A Guide for Nanowire Growth Nathan O

COMMENTARY COMMENTARY A guide for nanowire growth Nathan O. Weissa and Xiangfeng Duanb,c,1 the limits of conventional lithography (4, 5). Departments of aMaterials Science and Engineering and bChemistry and Biochemistry and c Nanotechnology has seen a convergence of California NanoSystems Institute, University of California, Los Angeles, CA 90095 these two strategies, cherry picking the advantages of either to realize novel architec- ture design at scales beyond the capabilities Human fascination with growth has been ultimately, it can be tamed by exploiting of one technique on its own. around since the Neolithic Revolution began these forces through engineering to pro- Nanowires like to grow in a similar fashion more than 10 millennia ago. With it grew the duce bountiful results. to trees: straight up. The recent develop- science and understanding behind the un- At the crux of advanced electronic fabri- ment of the vapor-liquid-solid (VLS) growth derlying mechanisms found in nature, which cation is the intersection of top-down and method has propelled the field to new heights, fi are often applied toward practical bene ts bottom-up approaches toward building nano- where a nanoscale liquid drop of catalyst such as bigger produce or higher yields. For structures for practical functionality in an facilitates the (typically) vertical growth of example, farmers plant seeds in specificand economical way. Living organisms are the high-quality, solid nanowires using vapor optimally spaced positions that allow dense original bottom-up assemblers: multitudinous phase reagents (Fig. 1D) (6). However, ver- and organized fields to develop (Fig. 1A), nanoscopic pieces (proteins, DNA, etc.) work tical nanowires are not necessarily very prac- whereas villagers in Cherrapunjee, India, grow together to make microscopic machines tical for electronics, as most device structures tree roots across rivers to form living bridges. (cells) that self-replicate and assemble to- and fabrication techniques require horizontal Other aesthetic applications include guided gether to form all kinds of life. On the other geometries. This is where guided growth growth of vines over an arbor (Fig. 1B)or hand, top-down is a crude tactic not gen- becomes an essential method to dictate the even living sculptures and “tree shaping” erally found in nature: we cut down trees, orientation of a nanowire: with a precisely (Fig. 1C). The same underlying principles grind down stones into tools, and use engineered system, they will grow along pre- apply to nanowires: guiding growth using otherwise blunt subtraction to create a de- defined geometries. Positioning the nano- catalyst seeds along a template, as devel- sired product. In terms of horticulture, this wires in a well-organized, dense array is oped by Schvartzman et al. and presented is analogous to pruning or cutting away critical in the fabrication of integrated cir- in PNAS (1). At the nanoscale, this becomes branches, in contrast to training and di- cuits. Growing the nanowires directly into an indispensable tool for synthesizing the rected growth. The electronics industry has place for each device avoids any further precisely ordered patterns required by ad- been dominated by top-down fabrication alignment steps. Novel strategies for scal- vanced electronic applications. Nature is methods of traditional semiconductor tech- able and efficient synthesis of ordered nano- often a model to mimic (2, 3) but is also nologies; however, bottom-up growth is wire arrays have been developed, such as a barrier that technology must overcome; seen as an inevitable necessity to breach nanoimprint lithography (7, 8) and postgrowth assembly using mechanical (9, 10) or electromagnetic (11) forces and self- assembly (12). They have their advantages, butnoneachievethesamedegreeofalign- ment and control as guided growth. Nanowire growth is influenced by its en- vironment, just like trees tend to grow toward the most sunlight and vines wrap themselves around pillars. Many advanced technologies have used guided growth principles at microscopic scales, such as surface modifica- tions and 3D scaffolds designed to guide cell growth and tissue engineering (13). Other 1D nanostructures have been synthesized using template surfaces, such as chemical vapor deposition growth of carbon nanotubes on quartz (14) and epitaxial growth of graphene nanoribbons on nanofaceted silicon carbide (15). A novel approach toward controlling nanowire growth uses substrates that are Fig. 1. Guided growth in nature and nanowires. (A) Vineyard: Farmers create linear patterns by planting seeds in patterned with a template of nanoscale crys- precise locations. (B) Grand Arbor in South Bank Australia: Vines growing along a horizontal template. (C) “Basket tallographic surfaces. Such patterned facets Tree”: The art of tree sculpture, a combination of guided growth, training, and grafting, is used to slowly manipulate the formations that trees can make. This piece of art was created by Axel Erlandson, completed in 1947, and is located in Gilroy Gardens, CA. (D) Tilted SEM images of vertically grown forest of VLS nanowires. (E) SEM image of guided Author contributions: N.O.W. and X.D. wrote the paper. horizontal growth on a crystal surface, showing highly parallel synthesis of precisely located nanowires with end-to- The authors declare no conflict of interest. end registration. (F) This process enables direct integration of the nanowires into more complex circuits such as this SEM image of a three-bit address decode. Photos courtesy of and copyrighted by (A) Stefan Bauer, (B) Lee Mylne, and See companion article on page 15195. (C) Richard Reames. (D) Reproduced with permission from ref. 6 and copyrighted by the Association for the Ad- 1To whom correspondence should be addressed. E-mail: xduan@ vancement of Science 2001. (E and F) Reproduced from ref. 1. chem.ucla.edu. www.pnas.org/cgi/doi/10.1073/pnas.1313743110 PNAS | September 17, 2013 | vol. 110 | no. 38 | 15171–15172 Downloaded by guest on September 24, 2021 create an energetically preferred growth vec- To demonstrate the feasibility of build- size. This may prove to be a serious limita- tor, allowing nanowires to only grow along ing integrated circuits using this guided tion, as the variability in performance, such the predefined channel. Seminal work by growth approach, Schvartzman et al. per- as conductance and transconductance, will Melosh et al. (16) used perpendicular ep- form a proof-of-concept array of more suffer greatly from a large distribution in itaxial growth on specific crystal facets, than100 top-gated transistors with a yield nanowire size. Applications will also require demonstrating reasonable scalability and of 85%. They have further demonstrated the nanowire arrays to be patterned as densely extremely high precision. However, this ap- a three-bit address decoder with 14 transis- and precisely as possible, whereas this VLS proach is limited by the lithographically tors integrated into a single circuit (Fig. 1F), growth may be limited by surface diffusion patterned top-down features and requires mobility of the liquefied gold nanoparticles. molecular beam epitaxy techniques that are Concurrently positioning Concurrently positioning nanowires dur- typically more challenging and less practi- ing growth offers a number of clear advan- cal than VLS growth strategies. nanowires during tages over postgrowth assembly techniques More recently, the Joselevich group de- growth offers a number that are unlikely to be outdone. However, veloped a horizontal VLS growth method inevitably, this approach comes with its own that yields high-quality nanowires that are of clear advantages over inherentdrawbacksaswell.Forexample,the parallel to the surface (17). Here, the crystal- postgrowth assembly particular substrate/nanowire material com- lographic facets resemble tracks for the nano- techniques. bination requirements can greatly limit com- wires to extend down. Further progress is positional versatility. Further refinement of reported here in PNAS using ZnO nanowires illustrating the potential mass scalability of the process is still necessary to improve the on a (1102) R-plane sapphire substrate (1). this technique. consistency, precision, yield, and density of Because the orientation of the nanowire is The improvements shown in this report the grown nanowires. The translation of fi de ned by the crystallographic orientation make it a promising complementary route to technology from a 14-transistor circuit to of the substrate, it enables an unprecedented microchip production. In particular, the de- one with billions of transistors found on accuracy, with 99% of the nanowires aligned terministic positioning of nanowires in a modern day microchips is no trivial task. ± E fi within 0.1° (Fig. 1 ), signi cantly better parallel process allows for scalable direct Other major hurdles in the pursuit of bottom- than state-of-the-art postgrowth assembly integration with existing top-down manu- up fabrication of integrated circuits will approaches (18). Furthermore, this tech- facturing. However, these technologies are hopefully be overcome in time; this work nique attains precise lengths, as all of the still in their early stages, and many ob- marks one step in the long journey. In ad- nanowires grow at the same time and stacles remain before it becomes viable at dition to microprocessors, semiconducting same rate, yielding length deviations less any practical scale. To truly replace the dense nanowire electronics have

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