Site-selective Growth Research Articles

Chemically Controlled Bending of Compositionally Anisotropic Microcylinders

Soft materials that can undergo mechanical actuation in response to external stimuli, such as changes in temperature, light, pH value, or ionic strength, have attracted increasing attention because of their potential use as thinfilm actuators, smart sutures, and soft robots. These materials typically require specialty polymers, such as shape-memory polymers or use macroscopically layered films. In layered films, the anisotropic distribution of two polymers, or a polymer and a metal, is essential. This creates a mismatch in mechanical properties that gives rise to a defined bending. In principle, this concept is not limited to macroscopic multilayer films, but can be achieved with colloidal materials, as long as the required anisotropy can be realized and different parts of the colloidal object will respond differently to the external stimulus. In recent years, compositionally anisotropic microand nanoparticles have been devised using a range of different synthesis methods including microfluidic and lithographic techniques, particle replication in low surface energy templates, selective crosslinking of polybutadiene segments in terpolymers, lithographic patterning of microspheres, electrochemical and photochemical reduction, templating of porous membranes and nanotubes, surfactant aided growth, graft polymerization, and processes based on controlled surface nucleation. Alternatively, electrohydrodynamic co-jetting is a method to prepare particles and fibers with multiple compartments by transferring fluids through a set of capillaries that can process dissimilar materials. In the past, electrohydrodynamic co-jetting has resulted in particles with multiple compartments that contain different polymer blends, dyes, low-molecular weight additives, reactive molecules and even inorganic nanoparticles. If a reactive additive, such as a functionalized polymer, is added to one of the compartments, selective surface modification is possible and can result in spatially controlled immobilization of proteins or peptides. Because different compartments can be loaded with dissimilar materials, entirely new sets of functions can arise from unique synergistic effects, not just from the addition of the properties of the individual compartments. Herein, we report a new type of compositionally anisotropic microcylinders, where defined compartments within the same microcylinder undergo differential expansion due to the site-selective growth of a surface layer. The asymmetric expansion creates surface stresses resulting in significant and controllable bending of the microcylinders, which depends on the particle geometry and the architecture of the surface layers. Using finite element simulations, we verify the observed bending trends and derive a family of performance curves that predict a wide-range tunability of the actuation stroke based on the cylinder geometry and the amount of swelling. The microcylinders are fabricated based on electrohydrodynamic co-jetting followed by microsectioning. In brief, an electric field is applied to a compound droplet comprising two or more polymer solutions generated by laminar flow from a side-by-side arrangement of capillary needles. We have previously demonstrated the synthesis of particles and fibers from chloroform-based solutions of lactic acid polymers. In the case of fibers, high viscosities, combined with high solvent volatility and low charge-to-volume ratios, can result in an extremely linear and controlled jet migration without the bending and whipping instabilities commonly observed in charged jets. This situation enables the production of multicompartmental microfibers, which not only exhibit monodispersity with respect to diameter, but can also be aligned on rotating collectors. Such highly aligned fiber scaffolds can then be cut into monodisperse microcylinders. Importantly, particle diameters are controlled by altering the solution and process parameters during electrohydrodynamic co-jetting, while control over cylinder length is achieved by the microsectioning step. Spatioselective functionalization of one or more compartments of the cylinders has been achieved by incorporation of poly(lactide-co-propargyl glycolide) as an additive during fabrication of the microcylinders, and subsequent modification with biotin and streptavidin by click chemistry. As shown in the Supporting Information, Figure S1, we incorporated a poly[lactide-co[*] Dr. S. Saha, N. Clay, A. Donini, Prof. J. Lahann Department of Chemical Engineering University of Michigan, Ann Arbor, MI 48109 (USA) E-mail: lahann@umich.edu

Type IV collagen-initiated signals provide survival and growth cues required for liver metastasis

The liver is a major site of metastasis for human malignancies, yet the factors that regulate tumor cell survival and growth in this organ remain elusive. Previously, we reported that M-27(IGF-IR) murine lung carcinoma cells with ectopic insulin-like growth factor-1 (IGF-I) receptor overexpression acquired a site-specific, liver-metastasizing potential. Gene expression profiling and subsequent RNA and protein analyses revealed that this was associated with major changes to the expression of extracellular matrix (ECM) protein-encoding genes including type III, IV and XVIII collagen genes, and these changes were also observed in the respective tumors in vivo. Because type IV collagen was the most prominently altered ECM protein in this model, we further analyzed its functional relevance to liver metastasis. M-27 cells stably overexpressing type IV collagen α1 and α2 chains were generated and their growth and metastatic properties investigated. We found that these cells acquired a site-selective growth advantage in the liver and this was associated with cell rescue from anoikis in a collagen IV/α2 integrin/FAK-dependent manner and increased responsiveness to IGF-I. Conversely, collagen IV or focal adhesion kinase (FAK) silencing by small-interfering RNA in highly metastatic tumor cells enhanced anoikis and decreased liver metastases formation. Moreover, analysis of human surgical specimens revealed uniformly high collagen IV expression in 65/65 hepatic metastases analyzed, regardless of tissue of origin, whereas it was variable and generally low in 50/50 primary colorectal carcinoma specimens examined. The results suggest that collagen IV-conveyed signals are essential cues for liver metastasis in diverse tumor types and identify mediators of collagen IV signaling as potential therapeutic targets in the management of hepatic metastases.

Investigation of pre-structured GaAs surfaces for subsequent site-selective InAs quantum dot growth

In this study, we investigated pre-structured (100) GaAs sample surfaces with respect to subsequent site-selective quantum dot growth. Defects occurring in the GaAs buffer layer grown after pre-structuring are attributed to insufficient cleaning of the samples prior to regrowth. Successive cleaning steps were analyzed and optimized. A UV-ozone cleaning is performed at the end of sample preparation in order to get rid of remaining organic contamination.

Current quantization in an optically driven electron pump based on self-assembled quantum dots

The electronic structure of self-assembled semiconductor quantum dots consists of discrete atom-like states that can be populated with a well-defined number of electrons. This property can be used to fabricate a d.c. current standard that enables the unit of ampere to be independently defined. Here we report an optically pumped current source based on self-assembled InAs/GaAs quantum dots. The accuracy obtained so far is 10−1 and is limited by the uncertainty in the number of dots. At 10 K the device generates a current difference of 2.39 nA at a frequency of 1 kHz. The accuracy could be improved by site-selective growth techniques where the number of dots is fixed by pre-patterning. The results are promising for applications in electrical metrology, where a current standard is needed to close the so-called quantum metrological triangle. The atom-like electronic structure of semiconductor quantum dots makes them ideal for storing well-defined numbers of electrons. This in turn can be used in the development of standards for current, to independently define the ampere.

Growth kinetics in position-controlled and catalyst-free InAs nanowire arrays on Si(111) grown by selective area molecular beam epitaxy

We investigated the interwire distance dependence on the growth kinetics of vertical, high-yield InAs nanowire arrays on Si(111) grown by catalyst-free selective area molecular beam epitaxy (MBE). Utilizing lithographically defined SiO2 nanomasks on Si(111) with regular hole patterns, catalyst-free and site-selective growth of vertically (111)-oriented InAs nanowires was achieved with very high yields of ∼90 percent. Interestingly, the yield of vertically ordered nanowires was independent of the interwire distance and the initial growth stages. Significant size variation in the nanowires was found to depend critically on the interwire distance and growth time. Two growth regimes were identified—(i) a competitive growth regime with shorter and thinner nanowires for narrow interwire distances and (ii) a diffusion-limited growth regime for wider distances, providing good estimates for the surface diffusion lengths. Surprisingly, despite these size-dependent effects the nanowire geometries remained unaltered with uniform, almost nontapered morphologies even over large variation in nanowire density (∼mid−106–109 cm−2 range). X-ray diffraction further confirmed the vertical (111) directionality with low crystal tilt by rocking curve widths (ω scans) as low as ∼0.6°. These findings demonstrate the capability to precisely tailor the position and size of well-oriented III-V semiconductor nanowires through noncatalytic MBE selective area growth and provide an important step toward fully integrated, uniform vertical III-V nanowire array-on-Si devices.

Minimization of wafer bowing in GaN-based vertical light-emitting diodes by selective area growth using metal-organic chemical vapor deposition

The effect of selective area growth (SAG) on wafer bowing of GaN-based light-emitting diodes (LEDs) is investigated. The SAG of LED structures was carried out on a silicon dioxide (SiO 2) mask pattern with periodic 1000×1000 μm openings, along the sapphire 〈1−1 0 0〉 and 〈−1−1 2 0〉 directions. The morphology of a selectively-grown n-GaN epilayer was examined in relation to various growth parameters such as temperature, pressure, and V/III ratio. Under optimized growth conditions, formation of a ridge-shaped epilayer with a v-pit free smooth surface was realized. Furthermore, the ridge-shaped vertical LED structure, after the removal of the sapphire substrate by laser lift-off (LLO) showed less wafer bowing compared with conventional vertical LED structures. This is attributed to the suppression of lateral strain and dislocations during the site-selective growth process, due to a reduction in the lateral dimensions.

Site-selective synthesis of in situ Ni-filled multi-walled carbon nanotubes using Ni(salen) asa catalyst source

The synthesis of Ni-filled multi-walled carbon nanotubes was performedby atmospheric pressure chemical vapor deposition with propane on Si at850 °C using a simplemixture of (N, N′-bis(salicylidene)-ethylenediiminato) nickel(II), commonly known as Ni(salen), and aconventional photoresist. Analysis of the carbon nanotubes using scanning electronmicroscopy together with high-resolution transmission electron microscopy show that thenanotubes have grown by a tip-growth mechanism and exhibit a multi-walled structurewith partial Ni filling. The high quality of the Ni-filled nanotubes is evidencedby Raman spectroscopy. The magnetic properties of Ni-filled nanotubes wereanalyzed using a superconducting quantum interference device which revealed theirferromagnetic behavior with large coercivity. A scalable as well as site-selective growth ofhigh quality Ni-filled carbon nanotubes is achieved by a simple photolithographicmethod.

Growth of zinc oxide nanoflowers by thermal evaporation method

An alternative method for site-selective growth of ZnO nanostructures that does not use an Au catalyst or a ZnO thin-film seed layer is presented. Well-aligned ZnO nanoflower structure arrays were directly fabricated on silicon substrates through zinc powder evaporation, which uses a simple thermal evaporation method without a catalyst. The collected ZnO nanoflowers were then characterized through X-ray diffraction (XRD) and scanning electron microscopy (SEM). The optical properties of these nanostructured materials are also discussed.

Intentionally positioned self-assembled InAs quantum dots in an electroluminescent p–i–n junction diode

An intentional positioning of optically active quantum dots using site-selective growth by a combination of molecular beam epitaxy (MBE) and focused ion beam (FIB) implantation in an all-ultra-high-vacuum (UHV) setup has been successfully demonstrated. A square array of periodic holes on GaAs substrate was fabricated with FIB of 30 keV Ga + ions followed by an in situ annealing step. Subsequently, the patterned holes were overgrown with an optimized amount of InAs in order to achieve site-selective growth of the QDs on the patterned holes. Under well-optimized conditions, a selectivity of single quantum dot growth in the patterned holes of 52% was achieved. Thereafter, carrier injection and subsequent radiative recombination from the positioned InAs/GaAs self-assembled QDs was investigated by embedding the QDs in the intrinsic part of a GaAs-based p–i–n junction device. Electroluminescence spectra taken at 77 K show interband transitions up to the fifth excited state from the QDs.

Anisotropic growth of organic semiconductor based on mechanical contrast of pre-patterned monolayer

Site-selective anisotropic growth of perylene film is achieved by using a striped Langmuir–Blodgett (LB) monolayer as an alignment layer. The stripes significantly increase the grain size along the stripe direction due to increased diffusion length. By controlling the grain boundaries and the molecular alignment, a mobility anisotropic ratio of ∼10 for the current flow parallel and perpendicular to the stripes has been observed.

Catalyst patterned growth of interconnecting graphene layer on SiO2/Si substrate for integrated devices

The authors report a simple site selective growth for producing interconnecting suspended graphene on SiO2/Si substrate. The outcome of the process depends on the thickness of the catalyst (Ni) and process ambient on SiO2/Si wafer by low pressure fast heating chemical-vapor deposition at 820 °C for periods from 30 s to 10 min. Raman spectroscopy revealed that the graphene grown on the substrate consists of from 1 to <20 layers, with the number of layers depending on the thickness of the catalyst (Ni). Also, the thickness of Ni catalyst determines whether the graphite layers (GLs) are grown in a suspended form or adhered to the substrate. The possibility of producing qualified as-grown GL supported on a wafer without further processing, such as transferring to another substrate, should contribute to further scientific research and development of graphene.

Site-Selective Growth of Highly Oriented ZnO Rod Arrays on Patterned Functionalized Si Substrates from Aqueous Solution

Site-selective deposition of highly oriented ZnO rod arrays was successfully achieved in an aqueous solution of zinc nitrate and hexamethyltetramine (C6H12N4) at low temperature. A patterned self-assembled monolayers (SAMs) surface was used as the template. ZnO nanorods were selectively deposited on the -CH3 terminated SAMs but were hindered on the OH-covered regions due to the preferential binding of C6H12N4−H+ complex with -OH endgroups. Continuous stirring during the reaction played a role in improving the crystal orientation and crystalline quality of ZnO nanorods. A little NH4Cl aqueous solution had a positive effect to increase the nucleation density of ZnO crystals, and thus enhance their order degree. These results provide a one-step process to fabricate micropatterns of thin films from a harmless substance at ordinary temperature and atmospheric pressure.

Study on Site-Selective Growth of Carbon Nanotubes and Diamond Films With Barrier Layers

Microwave plasma jet chemical vapor deposition is used to grow array types of carbon nanotubes and diamond films on silicon substrates. In this study, we report a patterned growth of carbon nanotubes and diamond films using the barrier layer method, wherein the selective areas vary from 20×20 μm2 to 30×80 μm2. The field-emission measurement shows that the turn-on electric field for patterned carbon nanotubes and diamond films are 1.1 V/μm and 5 V/μm, respectively. Therefore, this method of site-selective growth of carbon nanotubes and diamond films can be applied to the production of field emitters.

Sequential Electrochemical Oxidation and Site-Selective Growth of Nanoparticles onto AFM Probes

In this work, we reported an approach for the site-selective growth of nanoparticle onto the tip apex of an atomic force microscopy (AFM) probe. The silicon AFM probe was first coated with a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) through a chemical vapor deposition (CVD) method. Subsequently, COOH groups were selectively generated at the tip apex of silicon AFM probes by applying an appropriate bias voltage between the tip and a flat gold electrode. The transformation of methyl to carboxylic groups at the tip apex of the AFM probe was investigated through measuring the capillary force before and after electrochemical oxidation. To prepare the nanoparticle terminated AFM probe, the oxidized AFM probe was then immersed in an aqueous solution containing positive metal ions, for example, Ag+, to bind positive metal ions to the oxidized area (COOH terminated area), followed by chemical reduction with aqueous NaBH 4 and further development (if desired) to give a metal nanoparticle-modified AFM probe. The formation of a metal nanoparticle at the tip apex of the AFM probe was confirmed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA).

Fe catalytic growth, microstructure, and low-threshold field emission properties of open ended tubular graphite cones

Large-area tubular graphite cones (TGCs) with an open end were successfully synthesized on Si substrates by microwave plasma assisted chemical vapor deposition using a thin Fe film as catalyst. As-grown TGCs are uniformly distributed on the Si surface with a density of 5.8×105∕cm2 and an average growth rate of 0.6μm∕min. Some of them are very sharp with an apex angle as small as 2°–3°. Combined observations by transmission electron microscopy and cross-sectional scanning electron microscopy clarified that the TGCs possess a hollow nanotube core, a metal-free open tip and that the Fe catalyst is located in the root, strongly supporting the base-growth mechanism of TGCs. Scanning confocal micro-Raman spectroscopy along an individual TGC shows that the integrated intensity ratio of D to G band of individual TGC at tips (0.12) is an order of magnitude less than that at roots (1.2), confirming that the tip is highly crystalline whereas the root is of poor quality. Their field emission performance exhibits a turn-on field as low as 1.8V∕μm and highly stable emission characteristics. The present study opens the way to site selective growth of TGCs and demonstrates its use as a potentially excellent emitter.

Synthesis of Locally-Ordered Carbon Nanotube Arrays from Patterned Catalyst by Self-Assembly Technique

Site-selective growth of multi-walled carbon nanotubes (MWCNTs) from an iron oxide nanoparticle catalyst patterned by drying-mediated self-assembly technique is present. The ethanol solution of the iron nitrate was employed as catalyst precursor. The silicon wafer was mounted catalyst precursor by dip-coating. After solvent evaporation at room temperature, the catalyst pattern formed. The catalyst pattern was subjected to chemical vapor deposition of ethanol vapor after oxidation of iron nitrate. The patterned array of MWCNTs was obtained with a dot size of around 5 (m and the distance between dots of about 25 (m. The locally-ordered patterns of MWCNTs were found. The present method offers a simple and cost-effective method to grow carbon nanotubes with self-assembled patterns.

Highly oriented ZnO rod arrays on Si substrates from aqueous solution

Ordered zinc oxide (ZnO) rod arrays with very high orientation were fabricated on Si substrates by using a solution method. The substrate surfaces were functionalized by Self-Assembly Monolayers (SAMs). In the very early growth stage, the oriented ZnO crystals had already grown, which appeared to be the main reason why ZnO nanorods showed very high orientation. The un-dense and un-uniform SAMs provided a surface that was heterogeneous to ZnO nucleation. Consequently, highly oriented ZnO rods were selectively grown on the “coin-like” SAM-uncovered regions. The route developed here can provide some helpful information to control the nucleation and orientation of ZnO in aqueous solution. Also, the site-selective growth mechanisms can indicate a clue to grow patterned highly oriented ZnO nanorod arrays by the organic template.

Site-specific growth of ZnO nanowires from patterned Zn via compatible semiconductor processing

An alternative method for site-selective growth of ZnO nanowires without the use of an Au catalyst or a ZnO thin-film seed layer is presented. Using conventional lithography and metallization semiconductor processing steps, regions for selective nanowire growth are defined using Zn, which acts as a self-catalyst for subsequent ZnO nanowire growth via a simple thermal oxidation process. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction reveal that the nanowires grown by this technique are single-crystalline wurtzite ZnO. Room temperature photoluminescence exhibits strong ultraviolet emission from these nanowires, indicating good optical properties. A series of experiments was conducted to elucidate the unique growth behavior of these nanowires directly from the Zn grains and a growth model is proposed.

Fabrication of Monodisperse Asymmetric Colloidal Clusters by Using Contact Area Lithography (CAL)

We report a new fabrication method of asymmetric colloidal clusters by using contact area lithography with site-selective growth. Nanometric surface patterns (approximately 44, 60, and 81 nm in diameter) were prepared by coating surfaces with self-assembled monolayers (SAMs; octadecyltrichlorosilane (OTS) in this study) except the contact area either between colloidal particles or between colloids and substrate. Nanoscale site-specific heterogeneous nucleation and growth of oxide materials of titanium were studied using the patterns of OTS-SAMs onto the either flat or curved surfaces of SiO2. Experimental results suggest that a combination of the large difference in the surface energy between the growing and surrounding surfaces and the diffusion-controlled growth leads to complete nanoscale site specificity. We also fabricated superstructrures of silica spheres with hemispheres of titania (<20 nm in dimension) on their surfaces and discussed the optical properties of colloidal films consisting of the monodisperse asymmetric colloidal clusters in terms of photonic band gap.

Site-selective growth of self-assembled InAs quantum dots on focused ion beam patterned GaAs

A site-selective growth of self-organized InAs quantum dots (QDs) employing a combination of in-situ focused ion beam (FIB) implantation and self-organized molecular beam epitaxy (MBE) growth has been successfully demonstrated. First, a buffer layer of GaAs was grown by MBE before a square lattice of shallow holes with a pitch of 1–2 μm was fabricated by FIB implantation of Ga and In ions. Before InAs deposition, different procedures to remove the implantation damage were tested. The best results were obtained after re-evaporation of 3–5 nm GaAs. Another critical parameter that was carefully optimized is the deposited InAs amount. For the optimized process it was possible to induce growth of single QD in the hole with more than 50% probability. Between the shallow holes of 2 nm depths, no QD growth was observed. The optical quality of positioned dots was investigated by photoluminescence spectroscopy, employing a separate sample where the QDs were overgrown by GaAs cap layer. Excited-state interband transitions up to n=5 were observed from positioned QDs.