Seed-Initiated Anisotropic Growth of Unidirectional Armchair

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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 Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04240 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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Seed-Initiated Anisotropic Growth of Unidirectional Armchair Graphene Nanoribbon Arrays on Germanium

Austin J. Way, Robert M. Jacobberger, Michael S. Arnold*

Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States

* E-mail: [email protected]

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 1 ACS Paragon Plus Environment

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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

Graphene 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, 1119

However, the research and 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 2 ACS Paragon Plus Environment

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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 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 cyclodehydrogenation,3, 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 Ge110 or Ge110 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 2x104 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, 3 ACS Paragon Plus Environment

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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 features,38-40 from the edges of h-BN crystals,41, 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 Ge110 or Ge110; 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 < 2, which is problematic because these crystals with lower aspect ratios do not have a bandgap due to their large widths. The ability to overcome these challenges is limited by a lack of understanding of the factors governing graphene synthesis on Ge(001). The appearance of crystals with low-aspect ratio and nanoribbons with two nearly perpendicularly-aligned orientations may be related to the crystallographic orientation of the graphene nuclei. One clue is the fact that the armchair direction of the graphene lattice is rotated by 2.8° from the family of Ge〈110〉 directions in nanoribbons, versus 13.2° 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 4 ACS Paragon Plus Environment

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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 5 ACS Paragon Plus Environment

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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 Ge〈110〉, the growth anisotropy is maximized, resulting in the formation of ribbons 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 dependence of crystal shape on the orientation of the graphene lattice with respect to Ge(001). This knowledge is then employed to fabricate regularly-spaced, unidirectional arrays of nanoribbons and to significantly improve their uniformity in width and length. Rationally-designed arrays of circular seeds of monolayer graphene are fabricated on Ge(001) following the procedure depicted in Figure 1 and described in detail in the Supporting Information. Briefly, to create the seeds, graphene with incomplete monolayer coverage is first grown on Cu foil via atmospheric-pressure CVD,64 resulting in hexagonal crystals of graphene that are roughly 20 µm in extent (Figure 1a). These hexagonal crystals are chosen as a basis for patterning seeds because they have zigzag edges,49 providing a frame of reference for controlling and quickly determining the lattice orientation of the seeds. These hexagonal crystals are transferred onto the surface of a Ge(001) single-crystal wafer by employing a wet transfer technique that involves etching of the Cu foil and the use of a sacrificial polymer support layer (Figure 1b). After transfer, the orientation of each hexagonal crystal, and thus the orientation of the graphene lattice, with respect to Ge〈110〉, is quantified. Next, arrays of circular aluminum (Al) dots with diameter of either 60, 75, or 90 nm are patterned via electron-beam lithography, development, thermal deposition, and lift-off to serve as etch masks (Figure 1c). Oxygen plasma etching is used to remove the exposed regions of graphene that are not masked by the Al, and then the Al is 6 ACS Paragon Plus Environment

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removed in H3PO4, leaving arrays of circular graphene seeds with known crystallographic orientation on Ge(001) (Figure 1d). The graphene seeds are undercut during the reactive ion etch by roughly 5 nm (Supporting Information Figure S1), resulting in seeds that are slightly smaller than the Al dots, with diameters of approximately 55, 70, or 85 nm.

Figure 1. Fabrication of graphene nanoribbon arrays via seed-initiated growth. Each panel contains a schematic (top) and an SEM image (bottom) corresponding to the major steps in the process flow. (a) Hexagonal crystals of monolayer graphene are grown on Cu foil. (b) The hexagonal graphene crystals are transferred onto Ge(001). (c) Arrays of Al dots are patterned on top of the hexagonal graphene crystals on Ge(001) via electron-beam lithography, development, Al deposition, and lift-off. (d) The exposed graphene that is not protected by the Al masks is etched using an oxygen reactive ion plasma and then the Al masks are etched with H3PO4, resulting in an array of circular graphene seeds on Ge(001). (e) Graphene is grown from the seed array via CVD. Scale bars are 80 µm (a), 40 µm (b), 4 µm (c), and 200 nm (d,e).

Prior to the growth of graphene from the edges of seeds, the Ge(001) substrates containing graphene seeds are annealed at 910 °C in a flow of 200 sccm of Ar and 100 sccm of H2 for 30 – 90 min in an atmospheric-pressure CVD chamber to remove oxide and other impurities from the Ge surface. This

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anneal also reduces the size of the seeds, as discussed further, below. Immediately after annealing, 2.0 sccm of CH4 is added to the flow of Ar and H2 for 1.5 h to initiate the growth of graphene (Figure 1e). Figures 1-4 show representative scanning electron microscopy (SEM) images following seedinitiated growth. Graphene grows preferentially from the edges of the seeds (whereas secondary nucleation is suppressed to a density of only 0.02 – 0.2 µm-2), and the position of the resulting crystals is precisely controlled by the position of the seeds, enabling the growth of arrays of graphene crystals. All of the resulting crystals share two similarities: their interior angles are approximately 60 and 120° and their edges are approximately armchair (i.e., parallel to C-C bonds) – as determined by transmission electron microscopy and selected-area electron diffraction (Supporting Information Figure S2). Graphene crystals with armchair edges preferentially form on Ge(001) because the growth velocity of armchair edges is slower than all other edge orientations. Crystals that are missing from the arrays are primarily absent because of imperfections in our seed formation protocol (mainly, Al dots are removed during the lift-off process, which prevents the definition of seeds during the subsequent reactive ion etch). While some of the intended graphene seeds are not defined due to removal of the Al dots during patterning, remarkably, 98.7% of graphene seeds that are defined result in the growth of graphene crystals (Supporting Information Figure S3). There are significant variations in aspect ratio and orientation of the crystal edges (with respect to Ge〈110〉), depending on the orientation of the seed lattice. These differences are characterized in detail in Figure 2 as a function of the angle of misalignment, , which is the smallest angle that can be

formed between any of the three armchair directions of the transferred seed and the nearest Ge〈110〉 direction. As described above, seeds with different are generated by rotating the orientation of the graphene crystals during their transfer from Cu foil to Ge(001). For example, seeds with of 2°, 7°,

and 15° are depicted in Figures 2a-c, respectively. Figures 2d-f show SEM images of graphene crystals grown from seeds with these orientations, using a seed diameter of ~ 55 nm and a pre-growth anneal time of 30 min. The lateral extent of all of the crystals is similar along the Ge〈110〉 direction aligned closest


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with the long edges of the crystal (i.e., the horizontal axis of the images in Figures 2d-f) regardless of ; however, the aspect ratio of the crystals significantly decreases with increasing . The orientation of the long edges of the graphene crystals with respect to Ge〈110〉 also varies with .

The aspect ratio of the resulting crystals is analyzed in more detail in Figure 2g, and the orientation of the long edges of the graphene crystals with respect to the nearest Ge〈110〉 direction,

, is quantified in Figure 2h. When the armchair direction of the seeds is near Ge〈110〉, for

example when is 2°, nearly all of the resulting crystals are ribbon-like (Figure 2d) and have an

aspect ratio of 4 – 5 (Figure 2g). Moreover,

ranges from 1 – 3° in this case (Figure 2h),

approximately matching . When is increased to 10 – 15°, nearly all of the resulting crystals

instead evolve as compact parallelograms (Figure 2f) with an aspect ratio of ~ 1 (Figure 2g). Moreover,

ranges from 10 – 15° (Figure 2h), again approximately matching . The approximate

match between

and in both of these cases is intuitive; when the armchair direction of the

seeds is pointed from Ge〈110〉, the long edges of the resulting crystals (which are approximately armchair edges) are also pointed roughly from Ge〈110〉 (i.e., the lattice orientation of the

graphene crystal closely matches the lattice orientation of the seed from which it grows). In contrast, when an intermediate of 4° or 7° is used, both compact crystals with low-

aspect ratio and ribbons with high-aspect ratio form (Figures 2e, g). Moreover, a wide range of crystal

orientations evolve (Figure 2e) with

varying from 1 – 15° (Figure 2h). This observation is

interesting because when the armchair direction of the seeds is pointed, for example, 7° from Ge〈110〉, one would intuitively expect the lattice of the resulting crystals (which have approximately armchair edges according to electron diffraction in Supporting Information Figure S2) to also point 7° from

Ge〈110〉. Instead, many different

are observed. There are two possibilities that might account for

this observation: (i) the seeds rotate prior to or during CVD or (ii) crystals with lattice orientations distinct from the seeds nucleate around the periphery of the seeds. Mechanism (i) is conceivable because the 9 ACS Paragon Plus Environment

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rotation of graphene domains as large as 20 µm and 500 nm has been previously observed on liquid Cu and h-BN surfaces, respectively;65-67 although, the energetic barriers governing rotation on Ge(001) are not yet clear.68 Regarding the possibility of mechanism (ii), a single set of electron diffraction spots is detected from nearly all of the structures, indicating that if mechanism (ii) is occurring, then each structure must be mostly comprised of a single graphene domain that encapsulates the seed, which by itself is not large enough to produce a second set of measureable diffraction spots.

The imperfect match of and

observed for intermediate of 4° or 7° is

actually observed for all (Figure 2h and Supporting Information).

tends towards

when is small (0 – 4°) or large (10 – 15°); however, the match is not perfect as the angular

distributions for

at a given broadly span several degrees. Interestingly, in all cases,

tends towards 2.5 and 12.5° (see vertical lines cutting through Figure 2h), which are

approximately the mismatch between the armchair direction of graphene and Ge〈110〉 of 2.8 and 13.2° observed for graphene crystals grown without seeds.35 These data indicate that graphene has a strong

tendency to grow with specific

regardless of , likely as a result of one of the two

mechanisms discussed above. Thus, while graphene crystals of all orientations can be grown via seed

initiated growth,

near 2.8 and 13.2° are the most likely to occur.


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Figure 2. (a-c) Schematics of graphene seeds on Ge(001) with of 2° (a), 7° (b), and 15° (c). Only

the top layer of Ge atoms is shown. The precise structure of the graphene seeds after patterning and pregrowth annealing is unknown. The seeds used in this work are also much larger, with diameters of 55 – 85 nm, than those shown in the schematics. (d-f) SEM images of representative graphene crystals grown from seeds with of 2° (d), 7° (e), and 15° (f). Scale bars are 400 nm. (g-h) Histograms showing

the distribution of aspect ratios (g) and

(h) for ribbons grown from seeds with = 2°, 4°,

7°, 10°, and 15° (plots from top to bottom). The pre-growth anneal time in d-h is 30 min.

Practically, if one’s goal is to grow nanoribbon arrays, it is clear from Figure 2 that nanoribbons with high-aspect ratio evolve only when is small, in which case the armchair direction of the

seeds approximately points along Ge110 or Ge110. This practical guideline is further emphasized in Figure 3a, in which the average aspect ratio is plotted against over the range of 0 – 30°. The


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increases from 0° to 6° to average aspect ratio decreases from 7.7 ± 5.2 to 3.3 ± 0.7 to 1.2 ± 0.1 as

15°, respectively, as the armchair direction of the seeds rotates away from Ge110. However, the aspect ratio then increases again as further increases to 30°, at which point the armchair direction of the

seed lattice becomes aligned with Ge110 (which is perpendicular to Ge110), and the long edges of the ribbons are rotated by roughly 90° compared with ribbons grown from seeds with of 0°.

Thus far, we have only discussed the aspect ratio and orientation of the long edges of seedinitiated graphene crystals. However, more information is embedded in the data characterizing the size and shape of the crystals. For each graphene crystal that evolves, there are three sets of armchair edges from which the crystal can grow, each with a different relative orientation with respect to either Ge110 or Ge110. The six-fold symmetry of graphene is broken when it is on a Ge(001) substrate with fourfold symmetry, making these three armchair directions inequivalent. Therefore, each of these edges grows at a different velocity, governing the shape of the crystal. For example, when one of the armchair directions is rotated from Ge〈110〉 by θAC, (0° < θAC < 15°) the other two armchair directions will be rotated from either Ge110 or Ge110 by 30° + θAC and 30° – θAC. The growth velocities in these inequivalent armchair directions are expected to be different.52 By measuring the distance between

parallel edges of graphene with various

, we next determine the dependence of the growth

velocity on θAC (Figures 3b-e). A representative graphene ribbon with its long edges rotated by 2° from the nearest Ge〈110〉 (i.e.,

= 2°) is shown in Figure 3c. The crystal has two sets of parallel edges, and thus, the crystal shape

is bound by only two of the three possible sets of armchair edges. By measuring the length of the red arrow, d1, which is the distance between edges bound by planes with θAC = 2° (red dashed lines), the growth velocity for the armchair facet with θAC = 2° is calculated as v1 = d1·(2·t)-1, where t is the growth time. Likewise, by measuring the length of the yellow arrow, d2, which is the distance between edges bound by planes with θAC = 30° – 2° = 28° (yellow dashed lines), the growth velocity for the armchair facet with θAC = 28° is calculated as v2 = d2·(2·t)-1. It is only possible to estimate a lower bound for the 12 ACS Paragon Plus Environment

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growth velocity for the third set of armchair edges with θAC = 30° + 2° = 32° (blue arrows, v3 = d3·(2·t)-1) because these edges have a faster growth velocity than armchair facets with θAC = 2° and 28°, and therefore, none of the crystal edges are bound by planes with θAC = 32° (blue dashed lines). Equivalent

analyses are shown in Figure 3d for a crystal with

= 10°, providing measures of growth velocity

for θAC = 10° and 20° and a lower bound of the velocity for θAC = 40°, and in Figure 3e for a crystal with

= 13°, providing measures of growth velocity for θAC = 13° and 17° and a lower bound of the

velocity for θAC = 43°. This analysis is repeated on 640 crystals with various orientations to generate the data characterizing growth velocity versus θAC shown in Figure 3b. The growth velocity is minimized for θAC of 0°, and then increases approximately linearly with increasing θAC (Figure 3b). Provided that the orientation of the graphene lattice relative to Ge(001) is known, these data can be used to determine the growth velocities of the three sets of armchair edges and to predict the shape of any graphene crystal on Ge(001). It is clear from Figure 3b that that largest contrast in growth rate occurs when one of the three armchair edges is parallel to either Ge110 or Ge110, yielding nanoribbons with high-aspect ratio. For growth without seeds, graphene nuclei form that are preferentially rotated by only 2.8° from both Ge110 or Ge110, giving rise to the spontaneous formation of ribbons with high-aspect ratio that are aligned approximately along both Ge110 or Ge110. Previous results indicate that the aspect ratio of these ribbons increases with decreasing growth rate (reaching as high as 70), indicating that the contrast between the growth velocity for θAC near 0° and θAC near 30° depends on the conditions used in the CVD reactor. For seed-initiated growth, we have the capability to orient the seed crystals either along Ge110 or Ge110. Thus, we have the capability to intentionally seed the growth of nanoribbons with high-aspect ratio and to unidirectionally control their orientation along either Ge110 or Ge110. While ribbons oriented both +2.8° and –2.8° from Ge110 are observed during unseeded growth, here the

spread in

is much smaller than 2×2.8°=5.6° (and is for example only ±1° for ribbons grown from

seeds with = 2° in any given array, which is similar to our error of measurement). This narrow


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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.

Figure 3. (a) Plot of aspect ratio versus

. Horizontal lines in the boxes define the 25th, 50th and

75th percentiles, whiskers indicate the 10th and 90th percentiles, and squares give the mean. (b) Plot of growth rate versus the angle between the armchair direction of graphene and Ge〈110〉. Black data points are physical measurements, while the gray data points are lower bounds. (c-e) SEM images of

representative ribbons with

= 2° (c), 10° (d), and 13° (e) in which Ge110 and Ge110 are

horizontal and vertical to the image, respectively. The magnitude of the red, yellow, and blue arrows indicates the relative growth velocity of the slowest, d1, second slowest, d2, and third slowest, d3, armchair direction of graphene. The red, yellow, and blue planes indicate the growth front of each armchair edge. The shape of the resulting graphene crystal is found by finding the minimum area of intersection of each growth front. Only a lower estimate for d3 can be determined, because this plane does not bound the crystal. Scale bars are 100 nm. The pre-growth anneal time is 30 min.


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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 of roughly 2° or 3°. The experiments, seeds with a diameter of ~ 55 nm are fabricated with a

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 flow of 200 sccm of Ar, 100 sccm of H2, and 2.0 sccm of CH4, and the widths and lengths of the ribbons are measured. As the anneal time increases, the ribbon width (Figure 4a) and length (Figure 4b) decrease and the aspect ratio (Figure 4c) increases. For example, as the anneal time increases from 30 to 90 min, the average width decreases from 80 ± 20 nm to 26 ± 9 nm and the average length decreases from 388 ± 61 nm to 286 ± 33 nm. The width decreases to a much larger extent than the length, and consequently the aspect ratio increases from 4.7 ± 1.4 to 12.4 ± 4.7. The SEM image in Figure 4d shows a representative ribbon array grown using an anneal time of 90 min. These results demonstrate that the aspect ratio can be maximized if the size of the graphene seed is reduced. If the graphene seed size is sufficiently small, we expect the aspect ratio to significantly increase (e.g., > 30), as previously observed for nanoribbons grown without seeds.35 Interestingly, for anneal times > 45 min, seeds with > 3° are completely etched in almost

all cases, suggesting that seeds with these orientations are less stable on Ge(001). This is consistent with


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the observation that under conditions in which seeds are not used, > 90% of the crystals form with

< 3°. The resistance to etching and enhanced stability of graphene crystals with and

< 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 < 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 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 direct-current 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 as-patterned seeds due to etching of the seeds before growth via annealing, as discussed above and shown in Figures 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


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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.

Figure 4. (a-c) Plots of width (a), length (b), and aspect ratio (c) versus anneal time for nanoribbons grown from seeds with ≤ 3°, in which the armchair direction of graphene and Ge〈110〉 are nearly

aligned. Horizontal lines in the boxes define the 25th, 50th and 75th percentiles, whiskers indicate the 10th and 90th percentiles, and squares give the mean. (d) SEM image of a nanoribbon array grown using a pregrowth anneal time of 90 min. Scale bar is 400 nm. (e) Plot of ribbon width versus etch mask diameter using an anneal time of 30 min and a growth time of 90 min. The diameter of the seeds is likely a few nanometers smaller than that of the Ni etch mask due to undercutting during the reactive ion etch, as described above and shown in Supporting Information Figure S1 for larger seeds. Squares give the mean and whiskers indicate one standard deviation. (f) SEM image of a nanoribbon array with an average width of 12.8 ± 5.6 nm and an average aspect ratio of 17.5 ± 7.2. The anneal time is 45 min and the growth time is 360 min. Scale bar is 100 nm.


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≤ The polydispersity in nanoribbons grown with seeds (

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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 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 homogenous. 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 non-uniform etching during pre-growth annealing. Therefore, it should be possible to reduce the nanoribbon polydispersity even further by using seeds that are more uniform in size and shape.


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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.

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 Ge110 or Ge110, the crystal shape anisotropy is maximized, driving the growth of nanoribbons with their long edges aligned along Ge110 or Ge110, 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 unidirectionallyaligned nanoribbon arrays and occurs under conditions in which the secondary nucleation of randomlypositioned nanoribbons is greatly diminished. Moreover, seed-initiated synthesis significantly reduces 19 ACS Paragon Plus Environment

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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 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.

Associated Content Supporting Information. 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. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author *E-mail: [email protected]. ORCID 20 ACS Paragon Plus Environment

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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.

Acknowledgements This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-SC0016007. A.J.W. also acknowledges support from a National Science Foundation Graduate Research Fellowship under Grant No. DGE-1747503. We also thank Vivek Saraswat for depositing the Ni etch masks via sputtering.

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Seed-Initiated Anisotropic Growth of Unidirectional Armchair

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