Phase Transition of Graphene-Templated Vertical Zinc

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Phase Transition of Graphene-Templated Vertical Zinc Phthalocyanine Nanopillars D. Leonardo Gonzalez Arellano, Edmund K. Burnett, Sema Demirci Uzun, Julia A Zakashansky, Victor K Champagne III, Michelle H George, Stefan C. B. Mannsfeld, and Alejandro L. Briseno J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03078 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Journal of the American Chemical Society

Phase Transition of Graphene-Templated Vertical Zinc Phthalocyanine Nanopillars D. Leonardo Gonzalez Arellano1, Edmund K. Burnett1, Sema Demirci Uzun1, Julia A. Zakashansky1, Victor K. Champagne III1, Michelle George1, Stefan C. B. Mannsfeld2, and Alejandro L. Briseno1,3*† 1. Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, MA, 01003, USA 2. Center for Advanced Electronics Dresden, Dresden University of Technology, Dresden, 01062, Germany 3. The Pennsylvania State University, Department of Chemistry, University Park, PA 16803 *Corresponding Author: [email protected] KEYWORDS: Crystallization, Nanopillars, Graphene, Phase transition, Chemiresistors.

ABSTRACT: We report on the graphene-assisted growth, crystallization, and phase transition of zinc phthalocyanine (ZnPc) vertically-oriented single crystal nanopillars. Post-crystallization thermal annealing of the nanostructures results in a molecular packing change while maintaining the vertical orientation of the single crystals orthogonal to the underlying substrate. Grazing incidence Xray diffraction and high resolution TEM studies characterized this phase transition from a metastable crystal phase to the more stable β-phase commonly observed in bulk crystals. These vertical arrays of crystalline nanopillars exhibit a high-surface-to-volume ratio, which is advantageous for applications such as gas sensors. We fabricated chemiresistor sensors with ZnPc nanopillars grown on graphene and demonstrated its selectivity for ammonia vapors, and improvement in sensitivity in the β-phase crystal packing pillars due to their molecular orientation increasing the exposure of the Zn2+ ion to the ammonia analyte. This work highlights the first morphology-retentive phase transition in organic single crystal nanopillars through simple post-processing thermal annealing. This study opens up the possibility of molecular packing control without large variations in morphology, a necessity for high performance devices and establishing structure-property relations.

INTRODUCTION Metal phthalocyanines (MPcs) are a benchmark class of organic semiconductors, having been extensively studied and utilized in a broad range of devices such as organic solar cells, 1-4 field effect transistors,5-6 light-emitting diodes, 7 and gas sensors.8 Furthermore, MPcs have excellent chemical stability and their crystal structure can be tailored by chemical modification,9 tuning substrate surface energy,10 and adjusting deposition temperature.11-14 Thin films deposited at room temperature typically assume a metastable α-polymorph packing, while a stable β-phase can be accessed through post deposition thermal annealing. 15 Vertically oriented organic nanowire arrays have been demonstrated have been proposed to be valuable nanostructures for the development of sensors,16-19 batteries,20 waveguides,21 photovoltaics,22 photodetectors,23 neuron pinning,24 bactericidal surfaces,25 or 1D heterostructures or organic/organic interfaces,26 furthermore, thanks to the templatedirected possibilities offered by graphene vertical nanostructures can be used to synthesize a wide range of structural motifs with adjustable properties.27 Here, we investigate the molecular packing of zinc phthalocyanine (ZnPc) vertically oriented nanopillars deposited at high temperatures onto gra-

phene. The graphene template orients the phthalocyanine molecules in a face-on configuration with a slip-stack parallel to each other. Upon annealing, the nanopillars undergo a phase transition akin to that observed on thin films from the new polymorph to the β-phase while preserving their vertically oriented morphology. Metal phthalocyanine thin films have been explored as gas sensors, 8, 28-32 determining that the reactivity to oxidizing 33-36 or reducing gases is dictated by the transition metal core. The advantage of the 3D nanostructured architectures relies in the role of surface area, molecular packing, crystallinity, and morphology in molecular detection,37-38 Here, our aim is to detect ammonia vapor, a significant contaminant in agricultural settings,39-40 and, a potent toxin that has deleterious effects on brain function in humans and animals.41-42

RESULTS AND DISCUSSION The deposition of zinc phthalocyanine (ZnPc) at room temperature on bare Si results in a thin film composed of polycrystalline grains approximately 40 nm in diameter, with a monoclinic packing characteristic of the α-ZnPc crystal phase.43-45 ZnPc undergoes a phase transition when the substrate is held at high temperatures during evaporation or with post-deposition thermal treatment above 320 ˚C, transforming

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the spherical grains into elongated crystallites parallel to the substrate; these crystallites are described as the β-phase polymorph of ZnPc.15 Both methods result in planar thin films. Our approach to growing vertically oriented ZnPc nanopillar arrays relies on the use of graphene and thermal evaporation with elevated substrate temperatures. A graphene coated substrate provides a favorable surface for ZnPc molecules to adopt a face-on configuration due to van der Waals and π-π interactions. We previously reported a new technique for the growth of these vertically-oriented single-crystalline nanopillars.46 First, an ultrathin crystalline film or “wetting layer” forms at slow deposition rates, followed by the evolution of a straininduced 3D nucleation at higher deposition rates and high substrate temperature, further deposition yields a network of vertical nanopillars described by the Stranski-Krastanov growth model. The schematic in Figure 1A illustrates the vertical physical vapor transport (vPVT) apparatus used in our study. The fur-

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nace consists of a vacuum-sealed glass tube that consists of two zones; 1) a hot sublimation zone, and 2) a cooler recrystallization zone. The temperature profile is designed to decrease gradually as the sublimed material moves towards the deposition zone to induce crystallization. The graphene-coated substrate was placed approximately 5 cm above the source material, facing the molecular beam, and the system was evacuated to a base pressure of 10-2 mbar during the five-minute deposition process, following the procedure detailed in our previous work.46 The schematic representation of the nanopillars obtained in Figure 1B is characterized by a high density of single crystalline ZnPc nanopillars (ZnPc-NP). Post-deposition annealing of the ZnPc-NP at 400 °C in a tube furnace under a nitrogen atmosphere for 10 minutes resulted in ZnPc crystals that remain vertically oriented yet increase in length and diameter while reducing nanopillar density as depicted in the right panel of Figure 1B.

Figure 1. Molecular evaporation setup for the deposition of ZnPc nanopillars on graphene, A) Schematic of the vPVT deposition system, the vacuum chamber consists of two zones, sublimation (450 °C), deposition and cooling (350 °C), B) schematic representation of ZnPc nanopillars as deposited with a density of 60 pillars/µ2 and after heat anneal treatment at 400 °C with a pillar density of 4 pillars/µ2. Insets show the molecular packing with respect to the substrate before and after annealing.

Field-emission scanning electron microscopy (SEM) was used to characterize the morphological transformation of the ZnPc-NPs upon thermal treatment. Figure 2A depicts the SEM image of as-deposited vertical crystals; the nanopillars are densely packed and with cross section areas of 120 nm2. The average nanopillar length is 1.46 µm ± 200 nm, with a density of roughly 61 ± 10 pillars/µm2. Post-deposition annealing at 400 °C in a nitrogen atmosphere causes the nanopillar network to undergo changes in morphology, shown in Figure 2B. Notably, the nanopillar arrays maintain their vertical orientation, however, the nanopillars present a coarser crosssection composed of multiple facets that grow from the lateral faces of the crystal, as shown in the inset in Figure 2B. The average ZnPc-NP length increases to 3 µm ± 300 nm with the pillar density drastically decreasing to approximately 4 ± 1 pillars/µm2. The vPVT deposition yields metastable ZnPc vertical crystals on graphene, this metastable phase forms a densely populated network of nanopillars as a result of the isotropic molecular vapor pressure near the substrate, the ver-

tical chemical gradient, and supersaturation.46 After annealing, the nanocrystals undergo morphological and density changes similar to the cluster coarsening observed in inorganic materials such as ZnO and Au after thermal annealing.47-49 This phenomenon known as Ostwald ripening is described by a thermodynamic driving force based on the Gibbs-Thomson equation. In the ripening process mechanism, the metastable phase supersaturation is depleted while the stable phase grows at the expense of the less stable phase.50-52 Thari et al., have described how competition between two crystal phases can induce the total dissolution of one of them in favor of the other for polymorphic systems such as L-glutamic acid (LGlu).53 In the phthalocyanine system, this competition of crystal phases is in agreement with reports by Yim et al., who showed a phase transition from the α→ β phase for phthalocyanine thin films annealed above 300 °C.54 Therefore, we can conclude that the sharp decrease in crystallite density and a significant coarsening of the crystal surface in the ZnPc-NP is indicative of the Ostwald ripening process.


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Figure 2. A) SEM image of ZnPc vertical crystals as deposited on graphene, and B) after annealing at 400 °C for 10 min in N2 environment. The insets show the detailed morphology of the single nanopillars, scale bar 500 nm.

Grazing-incidence X-ray diffraction (GIXD) measurements were carried out on the ZnPc-NP before and after thermal annealing to elucidate the molecular packing changes. The ZnPcNP diffraction pattern (Figure 3A) shows the as-deposited nanopillars are highly ordered and by measuring the diffraction peak positions, the unit cell parameters were extracted using a least-square-error fitting protocol.55-56 The unit cell of the as-deposited film shows it adopts a triclinic space group P1 with a square packing with Z: 0, Z': 0 and cell parameters of: a=13.7 Å, b=13.7 Å, c=3.8 Å, α: 118.8°, β: 108.4°, and γ: 81.1°. This atypical square unit cell has been observed in 2D ZnPc films deposited on Au surfaces measured by scanning tunneling microscopy.57 Similarly, when ZnPc molecules are deposited on graphene, they lie parallel to the substrate due to strong molecule/substrate interaction.46, 58-60 The molecular packing demonstrated a 1D “out of plane” π-stacking. The post-deposition thermally annealed ZnPc-NP display a significantly different diffraction pattern. The scattering peaks are present at different q values, indicating a change in the unit cell and molecular packing. The large angular arching of the peaks is indicative of an increase in azimuthal misorientation and a change in crystalline texture. Using the same fitting protocol, the unit cell extracted shows a transformation to a monoclinic P 21/n space group with Z: 2 Z': 0. The new lattice constants are: a=14.5 Å, b=4.84 Å, c=17.12 Å, α: 90°, β: 106.3°, γ: 90°, notably similar to the herringbone 1D packing

β-phase polymorph of ZnPc single crystals 14, 61 as shown in Figure 3B. In Table 1, the experimental lattice constants obtained from the as deposited and annealed nanopillars are summarized with α- and β- ZnPc polymorphs from 2D thin films shown for comparison. Figure 3C and D shows the molecular packing of the asdeposited nanopillars viewed down the [001] and [100] direction, respectively. The unit cell parameters are indicated, as well as the out-of-plane distance between molecules of 3.2 Å and the tilt angle between the plane of the molecule and the stacking axis of 28.7°, reciprocal of the α angle of the unit cell and the origin of the tilt in the nanopillars on the substrate. The top view of two stacked molecules in Figure 3E reveal that the two metallic centers are offset by 1.75 Å. Upon annealing, the crystal packing changes to fit a second molecule in the unit cell (Figure 3F), the out-of-plane stacking distance decreases to 2.86 Å, while the tilt angle between the plane of the molecule and the stacking axis becomes 10.8°, as shown in Figure 3G. The top view of two stacked molecules in the post-annealed crystal packing exhibits a much larger distance between the Zn2+ ion centers of 3.85 Å, shown in Figure 3H. The crystal packing obtained after thermal treatment closely resembles the reported values of ZnPc films in the β-phase, while it maintains its vertical orientation with respect to the substrate.



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Figure 3. A) GIXD pattern of as-deposited ZnPc-NP (triclinic), and B) 400 °C annealed ZnPc-NP (monoclinic) grown on Si/graphene substrates. C-E) Schematic of the crystal structure of ZnPc-NP on graphene as deposited projected on the (100) plane. F-H) Schematic of the crystal structure of ZnPc-NP on graphene annealed at 400 °C projected on the (001) plane.

Table I. Unit cell parameters of α and β phase ZnPc in films and nanopillars.

Parameter

a [Å]

b [Å]

c [Å]

α [°]

β [°]

26

-

24

-

93

19.3

4.9

14.6

-

120.5

ZnPc-NP as-deposited

13.7

13.7

3.8

118.8°

108.4°

81.1°

ZnPc-NP @400 °C

14.5

4.84

17.12

90°

106.3°

90°

ZnPc film α-phase a ZnPc film β-phase

a, b

γ [°]

a See Ref. 14 b See Ref. 57

To corroborate the GIXD results we conducted electron diffraction studies and crystallographic packing simulations.

Figure 4 shows the bright field transmission electron microscopy (TEM) image and corresponding selected area electron


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diffraction (SAED) of the ZnPc single-crystals before and after annealing. In Figure 4A, the TEM electron diffraction pattern (i.e. reciprocal lattice points) corresponds well to the

ZnPc’s triclinic unit cell when viewing down the (010) basal plane. The inset depicts the ZnPc single-crystal growth morphology as predicted using the

Figure 4. Left; Bright field TEM images of a ZnPc nanocrystal grown on graphene, the inset shows the selected area of diffraction (SAED) pattern of a single crystal, and the Bravais-Friedel-Donnay-Harker (BFDH) growth morphology prediction. Yellow boxes highlight the unit cell and the rotated correspondent in reciprocal space. Right; High resolution TEM (HRTEM) of the single crystal, inset shows the fast Fourier transform (FFT) filtered correction showing the lattice spacing of the crystal. A,B) As deposited, C,D) Annealed at 400 °C.

Bravais-Friedel-Donnay-Harker (BFDH) method.58 The theoretically predicted growth along the (001) axis agrees well with the electron diffraction data. In other words, one can see that the reciprocal unit cell resembles the rotated real space triclinic unit cell as highlighted in the yellow boxes. The dspacing of the (100) plane was determined to be 12.57 Å from the high-resolution TEM (HRTEM) image in Figure 4B. The bright field TEM image in Figure 4C displays a single crystal of ZnPc after annealing. The predicted BFDH morphology and the SAED appear as insets, the yellow box is indicative of the real space monoclinic unit cell rotated in reciprocal space. The d-spacing for the (101) reflection was defined as 9.27 Å from the HRTEM image in Figure 4D, in close agreement with the simulated XRD data d(101)=9.4 Å at 2θ=9.5°. Thus, we confirm that the molecular packing of the nanopillar is nearly identical to the bulk β-phase crystal structure, with the crystal growth axis corresponding to the [010] direction as indicated in the BFDH inset.

The nanoarchitecture described so far provides an excellent platform for gas sensing applications where high surface area is advantageous. Conductivity changes after gas exposure for metal phthalocyanine films has been attributed to molecules absorbed only at the air/MPc interface and at grain boundaries because the crystal structure is tightly packed and remains unchanged during gas exposure. 59-61 We investigated the gas sensing characteristics of the ZnPc-NP by fabricating chemiresistor devices consisting of glass slides with two gold electrodes 0.5 cm in width and 1 cm in length separated by a 500 µm channel gap. Graphene was transferred to the substrate on top of the electrodes followed by ZnPc deposition. The sensor was placed inside a sealed 400 mL headspace jar. A baseline was established for 5 min in an air atmosphere followed by a 120 µL injection of NH3 vapor, and the percent change in resistance, ∆R/R x 100 % (calculated as Rf-Ro/Ro x 100), was measured as a function of time. We compared a thermally evaporated two-dimensional (2D) 20 nm ZnPc thin film, to a device fabricated with ZnPc-NP of approximately 1.5 µm in



length. After 30 s of exposure, we observed that the 2D thin film had a minimal resistance increase in the presence of NH3 vapor with a ∆R/R of 120 %, in contrast the device with ZnPcNP exhibited a ∆R/R of 1235 % upon NH3 vapor exposure, these results agree with our estimation of surface area of 1000 times increase with the nanopillar system compared to a 2D film, highlighting the importance of the increase in surface area obtained with this vertically oriented structure for gas sensor applications. To test the sensor recovery and reversibility we subjected the ZnPc-NP device to a cyclic exposure and extraction of ammonia vapor and obtained a reproducible response over a period of 30 minutes, as shown in Figure 5A. The process consists of 5-minute cycles that correspond to the injection of 120 µL of ammonia vapor and subsequent extraction of the vapor with a vacuum pump. The black open squares represent the cyclic response of the as-deposited ZnPc-NP and show no detrimental changes as a function of test cycles, demonstrating good sensor reproducibility. Interestingly, after thermally annealing the ZnPc-NPs, in addition to their morphology and phase changes, a further increase in sensitivity is observed to yield a ∆R/R 300 % higher than the as-deposited ZnPc-NP system. The selectivity of the devices is also enhanced upon thermal annealing. Exposure to multiple organic vapors in the as deposited sensors resulted in negligible or failed responses, yet after annealing, the sensors increase their sensitivity to the tested analytes. ZnPc-NP show selective NH3 detection compared to a range of organic analytes, as shown in Figure 5B. The calibration curve shown in Figure 5C, corresponds to the signal obtained for annealed devices which reach a detection limit of 4 ppm. The resistance change in the ZnPc-NP devices is attributed to the interaction of the electron-donating vapor, ammonia, with the Zn2+ metallic core, as the metal coordinative bonds are the strongest intermolecular binding force for adsorption of the ammonia molecule onto the ZnPc. 31 The resistance increases due to the hole trapping within the p-type ZnPc-NP by electrons donated from the chemisorbed analyte. 62 We rationalize the modest sensor response of the native ZnPc-NP array to be due to the unfavorable molecular arrangement for analyte diffusion. From the crystal structures described in Figure 3 we observe a tight packing arrangement for the as-deposited crystals, particularly with respect to the metallic center of the molecule where the overlap distance is of only 1.75 Å compared to 3.85 Å after thermal annealing. As we noted earlier the unit cell of ZnPc-NP after the phase transition grows in volume from 591.98 to 1153.61 Å3. The combination of this volumetric expansion and a higher tilt of the molecules with respect to the substrate plane offers an easier path for analyte molecules diffusion and interaction with the metallic center of the phthalocyanine, thus increasing the overall sensitivity of the device.

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Figure 5. A) Cyclic response and recovery of ZnPc vertical crystals as deposited and after thermal treatment on exposure to 1000 ppm of NH3, the blue arrows indicate the moment NH3 is injected into the system and red arrows indicate the moment when the vacuum pump is activated to remove the analyte. B) Selectivity of ZnPc nanopillar sensors after thermal treatment, C) calibration curve of ZnPc vertical crystals.

CONCLUSION We show that ZnPc nanopillars grown on graphene can undergo a phase transition while maintaining their vertical morphology. The resulting crystal structure differs from common polymorphs obtained in thin films and bulk crystals, demonstrating an atypical metastable square close-packed crystallographic structure that grows orthogonal to the substrate. A phase transition can be induced to the more energetically stable β -phase polymorph upon annealing the nanostructures at 400 °C. Interestingly, the β -crystals remain vertically oriented


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with respect to the substrate maintaining their nanopillar morphology. The ZnPc nanopillar networks show a change in resistance when exposed to NH3 vapor due to a charge transfer interaction between the metallic core of the phthalocyanine and the electron-donating gas. In terms of sensitivity of ZnPc, the high surface area architecture of the ZnPc-NP offers advantages over a 2D thin film, and can be further enhanced by annealing the nanopillars, this is a result of a change in the crystal packing of the metastable polymorph resulting in vertically oriented β -phase crystals which yield an increase in sensitivity of 300%. The development of high performance vertically oriented nanocrystals has application in a broad range of devices, such as vertical transistors and devices that require high surface area.

Materials & Methods

Materials. All compounds were used as received. Zinc phthalocyanine dye content 97%, and poly(methyl methacrylate) average Mw ~120,000 were purchased from SigmaAldrich (St. Louis, MO). Copper foil (0.025 mm thick) Puratronic® (99.999%) was acquired from Alfa Aesar. Silicon (Si) [100] (boron doped, resistivity

Phase Transition of Graphene-Templated Vertical Zinc

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