks, on the undoped graphene was approximately 1.3 times larger than that on the p-doped graphene and was 1.5 times larger than that on the ndoped graphene (Figure 3b and Table S1, Supporting Information). These distinctions in the Rclying

values remained until the later growth stage (20 nm thickness). It implies that the pentacene

ad-molecules were able to travel longer on the undoped graphene to find the most energyfavorable nuclei to incorporate into. In addition, we also quantified the Rc-standing of the crystallites with the standing-up structure by analyzing the (00l)T peaks (Figure 3b and Table S1, Supporting Information). As compared to those of the lying-down domains, they were in the opposite trend, i.e., the high Rc-standing in the layers grown on the doped graphene and the negligible value in the layer grown on the undoped graphene. This is because the standing-up structure only arises above the critical thickness where the intermolecular interactions are dominant over the interfacial dispersion force with graphene,33-34 and the actual heights of pentacene islands were smaller on the undoped graphene at the same pentacene doses (Figures 2b and 3b). The AFM and GIXD data shown in Figure 2 and Figure 3 were obtained with electrically gated graphene templates, however, the pentacene films grown on graphene templates with polymer-substrate doping showed the analogous results (Figures S2 and S5, Supporting Information). The consistency between the results collected from two different doping methods implies that the growth of pentacene film was determined mainly by the electronic state of the graphene template, not by the substrate’s other influences, such as the wetting transparency, intercalated water layer or localized trap sites. To understand this interesting dependence, we investigated the correlations between the nucleation density Nc of pentacene thin film, an important parameter of pentacene growth 6


Page 6 of 30

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

dynamics, and the graphene charge density ng, a controlling parameter of the graphene doping state, by analyzing numerous 2 nm pentacene thin films grown on the graphene with finely tuned ng (Figure 4). The ng can be extracted from ng  C g (VG  VDirac ) / e

(1)

where Cg is the dielectric capacitance, VG is the bottom-gate voltage, and e is the electron charge.29 Note that VDirac of the GFET with an unmodified 300 nm SiO2 dielectric layer and no polymer-contact doping was ~20 V. The graphene charge density ng was found to determine the Nc of the pentacene thin film at 303 K in the way that Nc gradually increased as ng increased from 0, i.e., the condition in which the doping charge carriers were neutralized (Figure 4a). Because Nc is the product of various competing processes, including adsorption, diffusion, desorption and nucleation, it can be considered from the nucleation physics point of view as follows: Nc = Cexp (ENuc kBT),

(2)

ln Nc = ln C + ENuc kBT

(3)

where C is the constant prefactor at a fixed deposition rate, kB is the Boltzmann constant, T is the substrate temperature, and ENuc is the activation energy for a homogenous nucleation.35 The dependence of Nc on ng in equation 2 and 3 shows that ENuc was a function of ng (ENuc(ng)); ENuc(ng) linearly increased as ng increased (Figure 4b). For the “initially incomplete” nucleation, as is the case here (Figure 2c), ENuc can be approximated as E Nuc  Ei / 2  E A , where Ei is the intermolecular binding energy, and EA is the activation energy for the desorption of an admolecule.35-38 Notably, Ei is independent of the substrate and thus becomes constant in this estimation,37 so we could attribute the increasing correlation between ENuc and ng solely to the relationship between EA and ng. In turn, it indicated that EA is related linearly to ng with coefficient ξ as (4)

EA = ξng + EA0

7


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

where EA0 is the EA of a pentacene ad-molecule on undoped graphene. The coefficient ξ was estimated from here is 2.8×10-14 eV/cm2 (Figure 4b). We also investigated the dependence of Nc on the substrate thermal energy kBT for five graphene samples having different ng; graphene templates on P4VPh (ng = 7.2×1011 cm-2, ndoped), P4VP (ng = 1.7×1012 cm-2, n-doped), PMMA (ng = 1.0×1012 cm-2, p-doped), PVC (ng = 2.7×1012 cm-2, p-doped), and PS (undoped), respectively (Figure 4c). The slopes of ln(Nc) versus 1/(kBT) are the values of ENuc = Ei 2 + EA in these five graphene samples. Clearly, ENuc, so thus EA, increased linearly with ng from 410 meV to 500 meV (Figure 4d). The coefficient ξ was estimated from this linear relationship is 2.8×10-14 eV/cm2, and is consistent with the value extracted from the relation between Nc and ng at 303 K (Figure 4a, 4b). This consistency verified our calculations. Moreover, the fitted values of ENuc are reasonable because the values of Ei and EA0 are reported to be in the range of 250–300 meV and 150-200 meV, respectively.36, 39 Here, EA is the energy required for a pentacene ad-molecule to be desorbed; it is thus directly proportional to the interaction energy between that ad-molecule and the substrate. Therefore, the linear function between EA and ng means that the increase in graphene doping will strengthen the interactions between the graphene and the pentacene ad-molecules. The charge that the doping adds to the underlying graphene can induce an electronic polarization to the nearby pentacene ad-molecule. Given that the Coulomb interaction between the charge added to the graphene and the induced-dipole in the ad-molecule is always attractive, the interaction energy between the ad-molecule and graphene always increases as the charge carrier density in graphene increases.40 This behavior provides a physical reason for Equation (4). In addition to Nc, hi increased and θ decreased with increasing the charge carrier density in graphene, showing that the growth mode for a pentacene thin film gradually transformed from 2D to 3D as the graphene was doped (Figure 2b and Figure S3, Supporting Information). It is of particular interest because the 3D growth mode of pentacene preferentially results in standing-up molecular orientation as discussed above (Figure 3); the standing-up structure 8


Page 8 of 30

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

predominantly forms on thick pentacene nuclei where the intermolecular interactions are dominant over the interfacial interaction with graphene. These dependences of hi and θ on ng can be easily explained from the linear increase of EA with ng. The linear increase of EA with ng indicates that the diffusion barrier ED for the pentacene ad-molecules on the graphene also increased with increasing ng, which is attributable to the linear scaling between ED and EA.41 Accordingly, the undoped graphene loosely interacts with the pentacene ad-molecules, providing them with sufficient diffusivity for the lateral growth mode. The strong interactions with the doped graphene limit the surface diffusivity of the pentacene ad-molecules, meaning that the columnar growth mode is dominant.42-43 As the results, at a certain deposition dose, pentacene crystallites grown on the undoped graphene, compared to those grown on the doped graphene, had larger grains, lower heights and higher molecular order. These features are beneficial to the optoelectronic properties of the pentacene layer, as follows. Graphene’s excellent optical transparency and electrical conductivity make it useful as both a transparent electrode (or electrode modifier) and as a growth template for a deposited organic semiconductor layer in optoelectronic applications such as organic photovoltaics (OPV) and organic light-emitting diodes (OLEDs) with a vertical geometry.44-46 In OPVs, the generated excitons travel to the donor-acceptor interfaces, where they dissociate into free electrons and holes. These charge carriers are then transported vertically to the relevant electrodes, forming the electrical current. The efficiency of the exciton transport is related to the exciton diffusion length LD, and the efficiency of the charge carrier transport is related to the electrical conductivity . Here, we demonstrate that using a graphene template with a favorable electronic state can greatly increase these parameters in a pentacene active layer (Figure 5). The LD value of a pentacene layer was measured using static photoluminescence quenching (PLQ) and spectrally resolved photoluminescence quenching (SR-PLQ).47-48 The principle behind this method is to distinguish between the photoluminescence (PL) intensity of a pentacene sample with a blocking layer (PLB) and the PL intensity of a pentacene sample with 9


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

a quenching layer (PLQ). Once the excitons reach the pentacene/quenching interface, they dissociate, meaning that the PLQ is reduced compared with the PLB (Figure S6, Supporting Information). The degree of reduction 

PLB indicates the efficiency of the exciton diffusion PLQ

and separation. The exciton diffusivity LD is then determined by fitting  to the 1D steady-state equation: 

PLB   ' ( ) LD  1 PLQ

(5)

where ′ is the modified absorption coefficient (Figure 5a). Fabricated with the undoped graphene as a growth template, the pentacene film achieved an extraordinary LD ~92 nm, which is 50% higher than the LD obtained by using the p-doped graphene (~61 nm) and 65% higher than that obtained by using the n-doped graphene (~55 nm). These results agree with the corresponding crystal structure and grain-size data. In addition to LD, a high OPV efficiency requires that free electrons and holes be transported to the corresponding electrodes. Therefore, the active layer must be sufficiently conductive. We now demonstrate that, for an optimized charge carrier density, the graphene can template the growth of the pentacene to yield crystallites with substantial electrical conductivity. In this experiment, we used only pentacene thin films with a nominal thickness of 2 nm for better electrical signals and uniform lying-down molecular orientation. Conductive atomic force microscopy (C-AFM) was used to collect the current–voltage (I–V) characteristics of the specific pentacene grains sandwiched between a platinum-iridium (Pt/Ir)-coated tip and the bottom graphene layer. Within the measured voltage range (from -500 mV to 500 mV), those grains exhibited linear I-V characteristics, i.e., Ohmic conduction (Figure S8, Supporting Information). The electrical resistivity  of a pentacene grain can be calculated using the equation R  

hi , where A is the A

effective measured area for each grain or the area under the tip, and R is the resistance extracted 10


Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

from the measured linear I-V curve.49 For easier discussions, we switched the electrical resistance R and resistivity  to their respective reciprocal parameters, i.e., the electrical conductance G and conductivity :

G 

A hi

(6)

The average electrical conductivity  of the pentacene grains grown on a graphene template depends only on the pentacene itself, the orientation of -orbital overlap and the crystallinity of such grains, and can be quantified as the slope of the Equation (6) (Figure 5b). Pentacene grains grown on the undoped graphene had a very high average conductivity of ~8.5 × 10-3 S·m-1 which is nearly 2.7 times greater than that obtained by using the p-doped graphene and 8 times greater than that obtained by using the n-doped graphene. The exciton diffusion length and electrical conductivity, strongly relate to the molecular orientation and the degree of crystalline ordering of the active layer. According to the Förster energy transfer, the LD is a function of the dipole orientation factor , related to the molecular orientation, and the hopping distance a, associated with the proximity between the chromophores.47-48 Likewise, the electrical conductivity of a crystal grain depends on the molecular orientation and the degree of crystalline ordering within such grain. Because the molecular orientations of pentacene crystallites grown on graphene templates were similar, as confirmed by GIXD results (Figure 3 and Figures S4-S5, Supporting Information), the different LD and  values for the layers grown on the undoped, p- and n-doped graphene are attributed mainly to differences in the grain sizes and crystalline perfection. Given that the chromophores are much closer within a crystal grain than between multigrains, a pentacene layer with large and high quality grains is desirable to yield a higher hopping distance a and thus facilitate an exceptional LD. As previously discussed, for a same deposition dose, the pentacene grain sizes increased with decreasing the graphene charge carrier density. Therefore, on graphene with a sufficiently low charge carrier density, the pentacene ad-molecules were able to arrange 11


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

themselves into high-quality crystals with large grains that act as beneficial pathways for excitons. Whereas the higher  of the pentacene grains assembled on the undoped graphene was merely a result of the crystalline perfection that the undoped graphene template imparted to the grown crystallites (Figure 3 and Table S1, Supporting Information). Such findings about the exciton diffusion length and electrical conductivity of pentacene thin films with various morphologies and molecular kinetics suggest that the optoelectronic properties of an organic semiconductor active layer could be increased by carefully engineering the graphene template to produce the desired molecular order, grain size and growth mode. In summary, we observed that the molecular-level evolution of the morphology and crystal structure of pentacene thin films grown on a graphene surface varied with graphene’s electronic state. By in situ controlling the graphene charge carrier density during pentacene deposition, we discovered the sole dependence of the growth dynamic of pentacene on graphene’s charge density regardless of the underlying substrate. Furthermore, we revealed that such dependence can be attributed to linear relations between the interaction energy EA of a pentacene admolecule with graphene and the graphene charge carrier density. Using this understanding, we proposed how to design a 2D growth template to engineer the assembled organic semiconductor layer’s major features, including its molecular orientation, crystal structure and morphology, for boosting the layer’s optoelectronic properties. Insights obtained from this study constitute a step toward achieving flexible organic electronic devices by incorporating graphene as tunable, transparent template-electrodes.

12


Page 12 of 30

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 1. Schematic top and cross-sectional views of graphene templates for pentacene growth using (a) gate-bias doping or combined polymer and gate-bias doping and (b) polymer-substrate doping. Characteristics of the doped graphene templates by polymer doping: (c) Raman average spectra of the polymer-doped graphene; (d) plots of the Raman spectral parameters (2D: 2D peak position, I2D/IG: intensity ratio between 2D and G peaks, and G: G peak full-width at halfmaximum) vs. the G-peak position (G). Dashed trend lines indicate doping levels. (e) UPS spectra of the polymer-doped graphene samples (upper), and the estimated work functions of them (lower). (f) I-V characteristics of the FET based on the corresponding polymer-doped graphene.

13


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (a) AFM images of pentacene films grown on undoped and n-doped graphene along the increasing nominal thickness during the initial stage of deposition. The AFM images were taken in the same area for each sample. Scale bar: 0.4 µm. (b) Height analysis for seven locations in the AFM images in (a). Inset: surface coverage analysis. (c) Actual deposited volumes of pentacene thin films grown on undoped and doped graphene at each nominal thickness.

14


Page 14 of 30

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 3. Effects of graphene’s doping on crystalline structure of pentacene films. (a) 2DGIXD patterns of pentacene films of various thicknesses grown on p-doped, undoped and ndoped graphene. (b) Schematic of different evolution modes of growth and crystalline structure of pentacene films on undoped and doped graphene.

15


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Surface dynamics of pentacene ad-molecules vs. graphene charge carrier density. (a) Pentacene nucleation density vs. graphene charge carrier density from gate-bias, polymercontact doping and combined polymer-contact and gate-bias doping. (b) The difference in nucleation energy barrier of pentacene vs. graphene charge carrier density calculated from Figure 4a. (c) Pentacene nucleation density vs. thermal parameter 1/(kBT). (d) Nucleation energy barrier of pentacene vs. graphene charge carrier density calculated from Figure 4c.

16


Page 16 of 30

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 5. Optoelectronic properties of the pentacene films grown on the graphene with different doping. (a) The exciton diffusion lengths for the pentacene films were estimated from the plots of quenching ratios  vs. modified absorption coefficients ′. Inset: the static PL spectra of the pentacene films grown on the undoped graphene with a blocking layer (filled) and quenching layer (open). (b) The electrical conductance of the pentacene crystallites vs. A . Inset: scheme hi

of the C-AFM setup.

17


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:

Additional details on the experimental methods; figures showing the schematic process for graphene with polymer-contact doping, AFM images of graphene films on various polymer substrates, AFM images of pentacene films of various thicknesses grown on graphene templates, surface coverage and average island height of pentacene in the initial growth stage, 2D-GIXD patterns of pentacene thin-films grown on graphene templates, static spectra and SR-PLQ plot of standing up pentacene thin film grown on SiO2, C-AFM I-V characteristics of nominal 2 nm pentacene single crystallites bearing lying-down and standing-up orientation (Figures S1–S8); summary of crystallographic features of pentacene films grown on different graphene templates (Table S1) (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT This work was supported by a grant (Code No.2011-0031628) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science and ICT, Korea. 18


Page 18 of 30

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

REFERENCES (1) Mativetsky, J. M.; Wang, H.; Lee, S. S.; Whittaker-Brooks, L.; Loo, Y.-L. Face-on stacking and enhanced out-of-plane hole mobility in graphene-templated copper phthalocyanine. Chem. Comm. 2014, 50 (40), 5319-5321. (2) Ogawa, Y.; Niu, T.; Wong, S. L.; Tsuji, M.; Wee, A. T. S.; Chen, W.; Ago, H. Selfassembly of polar phthalocyanine molecules on graphene grown by chemical vapor deposition. J. Phys. Chem. C 2013, 117 (42), 21849-21855. (3) Wang, Q. H.; Hersam, M. C. Room-temperature molecular-resolution characterization of self-assembled organic monolayers on epitaxial graphene. Nat. Chem. 2009, 1 (3), 206-211. (4) Acevedo-Cartagena, D. E.; Zhu, J.; Trabanino, E.; Pentzer, E.; Emrick, T.; Nonnenmann, S. S.; Briseno, A. L.; Hayward, R. C. Selective Nucleation of Poly(3-hexyl thiophene) Nanofibers on Multilayer Graphene Substrates. ACS Macro Lett. 2015, 4 (5), 483-487. (5) Tian, T.; Shih, C.-J. Molecular Epitaxy on Two-Dimensional Materials: The Interplay between Interactions. Ind. Eng. Chem. Res. 2017, 56 (38), 10552-10581. (6) Marsden, A. J.; Rochford, L. A.; Wood, D.; Ramadan, A. J.; Laker, Z. P. L.; Jones, T. S.; Wilson, N. R. Growth of Large Crystalline Grains of Vanadyl-Phthalocyanine without Epitaxy on Graphene. Adv. Funct. Mater. 2016, 26 (8), 1188-1196. (7) Kim, D. H.; Park, Y. D.; Jang, Y.; Yang, H.; Kim, Y. H.; Han, J. I.; Moon, D. G.; Park, S.; Chang, T.; Chang, C.; Joo, M.; Ryu, C. Y.; Cho, K. Enhancement of Field-Effect Mobility Due to Surface-Mediated Molecular Ordering in Regioregular Polythiophene Thin Film Transistors. Adv. Funct. Mater. 2005, 15 (1), 77-82. (8) Kim, B.; Chiu, C. Y.; Kang, S. J.; Kim, K. S.; Lee, G. H.; Chen, Z.; Ahn, S.; Yager, K. G.; Ciston, J.; Nuckolls, C.; Schiros, T. Vertically grown nanowire crystals of dibenzotetrathienocoronene (DBTTC) on large-area graphene. RSC Advances 2016, 6 (64), 59582-59589.

19


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(9) Kratzer, M.; Teichert, C. Thin film growth of aromatic rod-like molecules on graphene. Nanotechnology 2016, 27 (29), 292001. (10) Rafiee, J.; Mi, X.; Gullapalli, H.; Thomas, A. V.; Yavari, F.; Shi, Y.; Ajayan, P. M.; Koratkar, N. A. Wetting transparency of graphene. Nat. Mater. 2012, 11 (3), 217. (11) Shih, C.-J.; Wang, Q. H.; Lin, S.; Park, K.-C.; Jin, Z.; Strano, M. S.; Blankschtein, D. Breakdown in the wetting transparency of graphene. Phys. Rev. Lett. 2012, 109 (17), 176101. (12) Nguyen, N. N.; Jo, S. B.; Lee, S. K.; Sin, D. H.; Kang, B.; Kim, H. H.; Lee, H.; Cho, K. Atomically Thin Epitaxial Template for Organic Crystal Growth Using Graphene with Controlled Surface Wettability. Nano Lett. 2015, 15 (4), 2474-2484. (13) Cheng, Z.; Zhou, Q.; Wang, C.; Li, Q.; Wang, C.; Fang, Y. Toward intrinsic graphene surfaces: a systematic study on thermal annealing and wet-chemical treatment of SiO2supported graphene devices. Nano Lett. 2011, 11 (2), 767-71. (14) Lin, Y. C.; Lu, C. C.; Yeh, C. H.; Jin, C.; Suenaga, K.; Chiu, P. W. Graphene annealing: how clean can it be? Nano Lett. 2012, 12 (1), 414-9, DOI: 10.1021/nl203733r. (15) Pirkle, A.; Chan, J.; Venugopal, A.; Hinojos, D.; Magnuson, C.; McDonnell, S.; Colombo, L.; Vogel, E.; Ruoff, R.; Wallace, R. The effect of chemical residues on the physical and electrical properties of chemical vapor deposited graphene transferred to SiO2. Appl. Phys. Lett. 2011, 99 (12), 122108. (16) Ashraf, A.; Wu, Y.; Wang, M. C.; Yong, K.; Sun, T.; Jing, Y.; Haasch, R. T.; Aluru, N. R.; Nam, S. Doping-Induced Tunable Wettability and Adhesion of Graphene. Nano Lett. 2016, 16 (7), 4708-12. (17) Hong, G.; Han, Y.; Schutzius, T. M.; Wang, Y.; Pan, Y.; Hu, M.; Jie, J.; Sharma, C. S.; Muller, U.; Poulikakos, D. On the Mechanism of Hydrophilicity of Graphene. Nano Lett. 2016, 16 (7), 4447-53. (18) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, 20


Page 20 of 30

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

S. Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nat. Nanotechnol. 2010, 5 (8), 574-578. (19) Chen, J.; Guo, Y.; Jiang, L.; Xu, Z.; Huang, L.; Xue, Y.; Geng, D.; Wu, B.; Hu, W.; Yu, G.; Liu, Y. Near-Equilibrium Chemical Vapor Deposition of High-Quality Single-Crystal Graphene Directly on Various Dielectric Substrates. Adv. Mater. 2014, 26 (9), 1348-1353. (20) Kang, B.; Lee, S. K.; Jung, J.; Joe, M.; Lee, S. B.; Kim, J.; Lee, C.; Cho, K. Nanopatched Graphene with Molecular Self-Assembly Toward Graphene–Organic Hybrid Soft Electronics. Adv. Mater. 2018, 30 (25), 1706480. (21) Lee, W. H.; Park, J.; Sim, S. H.; Lim, S.; Kim, K. S.; Hong, B. H.; Cho, K. Surfacedirected molecular assembly of pentacene on monolayer graphene for high-performance organic transistors. J. Am. Chem. Soc. 2011, 133 (12), 4447-4454. (22) Suk, J. W.; Lee, W. H.; Lee, J.; Chou, H.; Piner, R. D.; Hao, Y.; Akinwande, D.; Ruoff, R. S. Enhancement of the electrical properties of graphene grown by chemical vapor deposition via controlling the effects of polymer residue. Nano Lett. 2013, 13 (4), 1462-1467. (23) Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 2014, 26 (9), 1319-1335. (24) Khim, D.; Luzio, A.; Bonacchini, G. E.; Pace, G.; Lee, M.-J.; Noh, Y.-Y.; Caironi, M. Uniaxial Alignment of Conjugated Polymer Films for High-Performance Organic Field-Effect Transistors. Adv. Mater. 2018, 30 (20), 1705463. (25) Soeda, J.; Okamoto, T.; Hamaguchi, A.; Ikeda, Y.; Sato, H.; Yamano, A.; Takeya, J. Two-dimensional crystal growth of thermally converted organic semiconductors at the surface of ionic liquid and high-mobility organic field-effect transistors. Org. Electron. 2013, 14 (4), 1211-1217. (26) Schuettfort, T.; Watts, B.; Thomsen, L.; Lee, M.; Sirringhaus, H.; McNeill, C. R. Microstructure of Polycrystalline PBTTT Films: Domain Mapping and Structure Formation. ACS Nano 2012, 6 (2), 1849-1864. 21


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(27) Kim, N.-K.; Jang, S.-Y.; Pace, G.; Caironi, M.; Park, W.-T.; Khim, D.; Kim, J.; Kim, D.Y.; Noh, Y.-Y. High-Performance Organic Field-Effect Transistors with Directionally Aligned Conjugated Polymer Film Deposited from Pre-Aggregated Solution. Chem. Mater. 2015, 27 (24), 8345-8353. (28) Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S. K.; Waghmare, U. V.; Novoselov, K. S.; Krishnamurthy, H. R.; Geim, A. K.; Ferrari, A. C.; Sood, A. K. Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor. Nat. Nanotechnol. 2008, 3 (4), 210-5. (29) Yan, J.; Zhang, Y.; Kim, P.; Pinczuk, A. Electric field effect tuning of electron-phonon coupling in graphene. Phys. Rev. Lett. 2007, 98 (16), 166802. (30) Lee, S. K.; Yang, J. W.; Kim, H. H.; Jo, S. B.; Kang, B.; Bong, H.; Lee, H. C.; Lee, G.; Kim, K. S.; Cho, K. Inverse transfer method using polymers with various functional groups for controllable graphene doping. ACS Nano 2014, 8 (8), 7968-7975. (31) Liang, S.-J.; Ang, L. K. Electron Thermionic Emission from Graphene and a Thermionic Energy Converter. Phys. Rev. Appl. 2015, 3 (1). (32) Yu, Y.-J.; Zhao, Y.; Ryu, S.; Brus, L. E.; Kim, K. S.; Kim, P. Tuning the Graphene Work Function by Electric Field Effect. Nano Lett. 2009, 9 (10), 3430-3434. (33) Chen, W.; Huang, H.; Thye, A.; Wee, S. Molecular orientation transition of organic thin films on graphite: the effect of intermolecular electrostatic and interfacial dispersion forces. Chem. Comm. 2008, (36), 4276-4278. (34) Götzen, J.; Käfer, D.; Wöll, C.; Witte, G. Growth and structure of pentacene films on graphite: Weak adhesion as a key for epitaxial film growth. Phys. Rev. B 2010, 81, 085440. (35) Venables, J. A.; Spiller, G. D. T.; Hanbucken, M. Nucleation and growth of thin films. Rep. Prog. Phys. 1984, 47 (4), 399.

22


Page 22 of 30

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(36) Pratontep, S.; Brinkmann, M.; Nüesch, F.; Zuppiroli, L. Correlated growth in ultrathin pentacene films on silicon oxide: Effect of deposition rate. Phys. Rev. B 2004, 69 (16), 165201. (37) Stadlober, B.; Haas, U.; Maresch, H.; Haase, A. Growth model of pentacene on inorganic and organic dielectrics based on scaling and rate-equation theory. Phys. Rev. B 2006, 74 (16), 165302. (38) Pratontep, S.; Nüesch, F.; Zuppiroli, L.; Brinkmann, M. Comparison between nucleation of pentacene monolayer islands on polymeric and inorganic substrates. Phys. Rev. B 2005, 72 (8) 085211. (39) Kim, K.; Santos, E. J.; Lee, T. H.; Nishi, Y.; Bao, Z. Epitaxially Grown Strained Pentacene Thin Film on Graphene Membrane. Small 2015, 11 (17), 2037-2043. (40) Wickenburg, S.; Lu, J.; Lischner, J.; Tsai, H. Z.; Omrani, A. A.; Riss, A.; Karrasch, C.; Bradley, A.; Jung, H. S.; Khajeh, R.; Wong, D.; Watanabe, K.; Taniguchi, T.; Zettl, A.; Neto, A. H.; Louie, S. G.; Crommie, M. F. Tuning charge and correlation effects for a single molecule on a graphene device. Nat. Commun. 2016, 7, 13553. (41) Seebauer, E. G.; Allen, C. E. Estimating surface diffusion coefficients. Prog. Surf. Sci. 1995, 49 (3), 265-330. (42) Zhang, Z. Atomistic Processes in the Early Stages of Thin-Film Growth. Science 1997, 276 (5311), 377-383. (43) Choudhary, D.; Clancy, P.; Shetty, R.; Escobedo, F. A Computational Study of the Submonolayer Growth of Pentacene. Adv. Funct. Mater. 2006, 16 (13), 1768-1775. (44) Jo, S. B.; Kim, H. H.; Lee, H.; Kang, B.; Lee, S.; Sim, M.; Kim, M.; Lee, W. H.; Cho, K. Boosting photon harvesting in organic solar cells with highly oriented molecular crystals via graphene–organic heterointerface. ACS Nano 2015, 9 (8), 8206-8219.

23


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(45) Singha Roy, S.; Bindl, D. J.; Arnold, M. S. Templating highly crystalline organic semiconductors using atomic membranes of graphene at the anode/organic interface. J. Phys. Chem. Lett. 2012, 3 (7), 873-878. (46) Zhang, L.; Roy, S. S.; Hamers, R. J.; Arnold, M. S.; Andrew, T. L. Molecular Orientation-Dependent Interfacial Energetics and Built-in Voltage Tuned by a Template Graphene Monolayer. J. Phys. Chem. C 2015, 119 (1), 45-54. (47) Lunt, R. R.; Benziger, J. B.; Forrest, S. R. Relationship between crystalline order and exciton diffusion length in molecular organic semiconductors. Adv. Mater. 2010, 22 (11), 1233-1236. (48) Lin, J. D. A.; Mikhnenko, O. V.; Chen, J.; Masri, Z.; Ruseckas, A.; Mikhailovsky, A.; Raab, R. P.; Liu, J.; Blom, P. W. M.; Loi, M. A.; García-Cervera, C. J.; Samuel, I. D. W.; Nguyen, T.-Q. Systematic study of exciton diffusion length in organic semiconductors by six experimental methods. Mater. Horiz. 2014, 1 (2), 280-285. (49) Wang, Y.; Torres, J. A.; Stieg, A. Z.; Jiang, S.; Yeung, M. T.; Rubin, Y.; Chaudhuri, S.; Duan, X.; Kaner, R. B. Graphene-assisted solution growth of vertically oriented organic semiconducting single crystals. ACS Nano 2015, 9 (10), 9486-9496.

24


Page 24 of 30

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 1 195x212mm (300 x 300 DPI)


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2 254x260mm (300 x 300 DPI)


Page 26 of 30

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 3 254x300mm (300 x 300 DPI)


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4 254x225mm (300 x 300 DPI)


Page 28 of 30

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 5 129x225mm (300 x 300 DPI)


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC 169x127mm (300 x 300 DPI)


Page 30 of 30

Electric-Field-Tunable Growth of Organic Semiconductor Crystals on

Recommend Documents