Patent Application
20140220340
Branched nanowires
Branched nanowires, also known as nanotrees, are versatile building blocks that increase achievable complexity in 3D architectures. Developing techniques capable of enhancing directed growth of nanotrees is essential to fully realize their potential for nanodevices. Nanotrees often serve as distributed conductors within a device's active volume, and such percolation networks are applicable in water splitting or photovoltaics. Attaining the electrical connections between adjacent nanowires required for these applications can be accomplished by joining branches, forming 3D networks of nanowires. Unfortunately, producing interconnections between adjacent nanowire branches has, to date, required epitaxial growth, and periodically patterned catalyst seed layers.
Nanotree growth is typically achieved via secondary growth of branches on the side of a vertically aligned nanotree trunk. Vapor-liquid-solid (VLS) growth has been commonly used to direct nucleation and growth of nanotree trunks and branches, although dislocation driven trunk growth has also been demonstrated. Recent progress in controlling branch morphology includes adjusting branch density along the nanotree, placing branches selectively along the height of the trunk, growth of hyperbranched structures, and modulating the branch diameter during growth. Controlled branch placement on selected sides of the trunk has remained elusive. The capability to place branches on a selected side of the trunk would allow for improved control over connectivity in nanowire networks.
Indium tin oxide (ITO) is a transparent conductive oxide (TCO) which is commonly used in organic light emitting devices, flat-panel displays and touchscreens, and organic solar devices. The organic photovoltaic (OPV) community has shown interest in nanostructured ITO films for high surface area electrodes. OPVs are often limited either by light absorption or charge extraction. High surface area electrodes decouple these two limiting factors by allowing for increased absorption while maintaining short charge extraction distances. Conductive pathways throughout the active layer allow the active layer thickness to be increased beyond the exciton diffusion length, motivating development of nanostructured electrodes.
Nanostructured electrodes with increased surface areas have been fabricated using anodic aluminum oxide templates, organic vapor phase deposition and vapor-liquid-solid (VLS) growth of high aspect ratio nanowires. VLS is a crystal growth technique that results in high surface area nanostructures that grow primarily in one dimension. ITO nanowire films can be grown with a self-catalyzed VLS growth mode accessible at elevated substrate temperatures. Beyond applications as high surface area electrodes, ITO nanowires are applicable as gas sensors, protein molecule sensors and UV light sources.
During physical vapour deposition of ITO, self-catalytic indium-tin alloy liquid droplets form on the substrate's surface if the substrate is heated above the alloy's eutectic point. The liquid droplet collects impinging vapour atoms as it has high sticking coefficient (the ratio of atoms adsorbed to the total number of atoms incident on the surface) with respect to the substrate. Nucleation occurs preferentially at the droplet-substrate interface, restricting the nanowire's lateral growth. This results primarily in crystal growth in one dimension, with the primary axis (trunk) normal to the liquid-solid interface.
ITO nanowires exhibit distinctive growth of branches orthogonal to the axial growth direction of the nanowire, reflecting the cubic bixbyite crystal structure of indium oxide. The branching mechanism produces high surface area nanowires, which may improve electrical access to OPV active device layers. ITO nanowire growth has been reported with a large range of substrates, flux rates, vapour incidence angles and deposition techniques.
The vapor-liquid-solid (VLS) mechanism, whereby growth proceeds by precipitation from a liquid catalyst that concentrates the surrounding vapor, has been used to grow crystalline nanowires. Branched nanowires (or nanotrees) can be formed by placing catalytic droplets on the sides of nanowires during growth via a stochastic or engineered process, enabling bottom-up fabrication of complex three-dimensional architectures. Control over intra-wire morphology has also been investigated, with several groups reporting variations in nanowire diameters. Givargizov and others suggest an unstable model of self-oscillations based on droplet contact angle and surface roughness driven by droplet supersaturation. Most of these reports attribute rippled (or bamboo) nanowire structures to this self-oscillatory growth model. However, others have demonstrated discontinuous diameter changes through annealing-driven catalyst migration, switching between different crystal cross-sections during growth, and segmented nanowire morphologies controlled by carrier gas pulsing.
Recently, a geometrical modification of VLS growth through glancing angle deposition (GLAD) was developed. For the self-catalyzed ITO system, this technique (VLS-GLAD) provides additional control over nanowire diameter, spacing, and branching behavior.
Glancing-angle-deposition (GLAD) is a physical vapor deposition technique that utilizes a spatially modulated vapour flux to control thin film morphology.
A method of producing branched nanowires comprising exposing a substrate to a directional vapor flux, the vapor flux comprising a growth material, the substrate being exposed to the vapor flux under conditions of vapor-liquid-solid growth of the branched nanowires, the conditions including a temperature of the substrate, the vapor flux having a deposition angle representing the angle between a normal direction of the substrate and the direction of the vapor flux, an azimuthal angle representing an angle of the projection of the direction of the vapor flux in the plane perpendicular to the normal direction of the substrate, and a flux rate, and varying at least one of the deposition angle, the azimuthal angle, the flux rate and the temperature to control a property of the branched nanowires.
In various embodiments, there may be included any one or more of the following features: the branched nanowires may have crystal facets and at least one of the deposition angle, the azimuthal angle and the flux rate may be controlled to selectively and sequentially direct the vapor flux to a subset of the crystal facets of a nanowire of the branched nanowires. This method may be used to align in azimuth a plurality of the branched nanowires. The alignment of the plurality of nanowires may change with height. At least one additional source of vapor flux may be used. If there is at least one additional vapor source, at least one of the orientation, position, flux rate and material of the additional vapor source may vary in time. The nanowires may be grown on a substrate with an epitaxial relationship to the nanowires' growth material. The nanowires are grown on a substrate without an epitaxial relationship to the nanowires' growth material. The substrate may be one of a semiconductor, glass, smooth thin film, columnar thin film, or nanostructured thin film. The substrate may have surface properties causing the nanowires to cluster in a region of the substrate. The surface properties of the substrate may be geometrically designed to invoke a specific behavior in the growing nanowires. The azimuthal direction of the vapor flux may rotate continuously clockwise or continuously counterclockwise. The azimuthal direction of the vapor flux may oscillate with an amplitude between 0 and 360 degrees. The azimuthal direction of the vapor flux may change discretely.
All permitted combinations of the disclosed embodiments.
These and other aspects of the device and method are set out in the claims, which are incorporated here by reference.
Embodiments will now be described with reference to the figures, in which like reference characters denote like elements, by way of example, and in which:
A method of producing branched nanowires, the method comprising exposing a substrate to a vapor flux at a deposition angle under conditions of vapor-liquid-solid growth and varying at least one of vapor flux, deposition angle and azimuthal angle. Specific designs of branched nanowires are also disclosed.
A. Vapor liquid solid growth of nanostructures on a substrate where the incident vapor flux is directional and a deposition parameter is varied.
B. The method of embodiment A where the parameter is varied or modulated as a controlled function in time.
C. The method of embodiment A where a parameter for one or more vapor sources is varied.
D. The method of embodiment A where the parameter is the angle between the normal direction of the substrate and the direction of vapor flux.
E. The method of embodiment A where the parameter is the rate of vapor flux.
F. The method of embodiment A where the parameter is the azimuthal angle of the direction of vapor flux about the normal direction of the substrate.
G. The method of embodiment A where the parameter is the temperature of growth.
H. A method for growing crystalline nanowires, and dependent and/or connected epitaxial or non-epitaxial branches thereof, where a plurality of the nanowires are aligned in azimuth, aligned in altitude, and in both azimuth and altitude, through selectively and sequentially directing growth material's vapor to a subset of the growing nanowire's crystal facets
I. The method of embodiment H where the where the growth material vapor flux may be directed by one or more discrete sources, whose relative orientation, position, rate, and material may be a function of time
J. The method of embodiment H where the alignment of the nanowires, and dependent epitaxial or non-epitaxial branches thereof, changes with height
K. The method of embodiment H where the growth material's vapor arrival rate at a subset of the growing nanowire's crystal facets oscillates continuously around a fixed value, or in a sequence of fixed values
L. The method of embodiment H where the growing crystalline nanowires are grown on a substrate, with or without an epitaxial relationship to the nanowire's growth material, such as, but not restricted to, semiconductor, glass, smooth thin film, columnar thin film, or nanostructured thin films
M. The method of embodiment H where the growing nanowires are grown on a substrate with periodic or nonperiodic surface properties, such that the growing nanowires cluster in a given region
N. The method of embodiment M where the surface properties of the substrate are geometrically designed to invoke specific behavior in the growing nanowires.
O. The method of embodiment H where the growth material's azimuthal direction increases continuously.
P. The method of embodiment H where the growth material's azimuthal direction oscillates continuously, with an amplitude between 1 degree and 359 degrees, around a single azimuth.
Q. The method of embodiment H where the growth material's azimuthal direction increases discretely
R. The method of embodiment H where the growth material's altitude changes in discrete steps, producing layers with different branching character
S. All permitted combinations of the disclosed embodiments
Vapor-liquid-solid (VLS) growth is a method for nanowire structures. Vapor collects in the liquid droplet until it reaches a supersaturated state. A crystal solid is precipitated from the droplet, typically onto a substrate. The droplet is lifted by the growing nanowire, and through control of the droplet's chemical and physical properties, the nanowire's nature may be controlled. The phase diagram of the liquid alloy indicates what phase of crystal is precipitated in order to reestablish equilibrium.
VLS growth requires a growth material and compatible catalyst in terms of solubility, surface energy, and phase at growth temperature. The alloy of growth material and compatible catalyst in the case of a non-self-catalytic growth material must be liquid at growth temperature, and the precipitated solid must be the desired phase, as determined by the phase diagram of the alloy.
Exploitation of unique aspects of vapor-liquid-solid glancing angle deposition (VLS-GLAD) produces new nanostructure types and nanostructure control. The VLS-GLAD technique is extremely flexible, and should be compatible with any material capable of vapor-liquid-solid growth and on any substrate compatible with the material chemistry. Our technology in its various aspects can control branch height, orientation, and branch diameter through precise control of growth conditions. Substrate epitaxy is not required, but is complimentary.
The requirements for VLS catalysts according to Wikipedia are:
The catalyst must form a liquid solution with the crystalline material to be grown at the nanowire growth temperature.
The solid solubility of the catalyzing agent is low in the solid and liquid phases of the substrate material.
The equilibrium vapor pressure of the catalyst over the liquid alloy must be small so that the droplet does not vaporize, shrink in volume (and therefore radius), and decrease the radius of the growing wire until, ultimately, growth is terminated.
The catalyst must be inert (non-reacting) to the reaction products (during CVD nanowire growth).
The vapor-solid, vapor-liquid, and liquid-solid interfacial energies play a key role in the shape of the droplets and therefore must be examined before choosing a suitable catalyst; small contact angles between the droplet and solid are more suitable for large area growth, while large contact angles result in the formation of smaller (decreased radius) nanowires.
The solid-liquid interface must be well-defined crystallographically in order to produce highly directional growth of nanowires. It is also important to point out that the solid-liquid interface cannot, however, be completely smooth. Furthermore, if the solid liquid interface was atomically smooth, atoms near the interface trying to attach to the solid would have no place to attach to until a new island nucleates (atoms attach at step ledges), leading to an extremely slow growth process. Therefore, “rough” solid surfaces, or surfaces containing a large number of surface atomic steps (ideally 1 atom wide, for large growth rates) are needed for deposited atoms to attach and nanowire growth to proceed.
Some known VLS Materials are shown in the table below
Some growth materials require a distinct catalyst materials, others, like ITO, are self-catalyzing.
The number density of nanowires (i.e. number per area) can be controlled by selecting a self-catalyzed VLS material and deposit within VLS regime parameters. For indium tin oxide (ITO), nanotree density can be tuned by deposition angles between ˜50°≦α≦90°, and by flux rates between 0.05 nm s−1 and 3 nm s−1.
Branched nanowires, or nanotrees, can be formed in catalyzed and self-catalyzed systems via the placement of catalyst material on the sides of trunks. VLS-GLAD enables selective placement of catalyst material on desired growth surfaces for catalyzed and self-catalyzed systems, enabling enhanced branch placement and orientation control. This has been demonstrated for self-catalyzed VLS growth of ITO nanotrees.
Nanowire diameter and branch density along the axis of a nanotree may be tuned by varying deposition angle and flux rate. Nanowire trunk diameter increases with flux rate in the range of 0.05 nm s−1 to 3 nm s−1, and decreases with deposition angle between 70° and 90°.
Control of average branch placement about the azimuthal angle of a nanowire at a specific height is accomplished by adjusting the restricted azimuthal range of incident flux as a function of height, and requires control of the trunk's azimuthal orientation with respect to the material flux. The restricted azimuthal range can be constant, vary continuously, or in discrete steps as a function of height; each produces a different structure.
Branch diameter can be controlled along the branch axis by controlling the incoming flux to the branch in two ways: (i) adjusting the local shadowing of a branch through control of azimuth and deposition angle, and (ii) varying the flux rate.
Nanowire structure alignment in the substrate plane is achieved through controlling the trunk's azimuthal orientation. The azimuthal distribution of the material flux should be matched to the VLS material's crystal properties (2-fold, 3-fold, 4-fold or 6-fold, rotational symmetry), so that the azimuthal positions of the flux throughout the course of the deposition reflect the n-fold symmetry of the desired structures. Preferential branch alignment results in films of nanotrees grown using this technique. No epitaxial relationship with the substrate is required.
Density can be modified with film height by modulating a during growth between low (0°≦α≦˜50°) and high (˜50°≦α≦90°) values so that the morphology of the growth switches between randomly networked (low α values) and branched nanotrees (high α values).
Deposition angle (α): The angle between the source material flux and substrate normal.
Azimuthal angle (φ): The angle of azimuthal rotation (i.e. about substrate normal).
Flux rate: The rate of material vapor flux incident on the substrate. Measured in equivalent planar thickness per time (nm s−1).
Number density: The number of objects in a given area (areal number density), or number of objects along a certain distance (linear number density).
Trunk: The part of the branched nanotree that has branches attached to it, typically attached to a substrate or surface other than another nanotree.
Branches: The nanowires that are attached to another nanowire (or trunk).
Growth material: The material that the nanotree is comprised of.
Catalyst material: The material which makes up the catalyst droplet for VLS growth. The catalyst material is the same as the growth material for self-catalyzed systems.
Catalyst Seed: Catalyst material droplet that can grow a nanowire.
Self-catalyzed: A material system where the material necessary for a VLS catalyst droplet is the same as, or included in, the growth material.
Nanotree: A branched nanowire.
Deposition angle can be used to control of nanotree areal number density and branch linear number density as follows: select a material that undergoes self-catalyzed VLS. These include, but are not limited to indium tin oxide (ITO), gallium nitride, indium phosphide, zinc oxide, etc; and deposit material at glancing angles (˜50°≦α≦90°) onto substrate heated to required temperature for self-catalyzed VLS. The choice of a affects both the areal number density of the nanotrees and the linear number density of branches along the trunk.
Flux rate can be used to control nanowire density and branch density as follows: select a material as in the previous paragraph; and for a given a, the areal number density and diameter of the nanotrees can be adjusted by changing the flux rate. We have demonstrated this effect for flux rate values between 0.05 nm s−1 and 3 nm s−1. Increased flux rates produce nanotrees with larger diameters (trunk and branch), lower areal number density and lower linear number density of branches.
Branched nanowires can be formed via VLS-GLAD through the placement of catalyst material on the sides of trunks. For materials that are not self-catalyzed a two-step process is necessary. Catalyst material can be deposited on the sides of a trunk, followed by deposition of growth material at appropriate VLS growth conditions.
A one-step process is possible when the growth material is self-catalyzed. Simultaneous deposition of the growth material and catalyst material will place catalyst droplets on the sides of trunks and grow them. This has been demonstrated.
VLS-GLAD allows for catalyst material to be placed selectively on desired growth surfaces, improving branch placement.
Flux rate can be used for branch and diameter control along the height of nanotrees. The types of branches and trunk/branch diameter at a given height can be controlled through flux rate using the following process steps. The process also controls the diameter of trunks in branch-free VLS systems.
For each height or layer (minimum of two), determine the required flux rate to achieve desired branching and diameter characteristics.
Deposit the first layer at the flux rate to produce the required diameter and branching character.
Change flux rate to that required for the next layer to produce the required branching and diameter character. The rate of change in flux rate will influence the transition between layers.
Repeat the steps of depositing a layer and changing the flux rate to that required for the next layer until the desired structure has been achieved.
The flux rates that have been shown to work are between 0.05 nm s−1 and 3 nm s−1 for ITO.
The density of branches along the height of the nanotrees can be modulated by changing the deposition angle during growth.
For each height or layer (minimum of two), determine the required deposition angle to achieve desired branching characteristics.
Deposit the first layer at the flux rate to produce the required branching character.
Change deposition angle to that required for the next layer to produce the required branching character. The rate of change in deposition angle will influence the transition between layers.
Repeat the steps of depositing a layer and changing the deposition angle until the desired structure has been achieved.
Control of branch placement about the azimuthal angle of a nanowire is accomplished by adjusting the restricted azimuthal range of incident flux as a function of height, and requires control of the trunk's azimuthal orientation with respect to the material flux. There are three distinct processes which restrict azimuthal orientation of the branches, and a fourth general category that combines the azimuthal restriction with deposition angle and rate modulation. These methods described here focuses on self-catalyzed materials, analogous procedures for catalyzed VLS materials are available.
Branch placement has been demonstrated for the self-catalyzed ITO system, and should be extendable to both catalyst seed placement and branch growth for catalyzed systems.
Branches can be placed selected sides of a trunk at all heights by the following:
Determine the required deposition angle and flux rate to achieve desired branching characteristics.
Deposit the VLS material without changing azimuth.
We have demonstrated this structure for different deposition angles.
Branches can be placed on different sides of the trunk as a function of height. The average side a branch is on may continuously vary as a function of height. Examples include chiral (azimuth increases in one direction), bi-directional (azimuth increases in one direction, then reverses), or oscillatory (azimuth oscillates around a single azimuth, as in PhiSweep). These may be achieved using the following process steps.
Determine the required deposition angle and flux rate to achieve desired branching characteristics.
Deposit the VLS material using the desired azimuthal pattern
We have demonstrated this structure for azimuthal rotation rates between 1 nm/rotation and 1000 nm/rotation. The bi-directional and oscillatory modes achieve preferential growth on three azimuthally adjacent branching directions, or two opposing branching directions.
Branches may be placed on different sides of the trunk as a function of height, with the average side varying periodically as a function of height. Examples include chevron or polygonal spiral structures, where the azimuthal direction of the vapor flux changes periodically.
For a self-catalyzed VLS material, this may be achieved with the following steps.
Determine the required deposition angle and flux rate to achieve desired branching characteristics.
Deposit the VLS material using the desired azimuthal pattern.
We have demonstrated chevron, or two-fold periodically varying orientations. Analogous procedures are available for catalyzed VLS materials.
The techniques described here can be used in combination to provide additional control over branch placement. For example, flux rate and azimuthal orientation can be used in combination to control branch density as a function of height.
Branch diameter can be controlled along the branch axis by controlling the incoming flux to the branch.
To control branch diameter by shadowing, the following steps may be used.
Adjust motion in the φ and possibly α so that branches move in and out of the flux shadow cast by neighbouring structures.
The rate of motion must be optimized so that shadowing occurs at a longer time scales than the droplet kinetics; there must be sufficient time for precipitation out of the droplet to cause it to shrink and absorption of the droplet to cause it to grow during a shadowing cycle.
The rate of motion in step will be affected by other deposition conditions such as temperature, effective flux rate, and residual gas concentration. This has been demonstrated for ITO at flux rates in the range of 0.05 nm s−1 to 3 nm s−1 with rotation period dependent on the flux rate.
To control branch diameter by macroscopic flux control, the following steps may be used.
Modulate the flux rate at the vapor source to create starve and feed cycles at the droplet. For example, shuttering of the vapor source can be used to turn the flux on and off.
A combination of shadowing and macroscopic flux control can be used simultaneously to control rippling (branch diameter).
The local flux modulation rate performed by shadowing, macroscopic flux control or a combination thereof must be optimized so that starve-feed cycles at the droplet occur at longer time scales than the droplet kinetics; there must be sufficient time for precipitation out of the droplet to cause it to shrink and absorption of the droplet to cause it to grow during each cycle. Optimization of the flux modulation rate will be affected by other deposition conditions such as temperature, effective flux rate, and residual gas concentration.
Alignment of the nanowire structures in the substrate plane is achieved through control of the trunk's azimuthal orientation by the following steps.
Determine the in-plane symmetry for structures that are aligned with their growth axis parallel (on average) to the substrate normal. The structures can be branched or unbranched nanostructures deposited at some arbitrary deposition angle α. For example the structures might exhibit 2-fold, 3-fold, 4-fold or 6-fold, rotational symmetry in-plane of the substrate depending on the crystal structure of the material and the epitaxial relationship between the branches (if any) and trunk. This symmetry can be found by X-ray diffraction techniques (pole figures) or from direct imaging of the structures with plan-view electron micrographs. Imperfections from ideal symmetry may be seen in electron micrographs and are tolerated by this process.
From knowledge of the in-plane symmetry design the motion of the flux for a separate deposition so that the azimuthal positions of the flux throughout the course of the deposition reflect the n-fold symmetry of the structures. Lower order symmetries can be used, for example 3-fold flux for 6-fold structures and 2-fold flux for 4-fold structures.
Adjust the starting position of φ during the deposition to accommodate relative orientation with the substrate if desired.
Deposit a second film using the n-fold azimuthal symmetry of the flux. This can be accomplished in several ways.
With one vapor source, by rapidly moving between discrete cp positions, for example φ=0°, φ=90°, φ=180° and φ=270° in 4-fold cases. The speed of rotation between these positions is often important.
With multiple vapor sources, by simultaneously depositing from discrete φ positions.
With one vapor source, or multiple vapor sources, by designing some other motion pattern where the distribution of flux in the azimuthal direction reflects the natural symmetry of the structures.
Choice of motion made in depositing the second film will affect the film thickness required to achieve in-plane alignment. To achieve in-plane alignment with less film thickness, optimization is required. For example the speed of rotation and dwell times at each φ position can be adjusted. Additionally, the choice in optimization will also be affected by other deposition parameters, such as temperature, flux rate and residual gas concentration.
To modify density with film height, modulate α during growth between low and high values so that the morphology of the growth switches between randomly networked (low values) and branched (high values).
For each layer (minimum of two), determine the required deposition angle α to achieve either branched or randomly networked morphologies.
Deposit the first layer with the desired morphology at α1.
During deposition move to α2 to deposit the second layer. The rate of change in α will influence the transition between layers.
Repeat step of depositing a layer with α3, α4, α5 . . . until the desired structure has been achieved.
A new growth technique for indium tin oxide nanowires with increased control over feature size and spacing is reported. The technique is based on a unique combination of self-catalyzed vapour-liquid-solid (VLS) growth and glancing-angle deposition (GLAD). Spatial modulation of the collimated growth flux controls trunk width, number and orientation of branches, and overall nanowire density. Here we report the influence of growth conditions (including deposition angle, flux rate, nominal pitch and substrate temperature) on nanowire morphology, with specific focus on the effect of large deposition angles. Sheet resistance and transmission of the films were measured to characterize their performance as transparent conductive oxides. Hybrid nanostructured films grown in this study include high surface area nanowires protruding from a conductive film, ideal for transparent conductive electrode applications.
Our group has been studying the optical and device performance of nanostructured GLAD ITO thin films deposited using electron-beam evaporation.
By combining GLAD's control over the vapour flux with a heated substrate to induce VLS growth of ITO, we are able to precisely control ITO nanowire morphology in a hybrid crystal growth technique named VLS-GLAD. In terms of a parameter space of temperatures and deposition angles, The VLS-GLAD region resides at elevated temperatures and high deposition angles (α). GLAD has high deposition angles, and VLS has elevated temperatures. Ordinary planar deposition may have low temperatures and low deposition angles. Here we report a systematic characterization of the effects of flux rate, angle of vapour incidence relative to substrate normal (α), pitch (nominal film thickness deposited per substrate rotation) and substrate temperature on ITO nanowire morphology. The density of preferential nucleation sites, and subsequently trunk density, is determined by the extent of geometric shadowing (controlled by α) caused by catalytic liquid indium-tin alloy droplets during nucleation. In addition, unique ITO nanowire morphology is obtainable via manipulating deposition angle, pitch and flux rate during growth. We examine the effects of deposition parameters on the nanowire morphological properties expected to optimize OPV performance when used as an electrode, including trunk diameter, branch diameter (defined in
Experimental Setup
Film Fabrication
ITO nanowire films were deposited using a high vacuum (base pressure below 0.1 mPa) electron beam evaporation system with a custom substrate motion controller capable of precisely controlling a (angle of vapour incidence) and φ (angle of substrate rotation around substrate normal). Substrate temperature was controlled by two halogen lamps illuminating substrates on a copper chuck. Temperature was monitored by a cold-junction corrected (ASTM E2730-10 standard) thermocouple held approximately 1 cm above the substrate holder. This position exposed the thermocouple to the same vapour flux and radiative loading as the substrate, but still allowed for substrate rotation. The distance between the source and substrate was 42 cm. The ITO source composed of pure 3 mm to 12 mm pieces of In2O3:SnO2 (91:9% mol.; 99.99% purity; Cerac, Inc.). No process gas was added during deposition. The films were grown on Si wafers (<100>; p-type; University Wafer), quartz (Quartz Scientific), B270 glass (Howard Glass Co.) and commercial ITO on float glass (Delta Technologies, Ltd.). The films were deposited at across a range of depositions angles (30° to 90°), pitch values (1 nm nominal to 1000 nm nominal), flux rates (0.05 nm/s nominal to 2 nm/s nominal) and substrate temperatures (80° C. to 240° C.). We estimate effective source diameter to be approximately 1 cm. This large source area permits deposition even at deposition angle of 90°, as the source is not fully occluded by the substrate holder.
A two-stage annealing process was performed on the nanowire films deposited on quartz substrates to improve the properties as a transparent conductor. The first stage anneal was initiated with a ramp rate of 10° C. per minute from room temperature to 500° C. in atmosphere then held for 90 minutes. The samples were removed after passively cooling to 90° C. Sample transmissivity increased after this first stage, but conductivity was typically decreased. The second stage anneal was performed in a three-zone tube furnace. Forming gas consisting of 5% H2 (balance Ar) was flown over the samples at a rate of 100 standard cm−3·min−1 during anneal. The samples were ramped linearly to 375° C. in 90 minutes, held at 375° C. for 60 minutes, and then allowed to passively cool to below 90° C. before removal. Conductivity was typically increased after the second anneal stage with little change to the transmissivity.
Film Characterization
A scanning electron microscope was used to image the films (15 kV accelerating voltage). Films deposited on Si substrates were imaged without further coating, whereas films deposited on quartz, B270 glass, or commercial ITO on float glass were coated with approximately 10 nm Cr to reduce charging. Trunk diameter, droplet diameter, branch diameter, trunk density, and nearest neighbour distance were measured by analyzing cross-sectional and top-down SEM images of uncoated ITO nanowires on Si substrates. Length and diameter values were averaged over five measurements using image analysis software. Nearest-neighbour distance was estimated as the mean of a gaussian distribution fit to the distances between all unique trunk pairs for each image. X-ray diffraction patterns were taken on the films deposited on Si substrates using a Cu Kα, radiation, a 0.5 mm collimator, and an area detector at a distance of 15 cm. Transmission electron microscopy (TEM) was used to image select ITO nanowires, and to further analyze the crystal structure. Transmission spectra of the films on quartz substrates were taken from 185 nm to 3300 nm using a spectrophotometer. Film sheet resistance (quartz substrate) was measured using a four point probe. Transmission and sheet resistance measurements were conducted before and after each annealing stage. Film performance as a TCO was determined by Haacke's figure-of-merit, defined by:
where Rs is sheet resistance, T is optical transparency (averaged over 400 nm<λ<780 nm) and x=10, typically. Typical device grade planar ITO films have 2·10−3·Ω−1<FH<22·10−3·Ω−1.
Nanowire Morphology
Cross-sectional and top-down SEM images of VLS-GLAD ITO nanowire films deposited with an increasing deposition angle (α) were taken. At elevated α, larger droplets may starve smaller droplets of flux via an increase in geometric shadowing, leading to a decreased subset of droplets producing trunks. Increasing deposition angle results in the growth of fewer nanowires over the same substrate area, consistent with the competitive growth model used to describe reduced number density of features in GLAD films.
The relationship between deposition angle and number density of trunks, nearest neighbour distance, number of branches and trunk diameter is quantified in
Nominal flux rate has a strong impact on film morphology, as seen in
For all VLS-GLAD films grown across a large range of temperatures, flux rates, deposition angles, and rotation rates, the branch diameter seems to be limited to a portion of trunk diameter. This upper bound is located at a fraction of 1/√{square root over (2)} of the trunk diameter, representing the side length of the inscribed square in the trunk's circular cross-section.
Further investigation regarding the effect of temperature on nanowire morphology will be necessary in future work. It was observed that pronounced nanowire growth begins at temperatures between 130° C. and 165° C., for deposition angles of 50° and 85°. At a reduced temperature of 165° C. VLS growth is noted for lower flux rates. However, at higher flux rates the nanowires begin to appear misshapen or curled. Seemingly without sufficient temperature, the rate of crystallization at the liquid-solid interface cannot increase to match higher flux rates, at which point ballistic growth dominates over kinetic growth.
Pitch (nominal film thickness deposited per substrate rotation) did not appear to have a significant effect on nanowire morphology. However, as pitch approaches infinity (no rotation), preferential branch formation in the direction of incident vapor flux was observed as catalytic liquid droplets form favourably on the edge of the nanowire facing the vapor flux, resulting in branching towards the incident vapor flux. In-depth studies into the effects of substrate rotation will be conducted in future work.
Crystal Structure
Crystal structure of the films was analyzed with x-ray diffraction. Diffraction patterns for as-deposited samples on Si substrates were taken for all deposition conditions studied. A selection of the diffraction patterns chosen to represent the entire set of data is shown in
The diffraction data was analyzed for trends between the deposition conditions and the crystal properties of the films. This included the crystallite size, Smith and Snyder FOM, strongest peaks present, and percent crystallinity as measured by a ratio between the integrated intensity of the raw line profile and background substrate profile. We did not observe any conclusive trends in these metrics with the deposition conditions including analysis of deposition angle, pitch, nominal flux rate, flux impingement rate on the substrate, and temperature. Any apparent trends in
Transmission electron microscopy (TEM) was performed to further analyze crystal structure. Selected area electron diffraction (SAED) confirmed that the ITO nanowires were single crystal. Aligned crystal lattice planes were observed, with the planes continuing from the trunk into the branch. This supports the geometrical data in
Optical and Electrical Properties
Transmission spectra and sheet resistance were measured and used to calculate Haacke's figure-of-merit (FOM) [Equation 1] for the all of the films presented to determine their feasibility as a TCO electrode. The first anneal increased the transmission of the films and the second anneal decreased the sheet resistance of the films, collectively improving their Haacke's FOM by 1 to 2 orders of magnitude. Haacke's FOM decreased with increasing nominal flux rate at an alpha of 50°.
Having demonstrated the high performance of VLS-GLAD ITO films, we explored the flexibility of our system for different TCO applications. We were able to determine the advantages of various morphologies of nanowires for TCO applications, and design hybrid nanostructures using VLS-GLAD to fabricate ideal TCOs. Films deposited at large deposition angles have a large interfacial area, and good electrical access into an active layer, however use of this morphology depends on using a conductive substrate to provide a common electrode at the base. Dense, interconnected films deposited at small deposition angles have a high Haacke FOM, but do provide good electrical access to the active layer. A combination of these morphology's advantages is attainable by VLS-GLAD modulation during growth, described in the next section.
Hybrid Nanostructures
Modulating the flux rate and deposition angle during growth via VLS-GLAD allows for the precise fabrication of unusual hybrid nanowire structures. These hybrid morphology architectures will provide improved control over the optical and electrical properties of the films. Modulation of the flux rate can be used to adjust the trunk diameter and branch density during growth. In an example, the trunk diameter responded to the flux rate by contracting from 54±4 nm to 23±1 nm as the flux rate was modulated from 1 nm/s to 0.05 nm/s. Similarly, in the trunk diameter responded to the flux rate by contracting from 53±6 nm to 27±2 nm and then expanding back to 56±3 nm as the flux rate was modulated from 1 nm/s to 0.05 nm/s and back to 1 nm/s. The branch diameter and density also respond to the modulation of flux rate. Branching density increases with decreasing flux rate; resulting in a morphology resembling a lobstick tree in the first example, and a trunk with branches only near the center of the trunk in the second example. In a third example the deposition angle was modulated, where a highly interconnected network of nanowires grown at α=30° is transformed back to a nanowire forest morphology at α=85°. The control provided by VLS-GLAD allows morphologies and functional properties to be hybridized, providing the possibility to optimize the nanowire properties for a variety of applications.
The film in the third example in the above paragraph is a good candidate for OPV electrodes as the interconnected base of the film serves as a common electrode, and the nanowire forest serves to penetrate the device active layer. The film has a sheet resistance of 81.2 Ω·sq−1 and an average transmission of 90.8%, which results in a Haacke figure of merit value of 4.7×10−3Ω−1. This is a vast improvement over a film of nanowires deposited at high α(85°), which are open-circuits without an underlying conductive layer. Therefore, real-time VLS-GLAD modulation provides the ability to tailor nanowire films for OPV applications.
VLS-GLAD modulated films allow for the hybridization of various nanowire morphologies, including networks of protruding high surface area ITO nanowires connected via continuous under-layers of ITO, ideal for TCO applications. Advanced control over nanowire structure provided by VLS-GLAD should allow for optimization of optical and electrical properties for TCO applications. The VLS-GLAD technique allows the effects of various deposition parameters to be decoupled, which should improve fundamental understanding of nanostructured film architectures.
Combining vapor-liquid-solid growth with glancing angle deposition (VLS-GLAD) facilitates fabrication of branched nanostructures not possible with either technique alone. Indium tin oxide (ITO) nanostructures grown by VLS-GLAD produce extremely porous nanotree structures, where periodic branch diameter oscillations are sometimes observed. We explain this rippled branch growth with a simple model linking the physics governing branch growth to the process variables controlled in VLS-GLAD. The model is verified by inducing specific, aperiodic ripples onto growing ITO branches through macroscopic vapor flux control and manipulation of local shadowing.
Two key features distinguish VLS-GLAD: (i) a collimated (low divergence) flux which breaks flux symmetry at the growth surface, enabling geometrical shadowing effects to mediate the nanowire growth and (ii) three-dimensional substrate motion to orient the flux during growth. Continuous rotation about substrate normal modulates the incident flux on the branches as they pass through the trunk's shadow. In response, the branch diameter oscillates during growth producing a rippled surface. The ripple period is consistent between branches (σRSD=15%), as expected in the trunk-dominated shadowing model. Unlike the self-oscillatory mechanism, surface ripples produced by flux shadowing are controllable through engineering of substrate motion during growth. Here we present our observations of branch surface rippling, derive a model linking branch growth physics to experimentally accessible variables in the VLS-GLAD technique, and verify the explanation by inducing aperiodic ripples into growing branches. As changes in incident flux drive changes in the diameter, this model captures diameter control modulated by both motion-controlled shadowing and flux rate. The VLS-GLAD technique provides an opportunity to study VLS growth kinetics, and produce nanostructures that have been previously unachievable.
From mass conservation, droplet diameter is controlled by the vapor flux capture rate and the liquid-solid crystallization rate. Vapor flux capture rate is a function of evaporation conditions, adatom diffusivity, and most importantly for the work presented here, the local shadowing environment a growing nanowire experiences. Unlike typical isobaric VLS-growth systems, the collimated vapor approach used here allows for dynamic control over local vapor pressure. For a growing nanowire array, high deposition angles (α) produce shadows between adjacent nanowires, as shown in
For a constantly rotating substrate, the incoming flux absorbed by a branch droplet oscillates between minimum and maximum values which depend on the local nanowire environment. A droplet on the side of an isolated nanowire will experience periodic shadowing, resulting in periodic growing and shrinking of the droplet. The growing branch's diameter follows droplet diameter, as long as the droplet does not become completely depleted.
The schematic in
{dot over (V)}
in(t)=RAγ(t) (1)
where R is the material flux rate as measured at the QCM in nm/s and γ parameterizes the flux modulation due to rotation-induced shadowing or flux shuttering (0≦γ≦1). For continuous rotation, γ(t)=(sin(2πt/τrot)+1)/2 where τrot is the rotation period in seconds. As neighbouring trunks increase in height, a branch will eventually become completely occluded from the flux. This effect is incorporated as an envelope on {dot over (V)}in. For this work, we approximate the envelope as an exponential of the form 1−eα(t-t
We assume the branch crystallizes in a layer-by-layer, or birth and spread growth mode, with each layer having a circular cross-section of the same radius as the droplet and monolayer thickness of d. We further assume nucleation time dominates layer crystallization time, such that liquid-solid crystallization time (τLS) is independent of droplet size and constant for growth of an entire branch. In reality, the growth rate is likely dependent on droplet diameter, however, this assumption tolerable for small changes in droplet diameter. The rate of outgoing volume from the droplet is then:
{dot over (V)}
out(t)=dπr(t)2/τLS (2)
where r is the instantaneous radius of the both the droplet and the branch at the liquid-solid crystallization interface. For a growth of a branch of length L in time tmax, TLS=tmaxd/L. Note that this is not valid at extremely slow rotation rates where the droplet becomes extinct. At each time step ti of length Δt, the instantaneous droplet volume and droplet/branch radius were then calculated:
The proposed model reproduces the observed rippling behavior, as shown in
To explore the conditions for which these rippling effects can be observed, the same simulation was run with different rotation speeds.
Complex Ripple Control. To further validate the hypothesis that rippling is a result of flux starvation, more complex structures were grown by two different methods of controlling the incoming flux to the liquid droplet. This was achieved by controlling the local shadowing function (e.g. changing the rotation rate) and by attenuating the vapor flux directly (e.g. rate control or shuttering). The model indicates that both methods should have a similar effect.
We use the flux shutter profile shown in
We have demonstrated that flux starvation in VLS-GLAD enables controlled branch diameter modulation during ITO nanotree growth, consistent with a simple volume in/out model. Flux starvation can be induced through shadowing or by adjusting the flux rate directly, both general features of VLS-GLAD. The nanostructures made possible by the VLS-GLAD technique should be available to any material system compatible with branched VLS growth.
Droplet diameter is coupled to nanowire properties such as composition, phase, and crystallographic growth direction. Thus, modulation of growth diameter with VLS-GLAD may provide a mechanism to modulate these properties during growth as well. Surface rippling may prove to be a valuable technique to enhance active surface area, create high energy surface defects, or induce phase changes in the material during growth. This technique also allows for the placement of “time stamps” during growth which may allow for improved investigation of time-related growth kinetics.
Single crystal indium tin oxide (ITO) nanotrees with engineered trunk and branch orientations are grown using a recently reported technique combining vapor-liquid-solid growth and glancing angle deposition (VLS-GLAD). In this work, three unique capabilities of VLS-GLAD are demonstrated for the first time: (i) nanotrees are aligned without epitaxy, (ii) branches can be placed on select faces of the nanotree trunk, and (iii) branch orientation can be modified along the height of nanotrees. VLS-GLAD uses a collimated obliquely incident vapor flux to place material on desired growth surfaces, resulting in preferential branch growth on the sides of the nanotree trunk exposed to the flux at the time of nucleation. Dynamic control of the azimuthal orientation of the flux relative to a growing nanotree enables the preferential orientation of branches to be modulated along the height of the nanotree, which we have demonstrated with both continuous and discrete substrate rotation schemes. An electron diffraction investigation confirms that the nanotrees can be considered as a single crystal, with continuity of the crystal structure across the trunk-branch interface. Crystal texture of the films is characterized by x-ray diffraction pole figures. By limiting the flux to discrete positions, the films develop both out-of-plane and in-plane crystal alignment (biaxial texture). We interpret the in-plane orientation as the result of competitive growth which leads to evolutionary selection of similarly oriented structures. Control over in-plane nanotree crystal alignment and height dependent branch orientation should increase the achievable complexity in three dimensional nanowire architectures.
We have used the self-catalyzed indium tin oxide (ITO) system, where catalyst seeding and growth of branches and trunks can be carried out with a single material. In this work, we show that a collimated obliquely incident flux produces preferentially oriented branch growth on ITO nanotrees. Branch growth is enhanced on the side of the nanotree facing the flux and inhibited on the shadowed regions of the trunk. By modulating the azimuthal position of the substrate during growth, we show that preferential branch orientation can be changed along the height of the trunk. We demonstrate these effects in four classes of ITO nanotrees defined by their height dependent branching morphologies: unidirectional, periodic, chiral and isotropic. Additionally, we provide evidence to suggest that an observed preferential azimuthal (in-plane) branch alignment in nanotree films grown without epitaxy, accompanied by a biaxial crystal texture, may be primarily a result of an evolutionary selection mechanism seen in other GLAD work.
Films were grown at a nominal temperature of 240° C., deposition angle of 85°, and nominal flux rates of 2 nm s−1 and 0.2 nm s−1. Pitch is defined as the nominal thickness deposited per substrate rotation. Films grown at 2 nm s−1 were deposited with a nominal thickness of 1200 nm, and an infinite pitch p=∞ (no substrate rotation, unidirectional structure), p=1200 nm (1 continuous 360° rotation, chiral structure) and p=10 nm (120 continuous 360° rotations, isotropic structure). A periodically structured film was deposited using spin-pause deposition parameters (2 discrete 180° rotations, periodic structure). For the periodic film, the azimuthal angle was periodically alternated, for a total nominal thickness of 1200 nm (φ=0° for 0 nm-400 nm; φ=180° for 400 nm-800 nm; φ=360° for 800 nm-1200 nm). Films with the same azimuthal modulation schemes were also grown at 0.2 nm s−1 to a nominal thickness of 300 nm measured at the CTM (except for the 0.2 nm s−1 periodic film, which was grown to a nominal thickness of 450 nm).
Scanning electron microscopy (SEM), electron tomography, selected area electron diffraction (SAED), and x-ray diffraction (XRD) were used to characterize nanotree morphology, crystal texture, and their orientation relative to the flux directions. A Bruker D8 Discover with HiStar area detector acquired XRD line profiles and pole figures of the (400), (222), (440), and (622) diffraction peaks. A Cu Kα radiation source and 0.5 mm collimator was used for each acquisition, with the detector placed ˜15 cm from the focal point. Films grown at 2 nm s−1 exhibited stronger XRD signals than those at 0.2 nm s−1 and were used for the pole-figure study to reduce acquisition time. The pole-figure data was processed using GADDS software. An absorption correction was applied by estimating the film's volume density from substrate mass measurements before and after deposition and accounting for film thickness and area. Radial integration of the pole-figure peaks was performed in GADDS to compare intensity variation in the azimuthal direction to branch orientation data collected from top-down SEM images of the sample taken with a Hitachi S-4800 at 10,000 times magnification. The trunks and branches of the nanotrees typically both grow along a [100] crystal direction, with the trunks tending towards growth along the substrate normal and the branches orthogonal to the trunk. All identifiable branches from four images of each sample were analyzed to compare the azimuthal branch orientations relative to the initial vapor flux direction (φ0) with ImageJ. Azimuthal orientation of each trunk in films grown at 2 nm s−1 was assessed by measuring the orientation of the most predominant branch on each trunk in 8 top-down SEM images (10,000× magnification). The trunk orientation is uniquely defined between 0° and 90° due to the 4-fold symmetry around the <100> direction.
Electron tomography tilt series were acquired on a JEOL 2200 FS TEM/STEM, operated at an accelerating voltage of 200 kV. Bright field TEM (BF-TEM) images were collected from ±65° with 1° steps with tilting axis along the trunk. Lacey carbon grids, which were coated with a 2 nm thick discontinuous gold film, were used as substrates. Assuming that the relative position of the ITO nanotrees and the carbon film does not change, the discontinuous Au film provides us with many markers suitable for alignment with better precision than the size of the Au nanoparticles, i.e. well below a few nm. Reconstruction and visualization were performed using the TEMography™ software package from System In Frontier Inc. The Simultaneous Iterative Reconstruction Technique (SIRT) algorithm was used to minimize the effect of gold particles overlapped with the sample. Electron diffraction analysis was performed on a Hitachi HF3300 TEM/STEM, operated at an accelerating voltage of 300 kV. Cross-section and individual nanotree TEM samples were prepared on a Hitachi NB5000 Focused Ion & Electron Beam System. Nanotrees were scratched off and loaded onto a silicon substrate, and then coated with a 100 nm thick carbon film as a protection layer during the subsequent FIB processing. Individual nanotree samples were plucked out and mounted onto a Hitachi 3D-observation holder by a standard micro-pillar sampling with the tilt axis along the trunk, which enables diffraction analysis from a full 360 degrees.
ITO nanotrees were grown via VLS-GLAD using the four substrate rotation configurations described above and listed in table 1 below (unidirectional, periodic, chiral and isotropic). Substrate rotation is used to control the azimuthal modulation of the flux from the perspective of the growing film. In this work, φ is used to define the azimuthal position of the vapor flux with respect to the substrate. The azimuthal motion of the flux induces height dependent, preferential branching orientations. The unidirectional nanotrees exhibit an asymmetric branch morphology, with average branch growth primarily directed toward the collimated flux. Periodic modulation between 0° and 180° during growth results in nanotree structures with branches on alternating sides of the trunk, depending on the relative azimuthal orientation of the flux during growth. Slow continuous rotation at a 300 nm pitch produces chiral nanotree structures. The chirality of the branch orientation is expected to follow flux rotation, but this is difficult to confirm with edge view SEM images. As the rotation rate is increased to a 10 nm pitch an isotropic nanotree is obtained. Branches grow uniformly on all four sides of the nanotree, with no observed preferential orientation.
Table 1 shows deposition conditions and properties of nanotree films deposited at 2 nm s−1 to a nominal thickness of 1200 nm.
Branch orientation was measured with respect to the initial flux axis (φ0) from plan-view SEM images of the nanotree films grown at 2 nm s−1. Normalized radial histograms shown in
Preferential branch orientations are less pronounced in films grown with continuous substrate rotation [
Overall, branches tend to grow on the region of trunks exposed to flux at the time of nucleation. This suggests branch nucleation occurs preferentially within a limited proximity to where the trunk is directly exposed to the collimated vapor flux. Branch nucleation due to surface diffusion vertically and azimuthally about the trunk appears to be limited as shown by the abrupt change in branch orientation midway up the periodic nanotrees and reduced branching on the shadowed surface in
Volumetric reconstructions of an isotropic nanotree grown at 2 nm s−1 were determined. The relative orientation of the nanotree trunk and branches can be clearly seen: branches grow orthogonally on four sides of the trunk, and normal to the main trunk. The square faceting of the trunk cross-section is clearly visible in a cross sectional slice, and can be confirmed with TEM image of a cross sectional sample. The presence of a corner likely reduces the probability of surface diffusion azimuthally about the trunk due to the presence of a 3D Schwoebel-Ehrlich barrier agreeing with our observations of limited branch nucleation on shadowed surfaces.
As the trunks grow vertically, in the substrate normal direction, it is expected that geometric shadowing gradually starves branches of flux deeper in the film. Branches far beneath the nanotree tops are not expected to receive a significant amount of flux from re-evaporation because surrounding adatoms will have had time to thermalize and be incorporated into the solid state. Similarly, material from surface diffusion becomes increasingly limited as branches closer to the growth surface will act as sinks to captured flux. This effect places an upper limit on the time that a branch may continue to receive growth material and therefore limits branch length. The effect is apparent by the relatively uniform branch length with increasing trunk height, and agrees with our previous observations of reduced growth rate as branches become occluded from the flux.
Electron and x-ray diffraction were used to examine the crystal structure of individual nanotrees and texture of the nanotree ensembles, respectively. Diffraction profiles of the four nanotree morphologies discussed here are consistent with a cubic crystal structure (space group Ia3−, 01-089-4597). Identical electron diffraction patterns obtained from trunk, branch and their interface of a nanotree cross-section confirm that each nanotree is a single crystal and that branches exhibit epitaxial growth on the trunk, as shown in
X-ray pole figures of the (400), (222), (440), and (622) diffraction peaks were taken to characterize the overall crystal texture and its relationship to the flux direction and branch orientation for each film.
Of the four morphologies, only the periodic and unidirectional structures have a clear biaxial texture, showing both in-plane and out-of-plane orientation (
The observed out-of-plane texture is due to the preferred growth direction of ITO nanotrees grown via VLS. In-plane texture is observed for the unidirectional and periodic films, where the vapor-incidence plane is parallel to the (110) direction, and is reduced or lost when the substrate is continuously rotated for the chiral and isotropic films, respectively. Development of in-plane texture is often attributed to an evolutionary selection process in oblique deposition, and is discussed further below.
The x-ray and electron diffraction data indicate that the nanotrees are single crystal with the trunks and branches growing in orthogonal [100] crystallographic directions. Thus, in-plane crystal orientation should be accompanied by a preferential branch orientation that is consistent with a [100] epitaxial relationship between a trunk and branches. To confirm this relationship, (440) pole figures were radially integrated to produce polar-plots, where the radial distance corresponds to the diffraction intensity, and overlaid onto the branch histograms (
(440) pole figures exhibit peaks at ±45° and ±135° relative to the flux axis in the unidirectional and periodic films, establishing a link between the observed in-plane texture in these films and branch orientation [
A potential mechanism to drive the evolutionary selection process is the difference in flux capture cross-section of the square trunks. Nanotrees with a {100} trunk face oriented 45° from the flux axis (φ0) present the largest surface from the perspective of the collimated flux and therefore, capture more vapor from the flux that can diffuse to the trunk droplet and produce vertical growth. Therefore, nanotrees oriented with a vertex of their square trunk cross-section pointing towards the flux should exhibit the fastest vertical growth rate. The faster growth rate will cause increased flux shadowing for neighboring trees that do not exhibit the preferred orientation, further suppressing their growth. Preferred branch nucleation and growth on the {100} faces of the trunk oriented ±45° from φ0 may increase the flux capture cross-section of these orientations at a higher rate than other orientations, further amplifying their competitive advantage. Thus, only nanotrees oriented favorably towards the flux are expected to avoid extinction, and continue to grow in the film.
One consequence of the evolutionary selection process is that films consisting of relatively short nanotrees should not exhibit in-plane texture since trunks are randomly oriented in the early stages of growth prior to the extinction of less flux-favored orientations. Indeed, no in-plane texture is detected in a (222) pole figure of a unidirectional film grown under the same conditions but to less than one tenth the nominal thickness. A second consequence is that relaxation of the selection pressure should increase the number of nanotrees that survive, and this is observed in the high trunk density observed in the isotropic film seen in Table 1. A third consequence is that films with selection pressure should consist of taller nanotrees than those without, as the selected nanotrees should grow faster vertically due to increased flux capture distributed among fewer nanotrees. From Table 1, we can see that both the unidirectional and periodic nanotrees are taller than chiral and isotropic nanotrees. However, this description does not capture any effects from varying branch density or differences in trunk/branch diameter, and must be considered a preliminary hypothesis at this time.
We measured the azimuthal crystal orientation of trunks with respect to φ0 in order to investigate the competitive growth hypothesis. The 4-fold symmetry observed in the branch histograms provides evidence for biaxial texture in the unidirectional and chiral films [
Single crystal ITO nanotrees grown with VLS-GLAD have trunks and branches occurring in orthogonal [100] directions of ITO's cubic crystal structure. We show that branch placement can be changed along the height of the trunk during growth by modulating the azimuthal position of a collimated vapor flux relative to the trunk axis. Although we have demonstrated this effect in a self-catalyzed VLS system, this technique should be applicable to all other VLS systems where catalyst seeds can be distributed uniformly around the trunk surface. When the flux is constrained to one position or two discrete positions with 2-fold azimuthal symmetry, in-plane alignment of the structures develops. This results in biaxial texture of the nanotree ensemble, as well as 4-fold azimuthal symmetry of the branch directions. Typically, in-plane alignment of nanotrees depends on enforcing an epitaxial relationship with the substrate, but it has been demonstrated on amorphous substrates here. Using VLS-GLAD, it should be possible to induce biaxial texture in any VLS system without requiring epitaxy by matching the flux symmetry to the crystal symmetry around the growth axis. In general, the use of a directed, collimated vapor flux used in VLS-GLAD as opposed to the more common isotropic flux has advantages that should be useful in fabricating complex, interconnected, three-dimensional nanowire networks.
The surface properties of the substrate can be geometrically designed to invoke a specific behavior in the growing nanowires. A geometric design is placement of either self-catalyzed material or catalyst seeds on a substrate, substrate topography, or existing nanowires before or during a certain VLS growth step. Such arrangements may be produced by methods such as photolithography, nanosphere lithography, electron beam lithography, etching, or glancing angle deposition. The arrangement of catalyst droplets may be a repeating pattern, a set of multiple patterns, or aperiodic in nature. Examples of such patterns include complete or partial square, hexagonal, and linear arrangements. The geometry is selected to produce a desired two- or three-dimensional arrangement of nanowires after VLS growth. Chemical patterning or geometric patterns to induce nucleation could also be used, including microscale surface topography patterns.
Immaterial modifications may be made to the embodiments described here without departing from what is covered by the claims.
In the claims, the word “comprising” is used in its inclusive sense and does not exclude other elements being present. The indefinite articles “a” and “an” before a claim feature do not exclude more than one of the feature being present. Each one of the individual features described here may be used in one or more embodiments and is not, by virtue only of being described here, to be construed as essential to all embodiments as defined by the claims.
This application claims the benefit under 35 USC 119(e) of U.S. provisional application Ser. No. 61/730,206 filed Nov. 27, 2012.
| Number | Date | Country | |
|---|---|---|---|
| 61730206 | Nov 2012 | US |
