Seed-Initiated Anisotropic Growth of Unidirectional Armchair

Jan 30, 2018 - These results show that seed-initiated graphene synthesis on Ge(001) will be a viable route for creating wafer-scale arrays of narrow, ...
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Letter Cite This: Nano Lett. 2018, 18, 898−906

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Seed-Initiated Anisotropic Growth of Unidirectional Armchair Graphene Nanoribbon Arrays on Germanium Austin J. Way, Robert M. Jacobberger, and Michael S. Arnold* Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: It was recently discovered that the chemical vapor deposition (CVD) of CH4 on Ge(001) can directly yield long, narrow, semiconducting nanoribbons of graphene with smooth armchair edges. These nanoribbons have exceptional charge transport properties compared with nanoribbons grown by other methods. However, the nanoribbons nucleate at random locations and at random times, problematically giving rise to width and bandgap polydispersity, and the mechanisms that drive the anisotropic crystal growth that produces the nanoribbons are not understood. Here, we study and engineer the seed-initiated growth of graphene nanoribbons on Ge(001). The use of seeds decouples nucleation and growth, controls where growth occurs, and allows graphene to grow with lattice orientations that do not spontaneously form without seeds. We discover that when the armchair direction (i.e., parallel to CC bonds) of the seeds is aligned with the Ge⟨110⟩ family of directions, the growth anisotropy is maximized, resulting in the formation of nanoribbons with high-aspect ratios. In contrast, increasing misorientation from Ge⟨110⟩ yields decreasingly anisotropic crystals. Measured growth rate data are used to generate a construction analogous to a kinetic Wulff plot that quantitatively predicts the shape of graphene crystals on Ge(001). This knowledge is employed to fabricate regularly spaced, unidirectional arrays of nanoribbons and to significantly improve their uniformity. These results show that seed-initiated graphene synthesis on Ge(001) will be a viable route for creating wafer-scale arrays of narrow, semiconducting, armchair nanoribbons with rationally controlled placement and alignment for a wide range of semiconductor electronics technologies, provided that dense arrays of sub-10 nm seeds can be uniformly fabricated in the future. KEYWORDS: Armchair graphene nanoribbon, chemical vapor deposition, seed, array, polydispersity, germanium

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application of nanoribbons have been hindered by difficulties in synthesizing or producing nanoribbons with a high degree of structural precision and in controlling the organization of nanoribbons into large-area arrays. Top-down techniques in which nanoribbons are lithographically patterned and etched from continuous sheets of graphene offer the advantage of precise control over the placement and alignment of every nanoribbon within an array.20,21 However, due to the low resolution and bluntness of the lithography and etching tools that are used for patterning, top-down methods result in relatively wide nanoribbons with rough, defective edges, which degrades the excellent charge20−25 and thermal10,26 transport properties that are expected in nanoribbons with smooth, pristine edges. Bottom-up techniques can overcome these shortcomings to yield nanoribbons with sub-10 nm widths and smooth edges. For example, the unzipping of graphite27 and nanotubes28,29 can result in narrow nanoribbons with nearly atomically smooth edges, but these methods do not provide control over the

raphene nanoribbons that are narrower than 10 nm and that have well-defined armchair edges (i.e., the long edges of the nanoribbons are parallel to CC bonds) can be semiconducting with a bandgap that is sufficiently large for semiconductor electronics. The bandgap of armchair nanoribbons depends on their width family, which is classified as either n = 3p, 3p + 1, or 3p + 2, where n is the number of carbon atoms along the width of the ribbon and p is an integer.1 Theory predicts that all armchair ribbons are semiconducting with the largest and smallest bandgaps belonging to the 3p + 1 and 3p + 2 families, respectively.1 Experimental measurements of the bandgap of armchair ribbons have resulted in a wide range of values, which do not always match that predicted by theory; but precise determination of the bandgap can be complicated due to charge screening and surface states from the substrate on which the ribbons are supported.2−6 In contrast, unpatterned, two-dimensional graphene is a semimetal that does not have a bandgap.1 Nanoribbons can exhibit high charge carrier mobility,7 carrier velocity,8 current carrying capacity,9 and electrical8 and thermal10 conductivity and thus are promising candidates for applications including logic gates, high-frequency communication devices, optoelectronics, photonics, and sensors.8,11−19 However, the research and © 2018 American Chemical Society

Received: October 3, 2017 Revised: January 23, 2018 Published: January 30, 2018 898

DOI: 10.1021/acs.nanolett.7b04240 Nano Lett. 2018, 18, 898−906

Letter

Nano Letters

family of Ge⟨110⟩ directions in nanoribbons, versus 12.2° in crystals with low-aspect ratio. Thus, it is likely that the relative orientation between the graphene lattice and Ge⟨110⟩ dictates the growth anisotropy on Ge(001) to some extent. Controlling this relative orientation may be the key to realizing nanoribbons with high-aspect ratio and unidirectional alignment. While the crystal shape evolution of graphene on Ge(001) is not yet understood, the formation of graphene crystals with particular shapes on metal surfaces has been relatively extensively studied. On metal surfaces, the shape of graphene crystals can be predicted by using the kinetic Wulff construction,48 which characterizes the edge growth velocity as a function of edge orientation. The edge growth velocity is affected by factors including the energetic barriers for the attachment of carbon or hydrocarbon intermediates to specific active sites (e.g., armchair edges, zigzag edges, or kinks), the concentration of active sites, and the excess chemical potential.48 One practical implication is that the shape of graphene crystals evolve until they are bound by edges with the slowest growth velocity on a given surface. For example, graphene growth on Cu with high H2/CH4 flux ratio results in crystals that are regular hexagons with zigzag edges,49,50 which are the slowest growing edges.48 The shape evolution of dendritic graphene crystals on Cu foil can also be empirically described by the dependence of growth velocity on edge orientation.51 Furthermore, it is known that the growth velocity is heavily influenced by interactions between the edges of the graphene crystal and the terrace and step structure of the underlying substrate.52,53 As a result of these interactions, the symmetry of graphene can be broken by the substrate to drive the formation of crystal shapes with reduced symmetry, such as elongated hexagons, triangles, or rhombi. Using seeds to initiate growth may be a promising technique to understand and control the anisotropic synthesis of graphene nanoribbons on Ge(001). Low-aspect ratio, micron-scale graphene crystals with controlled placement have previously been grown on Cu foil from large seeds of multilayer graphene49 or poly(methyl methacrylate) (PMMA)54 with diameters of roughly 500−1000 nm. However, these relatively large, thick seeds are not suitable for the growth of nanoribbons with sub-10 nm widths and precise edge structures. The growth of hexagonal boron nitride (h-BN) has been seeded from the edges of large graphene crystals on Cu foil.55 Interestingly, the lattice orientation of h-BN is determined by the crystallography of the graphene from which the h-BN is grown and not by the crystallography of the Cu substrate. Thus, by controlling the orientation of graphene seeds on Ge(001), it may be possible for graphene to grow with lattice orientations that are not typically observed without the use of seeds. Furthermore, the seed-initiated growth of inorganic nanoparticles and nanowires using solution and vapor phase techniques has reduced polydispersity by initiating growth at nearly the same time and by minimizing secondary nucleation.56−63 In this work, we study and engineer the seed-initiated growth of graphene nanoribbons on Ge(001). The growth of graphene is seeded from the edges of small, circular graphene crystals. The use of seeds decouples nucleation and growth, allows graphene to grow with lattice orientations that do not otherwise form without seeds, and controls where growth occurs. First, we intentionally rotate the lattice orientation of the seeds with respect to the Ge(001) surface and characterize the growth rate and aspect ratio of the resulting graphene crystals. We discover that when the armchair direction of the seeds is aligned with

ribbon placement, alignment, or edge orientation and suffer from poor yield. Alternatively, nanoribbons with atomically precise width and edge structure can be synthesized via polymerization followed by cyclodehydrogenation3,30−34 but these techniques currently yield short ribbons (typically 20 nm in length) and do not yet offer precise control over the placement of ribbons or the formation of ribbon arrays. We recently discovered a scalable, bottom-up synthesis in which narrow, armchair graphene nanoribbons can be grown directly on Ge(001) substrates via chemical vapor deposition (CVD).35 In this synthesis, CH4 is decomposed on Ge(001) at ∼910 °C in a flow of Ar and H2, driving the stochastic nucleation and highly anisotropic growth of nanoribbons that are aligned roughly along either the Ge[110] or Ge[110̅ ] direction. The nanoribbons start as atomic-scale nuclei that form from the decomposition of CH4, and then grow slowly in width and relatively quickly in length, resulting in ribbons with aspect ratios as high as 70. This highly anisotropic growth is unique to Ge(001), and does not occur on Ge(111) nor Ge(110).35 Scanning tunneling microscopy (STM) has shown that nanoribbons as narrow as 2 nm can be grown via this approach and that their edges consist of relatively smooth armchair segments with roughness that varies by only 1−2 lattice constants of graphene over edge lengths of tens of nanometers.35,36 These nanoribbons have also exhibited exceptional charge transport properties in field-effect transistors (FETs) compared with nanoribbons grown by other methods, for example simultaneously demonstrating an on/off conductance ratio of 2 × 104 and an on-state conductance of 5 μS.37 The large-area transfer of these ribbons from the Ge(001) surface onto insulating substrates, such as SiO2, HfO2, and Si3N4, has also been demonstrated using both dry37 and wet (see Supporting Information) transfer techniques. Using CVD, graphene nanoribbons have also been grown on surface features38−40 from the edges of h-BN crystals41,42 and on patterned catalysts in which growth is confined to predetermined areas that define the ribbon dimensions.43−47 However, none of these methods yield sub-10 nm armchair nanoribbons. Thus far, it has not been possible to harness this anisotropic growth mechanism on Ge(001) to (1) control the placement of the nanoribbons into arrays, (2) achieve unidirectional alignment of the nanoribbons, or (3) initiate the growth of all of the nanoribbons at nearly the same time to reduce their polydispersity. Specifically, the nanoribbons nucleate at random locations; they grow so that they are aligned along either Ge[110] or Ge[110̅ ]; and they likely nucleate over time, contributing to polydispersity in their widths and lengths (e.g., the measured mean-normalized standard deviation in width ranges from 36 to 60%, depending on the growth conditions).35,36 Another complication is that whereas most graphene crystals (∼90%) on Ge(001) evolve into nanoribbons with aspect ratio >10, some crystals (∼10%) evolve into more compact parallelograms with aspect ratio 30), as previously observed for nanoribbons grown without seeds.35 Interestingly, for anneal times >45 min, seeds with θseed lattice > 3° are completely etched in almost all cases, suggesting that seeds with these orientations are less stable on Ge(001). This is consistent with the observation that under conditions in which seeds are not used, >90% of the crystals form with θcrystal edge < 3°. The resistance to etching and enhanced stability of graphene crystal crystals with θseed lattice and θedge < 3° and the anisotropic growth of these crystals could potentially all stem from the same mechanism, arising from strong interactions between the ribbon edges and the underlying Ge atoms when θseed lattice < 3°. For example, particularly strong chemical binding has previously been observed between the graphene edges and steps on the Ge(110) surface.53 The second method we use to decrease the seed size is to directly pattern smaller features using lithography. This route is

ratio and to unidirectionally control their orientation along either Ge[110] or Ge[11̅0]. While ribbons oriented both +2.8° and −2.8° from Ge[110] are observed during unseeded growth, here the spread in θcrystal edge is much smaller than 2 × 2.8° = 5.6° (and is for example only ±1° for ribbons grown from seeds with θseed lattice = 2° in any given array, which is similar to our error of measurement). This narrow spread indicates that only one orientation (either + or −2.8° from the nearest Ge⟨110⟩) preferentially evolves. However, the aspect ratio that can be realized via seeding is currently limited by the relatively large diameter of the seeds. Next, we explore the effect of the seed diameter on the nanoribbon growth. In order to reduce the seed diameter, we use two approaches: (1) etch the graphene seeds via annealing and (2) pattern smaller seeds with electron-beam lithography, which is enabled by using nickel (Ni) as an etch mask. First, we etch the seeds to decrease their size and then study the resulting nanoribbon growth. For these experiments, seeds with a diameter of ∼55 nm are fabricated with a θseed lattice of roughly 2° or 3°. The samples are then annealed at 910 °C for 30, 45, 60, and 90 min with a flow of 200 sccm of Ar and 100 sccm of H2 to etch the seeds from their edges. The graphene seeds become smaller as the anneal time increases, which is likely due to etching of the seed edges by hydrogen69,70 or oxidizing impurities in the gas feedstock.71 For example the average diameter of seeds that are 84 nm after patterning decreases to 74 nm after etching for 30 min (Supporting Information Figure S1). The size and shape of the seeds can no longer be well resolved via SEM with further increases in anneal time, but it is likely that the seeds continue to decrease in diameter. After annealing, growth is then conducted at 910 °C for 1.5 h with a 903

DOI: 10.1021/acs.nanolett.7b04240 Nano Lett. 2018, 18, 898−906

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used to drive growth on samples with seeds with diameter of ∼70 nm, whereas a flow of 210 sccm of Ar, 90 sccm of H2, and 2.0 sccm of CH4 at 910 °C for 510 min is used on samples without seeds to initiate nucleation and growth. Both approaches yield ribbons with similar mean widths of 94 and 73 nm, respectively. However, the standard deviation (σ) divided by the mean (μ) is reduced from 47% (Figure 5a) to 18% (Figure 5c) by initiating growth from seeds, indicating that the ribbons become more homogeneous. The length polydispersity also significantly improves from 29% (Figure 5b) to 13% (Figure 5d) as a result of seeding. This improvement indicates that much of the variation in ribbon width and length previously observed without seeds35 can be ascribed to the nucleation of ribbons over time (as opposed to variation in growth velocity), whereas seed-initiated growth significantly reduces polydispersity by providing nuclei from which graphene crystals can start to grow nearly simultaneously and by minimizing this secondary nucleation. The width polydispersity that still remains after seed-initiated growth can moreover be largely attributed to variation in the size of the graphene seeds. After annealing the graphene seeds for 30 min, the diameter of the seeds varies with a σ/μ of 8.8% (Supporting Information Figure S1), potentially as a result of imprecision in electron-beam lithography and reactive ion etching processes used to pattern the seeds and nonuniform etching during pregrowth annealing. Therefore, it should be possible to reduce the nanoribbon polydispersity even further by using seeds that are more uniform in size and shape. In conclusion, we demonstrate that seed-initiated growth can be used to understand and control the growth of graphene on Ge(001). Graphene crystals grow anisotropically on Ge(001) with a velocity that strongly depends on the orientation of each armchair edge with respect to Ge⟨110⟩. When one of the three sets of armchair directions of the seed lattice is roughly aligned with Ge[110] or Ge[110̅ ], the crystal shape anisotropy is maximized, driving the growth of nanoribbons with their long edges aligned along Ge[110] or Ge[11̅0], respectively. Ribbons grown without seeds nucleate at random locations and at random times, and problematically adopt two orientations. In contrast, seed-initiated growth enables the lithographically controlled placement of individual nanoribbons and the fabrication of unidirectionally aligned nanoribbon arrays and occurs under conditions in which the secondary nucleation of randomly positioned nanoribbons is greatly diminished. Moreover, seed-initiated synthesis significantly reduces polydispersity, leading to ribbons with more uniform widths and lengths by providing nuclei from which each graphene crystal can begin to grow at nearly the same. The area over which the seeding and unidirectional alignment of nanoribbons can be achieved using the methods outlined, here, is limited only by the area of the graphene single-crystals from which the seeds are patterned (roughly 300 μm2 in this study). In the future, it should be possible to realize seeding and unidirectional alignment on the wafer-scale by patterning seeds from graphene that is grown on a substrate on which it nucleates with one orientation, such as Cu(111)73,74 or from graphene that is grown from a single nucleation center.75 In order to achieve nanoribbons narrower than 10 nm, and therefore nanoribbons with technologically relevant bandgaps, approaches for fabricating uniform, sub-10 nm seeds with nearly monodisperse diameter will also be needed. Provided that dense arrays of sub-10 nm seeds can be uniformly fabricated in the future, our results show that seed-initiated

preferred over etching the seeds via annealing, as etching likely increases the polydispersity in the seed size before ribbon growth is initiated. Using Al dots as etch masks, the diameter of the seeds after patterning is limited to ∼50 nm due to the relatively poor adhesion of Al to graphene during the lift-off process, as described above. In order to improve the adhesion between the mask and the graphene, we instead use Ni as an etch mask, which is expected to have stronger adhesion to graphene than Al.72 The fabrication of graphene seeds using Ni etch masks follows the same procedure as provided above for Al etch masks, except the Ni dots are deposited with directcurrent magnetron sputtering and are etched in dilute aqua regia (see Supporting Information). The use of Ni etch masks enables direct patterning of seeds as small as ∼20 nm. With this approach, we vary the seed diameter from 20 to 60 nm and study its effect on the resulting ribbon width using an anneal time of 30 min and a growth time of 90 min (Figure 4e). We find that the nanoribbon width decreases with seed size, as expected, and nanoribbons with widths as low as 11.1 ± 3.7 nm can be achieved using a seed diameter of 19.2 nm (Figure 4e). The ribbon width can be narrower than the diameter of the aspatterned seeds due to etching of the seeds before growth via annealing, as discussed above and shown in Figure 4a−d. The SEM image in Figure 4f shows a representative ribbon array with an average width of 12.8 ± 5.8 nm and an average aspect ratio of 17.5 ± 7.2, which is obtained using an anneal time of 45 min and a growth time of 360 min. These results indicate that smaller seeds can be achieved in the future by employing more advanced lithographic approaches (e.g., by further increasing the adhesion of the mask to graphene) and that such smaller seeds can be used to realize even narrower nanoribbons. The polydispersity in nanoribbons grown with seeds (θseed lattice ≤ 3°) and without seeds is compared in Figure 5 and Supporting Information Table S1. A flow of 200 sccm of Ar, 100 sccm of H2, and 2.0 sccm of CH4 at 910 °C for 90 min is

Figure 5. (a−d) Histograms of width (a,c) and length (b,d) for nanoribbons grown without (a,b blue bars) and with (c,d red bars) graphene seeds. The width and length of the ribbons grown without seeds is 73 ± 34 nm (a) and 1126 ± 329 nm (b), respectively, and with seeds is 94 ± 17 nm (c) and 417 ± 53 nm (d), respectively. The values of σ/μ are provided in each panel. 904

DOI: 10.1021/acs.nanolett.7b04240 Nano Lett. 2018, 18, 898−906

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graphene synthesis on Ge(001) should be a viable route for creating wafer-scale arrays of narrow, semiconducting, armchair nanoribbons with rationally controlled placement and alignment. These wafer-scale nanoribbon arrays promise to be of interest for a wide range of semiconductor electronics technologies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b04240. Includes experimental methods, histograms of the graphene seed size before and after annealing, transmission electron microscopy and selected-area electron diffraction data, characterization of yield of ribbons achieved with seed-initiated growth, and comparison of σ/μ for growth with and without seeds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Austin J. Way: 0000-0002-7418-8590 Robert M. Jacobberger: 0000-0001-5947-5308 Michael S. Arnold: 0000-0002-2946-5480 Author Contributions

A.J.W. fabricated samples and performed the experiments and data processing. R.M.J. performed the TEM characterization. M.S.A. supervised the work. All authors contributed to data interpretation. A.J.W. and R.M.J. drafted the manuscript, and all authors discussed and revised it. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DESC0016007. A.J.W. also acknowledges support from a National Science Foundation Graduate Research Fellowship under Grant DGE-1747503. We also thank Vivek Saraswat for depositing the Ni etch masks via sputtering.

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DOI: 10.1021/acs.nanolett.7b04240 Nano Lett. 2018, 18, 898−906

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DOI: 10.1021/acs.nanolett.7b04240 Nano Lett. 2018, 18, 898−906