A New Twist on Nanowire Formation: Screw-Dislocation-Driven

PERSPECTIVE pubs.acs.org/JPCL

A New Twist on Nanowire Formation: Screw-DislocationDriven Growth of Nanowires and Nanotubes Song Jin,* Matthew J. Bierman, and Stephen A. Morin Department of Chemistry, University of Wisconsin;Madison, 1101 University Avenue, Madison, Wisconsin 53706

ABSTRACT We discuss a nanowire and nanotube formation mechanism in which axial screw dislocations provide self-perpetuating steps to enable one-dimensional (1D) crystal growth, unlike previously understood vapor-liquid-solid (VLS) or analogous metal-catalyzed growth. We initially found this mechanism in hierarchical pine tree PbS nanowires with helically rotating branches. We further applied it to ZnO, demonstrating that screw dislocations can drive the spontaneous formation of nanotubes, and used classical crystal growth theory to confirm that their anisotropic 1D growth is driven by dislocations. Dislocation-driven growth should be general to many materials grown in vapor or solution and is underappreciated. It will create a new dimension in the rational synthesis of nanomaterials. The resulting complex hierarchical nanostructures can be useful for solar energy conversion, and our understanding will allow large-scale synthesis of 1D nanomaterials for practical applications.

ne-dimensional (1D) nanomaterials1-7 such as nanowires (NWs), nanorods (NRs), and nanotubes (NTs) possess novel properties that have already found many applications8 in nanoelectronics,9,10 nanophotonics,11 renewable energy,12 and chemical and biological sensing.13 The fundamental understanding of their growth is critically important for achieving rational and controllable synthesis to advance these applications. The question of how 1D crystals grow has long fascinated scientists, starting from the earlier microscale whiskers14,15 and continuing on to contemporary NWs.16 From the perspective of crystal growth, the challenge in making 1D materials is breaking the symmetry of crystal growth to ensure that crystals grow in a highly anisotropic fashion (instead of forming thin films and bulk crystals with polyhedral shapes). The vapor-liquid-solid (VLS)15,16 mechanism and other analogous catalyst-driven mechanisms, such as solution-liquidsolid (SLS)17,18 and vapor-solid-solid (VSS)19 growth, have been the most commonly discussed mechanisms for bottom-up synthesis of NWs. In the VLS (or SLS) mechanism (Figure 1A), a nanoscale liquid droplet is formed between a nanoparticle metal catalyst and the desired precursor species due to eutectic phase behavior, and the further supersaturation of the precursor leads to the precipitation and growth of crystalline NW material. Here, the anisotropic 1D growth is driven by the liquid-solid interface. In this Perspective, we will discuss a “new” NW formation mechanism (and, more broadly, a formation mechanism of any 1D nanomaterials including NTs), in which axial screw dislocations provide the self-perpetuating steps to enable 1D crystal growth, unlike previously understood mechanisms that require metal catalysts. We initially found and clearly illustrated this mechanism in hierarchical nanostructures of lead sulfide (PbS) NWs resembling “pine trees”20 that were synthesized via chemical vapor deposition. Interestingly, the basic idea that axial screw dislocations could explain the rapid

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1D crystal growth observed in micrometer-sized filamentary crystals, or “whiskers”, had been discussed by Sears in the 1950s,14,21 well before VLS growth. In fact, in the original works elucidating the VLS growth mechanism by Wagner and Ellis,22 extensive effort was made to rule out dislocations as the driving force in metal-catalyzed silicon whiskers. Ironically, since then, the dislocation-driven mechanism has been rarely discussed, especially in modern NW literature.1,3 Here, we will describe our fascinating journey through these interesting nanostructures and the new insights that we have gained from our studies so far. We will further discuss the generality and significance of this growth mechanism, the potential applications of the resulting structures and these new understandings of 1D nanomaterials growth, and the implications in physical property investigations of dislocations.

In screw-dislocation-driven NW growth, axial screw dislocations provide the self-perpetuating steps to enable 1D crystal growth. VLS-Driven Hyperbranched Nanowires. Initially, we synthesized hyperbranched NW clusters of PbS or PbSe (Figure 1B, C) via chemical vapor deposition (CVD) reactions using PbCl2 and S (or Se) as the precursors.23 Several similar CVD syntheses of PbS or PbSe NWs were reported with24 or without25,26 Received Date: March 3, 2010 Accepted Date: April 7, 2010 Published on Web Date: May 06, 2010

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DOI: 10.1021/jz100288z |J. Phys. Chem. Lett. 2010, 1, 1472–1480


Figure 1. (A) Schematic illustration of vapor-liquid-solid (VLS) growth of nanowires (NWs). (B,C) Hyperbranched NW clusters of PbS generated by the in situ VLS growth mechanism (D). B-D are adapted from ref 23.

Figure 2. (A, B) Electron microscopy images of pine tree NWs of PbS. (C) Diffraction contrast TEM of a PbS pine tree showing that a dislocation is present in the trunk and absent in the branches. (D) Illustration of Eshelby twist in a dislocated NW. A, C, and D are adapted from ref 20 with copyright permission from AAAS.

intentional catalysts; the key difference (and likely an important contributor to the reproducible observations of diverse morphologies) in our reactions is the intentional coflow and modulation of hydrogen (H2) gas during the CVD growth, together with an inert carrier gas (argon). In the case of hyperbranched PbQ (Q = S, Se) NWs, we hypothesize that H2 reduces some PbCl2 precursor to elemental lead, which can form low-temperature eutectics with PbQ according to the phase diagrams and thus serve as VLS catalysts.23 Therefore, the formation of hyperbranched PbQ NWs with branches of similar lengths along various directions are essentially in situ self-catalyzed VLS NW growth with multiple generations of VLS branches growing in an epitaxial fashion (Figure 1D), which results in very dense networks of “dendritic” NWs with a nearly “cubic” outer envelope. Dislocation-Driven Nanowire Growth and the Formation of Nanowire Pine Trees. More interestingly, under slightly different conditions of hydrogen flow, we have further observed fascinating pine tree like NWs of PbS (Figure 2A,B) through very similar CVD reactions.20 These beautiful structures consist of a long (sometimes several hundred micrometers) trunk NW with four sets of regularly spaced, progressively shorter “branch” NWs (often with lengths of a few tens of micrometers) from the bottom of the tree to the top, thus making cone-shaped outer envelopes. Furthermore, the branches rotate around the trunk in a helical fashion, making intricate hierarchical structures. A similar rotating NW tree morphology made of lead selenide (PbSe) was also reported.27 It is striking to compare the orthogonal hyperbranched NW clusters with the nearly cubic envelope discussed above with these NW pine trees that are formed under only slightly

different H2 flow conditions or that sometimes simply coexist in the same reactions. These nanostructures demonstrate a fundamentally different NW growth mechanism that is driven by axial screw dislocations. To understand this concept, one has to consider the fundamentals of crystal growth processes.28 Layer-by-layer (LBL) crystal growth (Figure 3A) was the intuitive model for the extended periodic crystal structure of solids that was beginning to be appreciated in the 1920-1930s. However, due to the high-energy barriers required to nucleate new layers of atoms (which create step edges that facilitate the addition of more atoms), LBL crystal growth theory predicts that high “supersaturation” (that is, higher concentrations of gas-phase or liquid-phase precursors than what is needed based on equilibrium) is necessary to enable a reasonable crystal growth rate. This was in conflict with the experimental observations that crystal growth can actually happen under moderate supersaturations. In 1949,29 F. C. Frank solved this conflict by pointing out that crystals often have imperfections such as screw dislocation line defects (Figure 3B),30,31 which upon intersection with the crystal surface would make steps that propagate as spirals and thus become an endless source of crystal steps. This theoretical model, known as the Burton-Cabrera-Frank (BCF) theory,32 successfully explains the observed growth rates of crystals under moderate and low supersaturation. The surface dislocation spirals have since been clearly observed in many materials,33 and the dislocation-driven growth mechanism has been accepted and come to be known as Frank's mechanism of crystal growth. If we create the appropriate low supersaturation condition that allows dislocation-driven crystal growth but that does not


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radians per unit length) should be b R¼ πR2

where R is the radius of the cylinder and b is the magnitude of the screw component of the Burgers vector. Eshelby's theory is a fundamental concept in the discussion of the energetics of dislocations and a foundation for understanding the mechanical properties of dislocations.30,31 For a NW with a diameter of 100 nm and containing a screw dislocation with a magnitude of 1 nm, eq 1 predicts an Eshelby twist of about 7.3 o/μm, or about 49 μm per a full 360° rotation, consistent with the general observation. Statistical surveys of a large number of PbS trees20 (and PbSe trees27) confirmed the Eshelby theory and yielded reasonable Burgers vector magnitudes. General Understanding of Dislocation-Driven Nanowire Growth in Vapor or Solution Phase. We now set out to understand the general principles that dictate dislocation-driven NW growth with the goal of expanding it to other material systems and synthetic strategies. We have further systematically investigated the basic characteristics of screw-dislocationdriven NW growth using the synthesis of PbS NW pine trees as a model system.36 The highly anisotropic tree shape and rotating branches with Eshelby twist provide direct visual evidence of screw dislocations and relieve the nontrivial burden of observing dislocations in large numbers of samples using TEM. The optimal conditions for synthesizing PbS NW pine trees have been investigated with detailed studies of the effects of various growth parameters, such as hydrogen flow, temperature, pressure, and the growth substrates (silicon or glass) employed. We found that H2S, which is formed by the very favorable reaction of S with H2, is the actual precursor (instead of S directly) that reacts with PbCl2 to form the PbS product, and H2S can also react with other species in the system, such as with the silicon substrate to form SiS2.36 Statistical surveys showed that the dislocation-driven trunk has a constant growth rate of about 6 μm/min and the VLS driven branch NW has a growth rate of about 1.2 μm/min under the typical reaction conditions at 600 °C and 900 Torr and a H2 flow rate of 1.5 sccm. We have identified the onset of H2 flow plus the presence of fresh silicon (that comes from the silicon substrates employed in these syntheses) as the critical ingredients for reproducibly generating PbS NW trees for this specific reaction system. In a general context, dislocation-driven NW growth requires two basic ingredients, the creation (seeding) of screw dislocations and a suitable low supersaturation condition for promoting dislocation-driven growth over LBL growth and other growth modes. In the specific case of PbS trees, dislocations are spontaneously generated; it is likely that the initial spike in supersaturation of H2S (the actual sulfur precursor that determines supersaturation instead of S) triggered by the onset of H2 flow results in fast deposition of imperfect materials with slightly mismatched crystalline grains that can lead to the creation of dislocations. Maintaining suitable continuous H2 flow then provides a favorable low supersaturation of H2S that ensures that dislocation-driven NW growth dominates over (or at least is not overwhelmed by) other competing growth mechanisms. The dislocations then propagate anisotropically to form the PbS NW trunks.

Figure 3. Schematic illustrations for (A) layer-by-layer (LBL) crystal growth on a perfect crystal facet, (B) a screw dislocation that generates a step, (C) screw-dislocation-driven NW growth with a dislocation growth spiral on top, and (D) faster screwdislocation-driven trunk growth combined with slower epitaxial VLS-driven branch growth that produces pine tree NWs. C and D are taken from ref 20 with copyright permission from AAAS.

allow (or favor) LBL growth, then a cylindrical crystal with a dislocation growth spiral on the top and smooth side surface will grow rapidly along the axial direction driven by the screw dislocation but struggle to grow along the radial direction through LBL growth (Figure 3C). In this way, we break the symmetry of crystal growth and enable the highly anisotropic crystal growth needed for 1D NWs or whiskers. In other words, in screw-dislocation-driven NW growth, axial screw dislocations provide the self-perpetuating steps to enable 1D crystal growth. This is in contrast to VLS and other catalyst-driven NW growth mechanisms that are a special mode of LBL crystal growth facilitated by the liquid-solid interface (which has a lower-energy barrier than direct LBL growth from vapor phase). We have carried out careful microstructural characterization using diffraction contrast transmission electron microscopy (TEM) under the two-beam condition34 to conclusively show that axial dislocations with screw components are present in PbS NW trunks (Figure 2C).20 Furthermore, such dislocations were not observed in the hyperbranched PbS NWs or the branches of the PbS NW pine trees. We suggest that lead particles generated in situ can serve as self-catalysts to enable VLS growth of the branches that epitaxially grow off of the trunk. The cone shape of the nano pine trees is the result of simultaneous fast dislocation-driven trunk NW growth and the slower VLS-driven growth of branches nucleated at differing times (Figure 3D). The helical rotation of the branches is the consequence of the strain of the axial dislocation; the torque produced by the stress field of an axial screw dislocation in a cylinder is balanced by twisting the crystal lattice, a phenomenon known as the Eshelby twist.35 In a finite cylindrical rod containing an axial screw dislocation at the center (Figure 2D), according to elasticity theory, the Eshelby twist (R, the twist of the lattice in


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dislocation-driven NWs, one must exploit low supersaturation conditions that favor the dislocation-driven growth mechanism but not LBL or dendritic growth. We note that it is sometimes commented on in the literature that empirically low concentrations (supersaturations) seem to facilitate synthesis of NWs that do not require catalysts, and we suspect that what we explain here is the fundamental reasons behind such observations. This can provide a rational pathway to enable the catalyst-free synthesis of 1D nanomaterials. As a demonstration, we have improved the solution-phase growth of single-crystal NWs of zinc oxide (ZnO) driven by dislocations using flowed solutions of constant low supersaturation.37 There have been numerous reports on catalystfree aqueous solution growth of ZnO nanomaterials.38,39 Typically, hydrolysis of zinc salts in closed containers together with weak polyamine bases, such as hexamethylenetetramine (HMT), readily yields ZnO NWs, NRs, and sometimes NTs, with their morphology influenced by variations in reagent concentration, growth time, and temperature.39 To date, there has been no agreement or convincing explanation of the growth mechanism responsible for 1D growth in these solution syntheses. To test the hypothesis of screwdislocation-driven ZnO NW growth, we investigated the effects of supersaturation, particularly low supersaturation that favors dislocation-driven growth, on nanomaterial morphology and growth kinetics. However, commonly practiced closed-system hydrothermal growth of ZnO, where precursors are introduced at one time and allowed to react to completion, suffers from a significant decline in the concentration of soluble zinc species (thus supersaturation) during the course of the reaction (as illustrated in Figure 4B), which leads to irregular growth kinetics that are difficult to deconvolute. Therefore, we developed and consistently used a continuous flow reactor (CFR) to maintain constant supersaturation and enable indefinite reaction times (Figure 4B).37

Figure 4. (A) Schemes illustrating the progression of crystal growth rates for the three growth mechanisms (dislocation-driven, LBL, and dendritic) as a function of supersaturation, and (B) the comparison of the supersaturation profiles for close-system hydrothermal growth and flow reactor growth. (C-E) Morphological changes in 1D ZnO nanomaterials with increasing supersaturation in the low supersaturation regime around σ*. Adapted from ref 37 with copyright permission from AAAS.

Furthermore, this is helped by the fortuitous SiS2 side reaction that dictates the H2S concentration. Thermodynamic consideration and experiments showed that the silicon substrate reversibly reacts with H2S to form SiS2, and SiS2 can in fact serve as a viable precursor for PbS NW growth with the help of small amounts of H2 or H2O (moisture).36 Of course, the formation of pine tree NWs further demands that reaction conditions allow the continuous generation of VLS catalysts and maintain a supersaturation level still suitable for reasonable VLS NW growth to produce the branches, but this is not a necessary condition for “simple” dislocation-driven NW growth. This empirical understanding of dislocation-driven NW growth as exemplified by the CVD growth of PbS NW pine trees provides general guidelines for rationally designed dislocation-driven NW growth. There are no fundamental reasons that such growth cannot be done with other materials, in either vapor phase or solution phase, and that its implementation cannot be based on simpler control than the rather intricate chemistry seen in the PbS CVD reactions. The most essential and critical factor to dislocation-driven NW growth is controlling and modulating the supersaturation of the reaction precursors because dislocation-driven anisotropic 1D crystal growth is achieved by preferred growth at self-perpetuating spirals of axial screw dislocations under low supersaturations. If we elaborate on this using the framework of classical crystal growth,28,32 supersaturation (σ) is defined as c σ ¼ ln ð2Þ co

To intentionally grow dislocationdriven NWs, one must exploit low supersaturation conditions that favor the dislocation-driven growth mechanism but not LBL or dendritic growth. We first prepare highly defective nanoparticles of ZnO that are synthesized at high supersaturations, some of which could contain dislocations that can propagate the anisotropic crystal growth. We then introduce these particles into the low supersaturation environment of the flow reactor where dislocation growth is favored. (In the future, we can imagine some intentional control over this process by “engineering” screw dislocations on the substrates.40) Modifying such a mundane solution synthesis guided by crystal growth theory results in dramatic morphological changes (Figure 4C-E);37 with decreasing supersaturation, the diameter of the NRs/NWs

where c and co are the concentration and equilibrium concentration of the system, respectively.28 According to the BCF theory,32 dislocation growth, LBL, and dendritic growth progressively dominate crystal growth as supersaturation is increased (Figure 4A). Therefore, to intentionally grow


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magnitude of the Burgers vector (b)43 b2 μ R ln E¼ 4π r

where μ is the shear modulus and R and r are the outer and inner tube radii, respectively.31 As b increases, eventually, its associated strain energy is large enough to favor the creation of a new internal surface along the dislocation core, leading to spontaneous formation/growth of a hollow core. This is the reason behind the formation of micropipes in single crystals or thin films often observed in dislocation-prone materials such as SiC or GaN43,44 and sometimes termed “open-core dislocations”. For NWs, the axial screw dislocations contained within not only drive anisotropic growth but also cause the spontaneous formation of NTs due to large dislocation strain energy (Figure 5C). Single-crystal NTs can be the equilibrium morphology of 1D nanomaterials that contain dislocations with large strain energy.

Figure 5. (A, B) TEM images of solution-grown single-crystal NTs of ZnO. (C) Schematic illustration of dislocation-driven growth of a hollow NT. Adapted from ref 37 with copyright permission from AAAS.

decreases, and the aspect ratio increases. At very low supersaturation (precursor concentrations in the tens of μM regime for ZnO precursors), thin single-crystal NWs and NTs (see discussion below) with typical diameters of 15-30 nm are formed (Figure 4C), rivaling the morphology of those ZnO NWs formed by vapor-phase growth via the VLS mechanism. The presence of screw dislocations in these thin NWs is readily confirmed using diffraction contrast TEM. Dislocations can also be observed in the ZnO NRs grown under high supersaturations or under hydrothermal conditions, but their larger size and convoluted growth process make such observations much more difficult.37 Furthermore, by bridging the gap between modern nanomaterials research and fundamental crystal growth theory, we compared the prediction from the classical crystal growth theory on dislocation-driven (BCF theory) and LBL growth with the ZnO growth kinetics observed under carefully controlled low supersaturation conditions. The agreement between the observed kinetics and that predicted from BCF theory confirms that the anisotropic crystal growth of these 1D nanomaterials is indeed driven by screw dislocations.37 We also understand that such low supersaturation growth can shut down the LBL growth along the NW/NT side wall that accounts for radial broadening, therefore reducing the tapering and increasing the aspect ratios of the 1D nanomaterials produced. Dislocation-Driven Growth of Hollow Nanotubes. We have further elucidated the growth and direct formation of singlecrystal inorganic NTs due to screw dislocations with larger Burgers vectors. Single-crystal NTs or microtubes, particularly those made of inorganic materials, have been frequently observed in the literature;6 however, their growth mechanism is often not clearly explained and usually thought to be unrelated to that of NWs. In fact, hollow NTs of ZnO are the most commonly observed products when we carry out the synthesis in a flow cell under low supersaturation (Figure 5A,B).37 Larger-sized microtubes of ZnO are also observed at intermediate concentrations by us or by others.41,42 To explain the hollow structure, we need to understand that there is a strain energy (E, per unit length) due to a screw dislocation that is quadratically dependent on the


ð3Þ

Single-crystal NTs can be the equilibrium morphology of 1D nanomaterials which contain dislocations with large strain energy. The classical open-core dislocation model predicts a relationship between r and b as the result of balancing the surface energy (2πγr, where γ is surface energy) and the strain energy from eq 3 (both per unit length)43 sffiffiffiffiffiffiffiffiffiffiffiffiffi 8π2 γr bTUBE ¼ ð4Þ μ One can also derive a modified Eshelby twist equation for hollow tubes. However, the observed ZnO NTs were found to display very little twist, in contradiction to the Eshelby prediction. We realized that Frank's and Eshelby's treatments on the energetics of dislocation strain in the 1950s were for either hollow cores in bulk materials43 or solid cylinders,35 respectively, which overlooked the scenario of hollow tubes for which both the Eshelby twist and hollowing out process are viable pathways for relieving dislocation strain energy. When we consider the competition between hollow dislocation cores and Eshelby twist in relieving the dislocation strain energy by balancing all three energy contributions (the surface energy from the hollow inner tube, the lattice strain due to the dislocation, and the reduction in lattice strain due to the Eshelby twist, all per unit length), we can derive a revised expression of the Eshelby twist component of b as the difference between total b and the b going into making tubes37 sffiffiffiffiffiffiffiffiffiffiffiffiffi ! 8π2 γr R2 þ r2 bTWIST ¼ bTOTAL - bTUBE ¼ -1 ð5Þ μ R2 - r2 Evaluating eqs 4 and 5 for some specific cases makes it clear that our improved model predicts that thick-walled NTs should

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have very little Eshelby twist because most of the strain energy is relieved through hollowing out the crystal and thin-walled tubes can have more notable twist.37 We have used the specific example of ZnO as an illustration; however, the fundamental principles are general and can apply to NTs of many materials6 that are formed in either solution- or vapor-phase synthesis, especially those that cannot be explained by layered structures,45 templating,3,45 or the Kirkendall effect.46 The connections between supersaturation, the preferred crystal growth mechanism, the energetics of dislocations, and the final morphology present unifying concepts to explain and control the variety of reported 1D nanomaterial morphologies (NR, NW, NT). In certain contexts, screw dislocations can also be used to explain chiral carbon NTs47 or other NTs of layered structures.48 In fact, we could consider such NTs with layer structures as the most extreme cases of thin-shelled NTs (only an atomic layer or molecular layer thin) with dramatic Eshelby twist (the chirality and chiral angles in the traditional description). Furthermore, we are using dislocation growth and elasticity theory to explain the spontaneous formations of other intriguing nanostructure morphology and crystal growth phenomena. These studies will provide the general and unifying concepts for many nanomaterial morphologies that are commonly observed but are often unconvincingly explained. Generality of Dislocation-Driven 1D Nanomaterial Growth and Future Directions. Dislocation-driven NW growth is a fundamental advance that promises to create a new dimension in the rational design and synthesis of 1D nanomaterials. It is a general mechanism that should be applicable to many materials including PbQ, ZnO, GaN, InN, AlN, SiC, CdS, and many others, grown from both solution and vapor phase. While some of these materials (GaN, SiC) are known to be prone to have screw dislocations, some others are not. Besides the examples of PbS, PbSe, and ZnO confirmed by us so far, there is strong evidence for GaN in the recent literature.49,50 We are working on confirming that this mechanism is at play in many more materials. Dislocations are the “facts of life” for any crystalline material, and almost no crystals can be exempt from them.30,31 Often, the more important question is whether or not conditions are suitable for dislocationdriven NW growth to dominate and reveal itself. No one would have predicted beforehand that PbS would turn out to be the material that exemplifies the dramatic and rich morphologies that can result from dislocation growth. Despite originating earlier than the VLS mechanism, the dislocation growth mechanism has been far underappreciated in modern NW literature. We suggest that for many cases where the growth mechanism is inconclusively explained and especially when free of catalysts, the possibility of dislocation growth should be carefully considered and examined. However, we need to caution that it is well-know that dislocations are mobile and especially not stable in small volumes,30,31 so that postgrowth mechanical perturbation (such as the processes of removing the synthetic products from their growth substrate to prepare the TEM specimen) could work the dislocation out of the nanomaterials. This could explain the difficulty, the inconsistency, and perhaps the lack of reports in observing dislocations in the final NW products. Therefore, failure to


observe the dislocations consistently in samples using TEM should not be the definitive proof against dislocation-driven NW growth. Other signs that result from screw dislocations (or left behind by screw dislocations once existent in the lattice), such as hollow tubes and Eshelby twist, should also be carefully examined as supporting evidence for dislocation-driven growth. In contrast, it is obviously much easier to carry out a search for apparent catalyst caps, and once such catalysts are found, it is conceptually rather comforting to attribute the NW growth to the VLS mechanism. However, there might be cases when metal catalysts are employed and VLS growth is believed to be in operation, and yet, both dislocation-driven growth and VLS growth are in existence and competing with each other (so that dislocation-driven growth is overwhelmed and masked by the VLS growth). There are also numerous examples where even though metal catalysts were intentionally employed, the phase diagram cannot support the operation of VLS growth (or VSS growth); therefore, it could even be possible that metal catalysts merely “happen to be there” and play some auxiliary role in dislocation-driven growth. We suspect that many more examples of dislocation-driven NW growth will be discovered when we look more carefully and understand how to look for them.

Dislocation-driven NW growth is a general mechanism that should be applicable to many materials but has been far underappreciated in modern NW literature. Even though we have empirically found the synthetic methods to accomplish dislocation-driven growth for the case of PbQ (tuning hydrogen flow in CVD) and ZnO (creating defective seed crystals), it is presently not completely understood how the initial dislocation responsible for the anisotropic crystal growth originates. While some general discussions on the origins of dislocations exist30,31 and the intuitive explanations for the PbQ and ZnO cases (a sudden increase of supersaturation resulting in rapid and “imperfect” deposition of crystallites and films that leads to the creation of dislocations) are consistent with these general discussions, an experimentally observed and/or theoretical mechanistic understanding with atomic-level insights is currently lacking. These questions perhaps can be better pursued now with the much improved experimental and theoretical methods available than they could have in the earlier work on dislocation theory, which heavily relied on continuum mechanics. We would like to understand more about how screw dislocation growth spirals on crystal facets evolve into 1D objects with the dislocation spirals on top, perhaps under the influence of dramatic supersaturation changes or impurities.36 Such new insights might allow us to engineer screw dislocations to enable more rational and controllable dislocation-driven 1D nanomaterial growth in the future. More theoretical development on the specific applications of crystal growth theory in

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photogenerated carriers (Figure 6A,B).54 We further propose that more “complex” nanostructures, both in the form of hierarchically branching/hyperbranching NW structures20,23,55 and in the form of multicomponent NW heterostructures of diverse materials, are more promising for solar energy harvesting and conversion.53 The increased scattering improves light absorption, and the dendritic structure optimizes carrier collection in these hierarchical branching nanostructures (Figure 6C,D). The simultaneous operation of both the VLS and dislocation growth mechanisms, once understood and well-controlled, could enable controllable synthesis of elaborate hierarchical nanostructures (Figure 6E) for applications in solar energy conversion. Another advantage is that at nanoscale dimension, the lattice-match requirement between heterojunctions is relaxed, which can allow a much more diverse set of materials to be utilized for high-quality heteroepitaxial junctions that would be impossible in bulk films.53 This was demonstrated in our example of epitaxial growth of PbS NWs on single-crystal TiO2, mica, and NaCl.56 Complex nanostructures may be more ideal for photoelectrocatalysis because solar fuel production does not require external electrical contact but benefits from the high surface area and therefore can better take advantage of chemically synthesized nanostructures. However, the actual applications of these complex nanostructures can be challenging due to the drastic departure from the conventional paradigm of planar p-n junction solar cells. We need to investigate the fundamental challenges and explore potential strategies to utilize such branching nanostructures for solar energy conversion. Our fundamental studies on dislocation-driven nanomaterial growth not only enhance the understanding of anisotropic crystal growth but also open up the exploitation of large-scale/low-cost solution growth for rational catalyst-free synthesis of 1D nanomaterials for diverse applications. Particularly for the very important applications of NWs and NTs in renewable energy,12 such as solar energy conversion, energy storage, and thermoelectrics, large amounts of ecologically sustainable nanomaterials made at low cost are needed to address the terawatt-scale energy challenges that we face.57 While synthesizing NWs via a catalyst-driven high-temperature/vacuum CVD process is both important and necessary to understand the fundamental science and build the foundation for real applications,58,59 the practical solutions will likely have to come from nanomaterials prepared through processes far less expensive and energy intensive than those currently employed in the semiconductor industry. Catalyst-free aqueous solution growth is arguably the least expensive synthesis that can be carried out at a large scale. While ZnO was the specific example demonstrated so far, we have provided a general theoretical framework for controlling solution NW/NT growth37 that can be applicable to other abundant and stable semiconductor materials that are promising for solar energy applications.60 Some of these materials were previously considered to be “poor” semiconductors whose carrier density, mobility, or minority carrier diffusion length are unsuitable for solar energy applications, but they could become interesting if the new design concepts based on NWs and NW heterostructures can circumvent their issues. We envision utilizing these understandings on the dislocation-driven growth mechanism to

Figure 6. (A-D) Schemes illustrating the advantages of complex hierarchical NWs over vertical NWs and planar devices for solar energy conversion. An example of the “forest of PbS NW trees” is shown in (E). A-D are adapted from ref 53 with copyright permission from RSC, and E is adapted from ref 20 with copyright permission from AAAS.

the low supersaturation regime and for more complicated and realistic chemical systems and more mechanistic examination of the dislocation-driven growth process using, for example, state-of-art in situ electron microscopy51 could further our understanding of these 1D growths. These understandings will build the foundation for rationally designed and deliberately controlled dislocation-driven nanomaterial growth in the future. We will not be surprised that by following these understandings, more dislocation-drive NW/NT growth, and possibly even pine tree like NW growth, can be achieved in other material systems. Applications and Implications of Dislocation-Driven 1D Nanomaterial Growth. The 1D nanostructures that are grown by the dislocation mechanism open the door to more fundamental studies of the physical properties of dislocations. It is wellknown that the dislocations in bulk crystalline materials are largely responsible for the observed mechanical properties30,31 and have great influence on electronic, magnetic, and thermal properties.52 However, until now, the effects of dislocations on these properties have been studied through bulk measurements with estimated dislocation densities. The dislocation-driven NWs can provide a platform for the investigation of the effects of a single dislocation on the physical properties of materials at the nanoscale. The dislocations in these nanostructures may possess interesting physical properties, which, if understood and well-controlled, could lead to novel device applications. The complex hierarchical nanostructures shown here can have advantages in solar energy harvesting and 3-D nanoelectronics. Nanotechnology offers new approaches to solar energy conversion that promise higher efficiencies and lower costs.12,53 Nanowire materials are advantageous in photovoltaic (PV) and photoelectrochemical (PEC) device applications compared with conventional planar single-crystalline or thin-film materials because they have a long axis to absorb incident sunlight yet with a short radial distance to separate the


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develop the nanomaterial synthesis of unconventional semiconductors that can be carried out on a large scale in an economical manner in aqueous solutions.

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AUTHOR INFORMATION

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Corresponding Author:

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*To whom correspondence should be addressed. E-mail: jin@ chem.wisc.edu.

Biographies

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Song Jin received his B.S. from Peking University in 1997 and his Ph.D. from Cornell University in 2002 and has been an assistant professor of chemistry at the University of Wisconsin;Madison since 2004. He is interested in the chemistry and physics of nanomaterials. Jin studies the fundamental formation mechanisms of nanowires, their novel physical properties, and applications in solar and thermoelectric energy conversion, nanospintronics, and nanomedicine. Matthew J. Bierman received his B.S. from UW;La Crosse in 2003 and his Ph.D. from the University of Wisconsin;Madison in 2009 and is currently a postdoctoral fellow at the California Institute of Technology. His research is motivated by materials development for energy applications. His thesis work centered around the dislocation-driven nanowire growth mechanism, which was recognized with a 2009 MRS Student Gold Award. Stephen A. Morin received his B.S. from the University of Texas at Austin in 2005 and is expecting to receive his Ph.D. from the University of Wisconsin;Madison in 2010. His research is motivated by the challenges of synthesizing and assembling functional materials at the nanoscale. He adapts fundamental crystal growth concepts to low-temperature solution-based bottom-up nanomaterials synthesis/assembly strategies.

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ACKNOWLEDGMENT This research is supported by NSF (CAREER DMR 0548232). S.J. also thanks the Sloan Research Fellowship, Research Corporation Cottrell Scholar Award, DOE (DE-FG0209ER46664), Exxon Mobil Solid State Chemistry Fellowship, and DuPont Young Professor Grant for support. S.A.M. thanks UW; Madison NSEC (NSF DMR 0425880&0832760) and a 3M Graduate Research Fellowship for support. We thank our colleagues Y. H. A. Lau, D. Chernak, and J. Tong for their contributions to the research discussed in this Perspective.

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