Fabrication of Microlens Arrays with Well-controlled Curvature by Liquid Trapping and Electrohydrodynamic Deformation in Microholes

1. Xiangming Li, 
2. Yucheng Ding^*, 
3. Jinyou Shao^*, 
4. Hongmiao Tian, 
5. Hongzhong Liu

Article first published online: 22 MAR 2012

DOI: 10.1002/adma.2011046

The microlens array (MLA) or micromirror array (MMA) is one of the most important units in many optical devices and photoelectronic systems. The paper presents a process for fabricating an MLA with well-controlled curvature by liquid trapping and electrohydrodynamic deformation in microholes. The approach has been shown capable of generating large-area and high-quality MLAs or MMAs economically.

Elastomer Surfaces with Directionally Dependent Adhesion Strength and Their Use in Transfer Printing with Continuous Roll-to-Roll Applications

1. Sang Yoon Yang², 
2. Andrew Carlson²,
3. Huanyu Cheng³, 
4. Qingmin Yu⁴, 
5. Numair Ahmed⁵, 
6. Jian Wu⁶, 
7. Seok Kim⁵, 
8. Metin Sitti⁷,
9. Placid M. Ferreira⁵, 
10. Yonggang Huang³,
11. John A. Rogers^1,*

Article first published online: 19 MAR 2012

DOI: 10.1002/adma.201104975

In this paper we present mechanics and materials aspects of elastomeric stamps that have angled features of relief on their surfaces, designed to enable control of adhesion strength by peeling direction, in a way that can be exploited in schemes for deterministic assembly by transfer printing. Detailed mechanics models capture the essential physics of interface adhesion in this system. Experiments with cylindrical stamps that have this design demonstrate their potential for use in a continuous, roller mode of operatio

Measuring Binding of Protein to Gel-Bound Ligands Using Magnetic Levitation

Nathan D. Shapiro†, Katherine A. Mirica†, Siowling Soh†, Scott T. Phillips†, Olga Taran†, Charles R. Mace†, Sergey S. Shevkoplyas§, and George M. Whitesides*†‡

^†Department of Chemistry & Chemical Biology and ^‡Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, Massachusetts 02138, United States

^§ Department of Biomedical Engineering, Tulane University, New Orleans, Louisiana 70118, United States

J. Am. Chem. Soc., Article ASAP

DOI: 10.1021/ja211788e

Publication Date (Web): February 24, 2012

Copyright © 2012 American Chemical Society

This paper describes the use of magnetic levitation (MagLev) to measure the association of proteins and ligands. The method starts with diamagnetic gel beads that are functionalized covalently with small molecules (putative ligands). Binding of protein to the ligands within the bead causes a change in the density of the bead. When these beads are suspended in a paramagnetic aqueous buffer and placed between the poles of two NbFeB magnets with like poles facing, the changes in the density of the bead on binding of protein result in changes in the levitation height of the bead that can be used to quantify the amount of protein bound. This paper uses a reaction–diffusion model to examine the physical principles that determine the values of rate and equilibrium constants measured by this system, using the well-defined model system of carbonic anhydrase and aryl sulfonamides. By tuning the experimental protocol, the method is capable of quantifying either the concentration of protein in a solution, or the binding affinities of a protein to several resin-bound small molecules simultaneously. Since this method requires no electricity and only a single piece of inexpensive equipment, it may find use in situations where portability and low cost are important, such as in bioanalysis in resource-limited settings, point-of-care diagnosis, veterinary medicine, and plant pathology. It still has several practical disadvantages. Most notably, the method requires relatively long assay times and cannot be applied to large proteins (>70 kDa), including antibodies. The design and synthesis of beads with improved characteristics (e.g., larger pore size) has the potential to resolve these problems.

Constructing Anisotropic Single-Dirac-Cones in Bi1–xSbx Thin Films

Shuang Tang*† and Mildred S. Dresselhaus*‡

^† Department of Materials Science and Engineering,Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

^‡ Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

Nano Lett., Article ASAP

DOI: 10.1021/nl300064d

Publication Date (Web): March 20, 2012

Copyright © 2012 American Chemical Society

The electronic band structures of Bi_1–xSb_x thin films can be varied as a function of temperature, pressure, stoichiometry, film thickness, and growth orientation. We here show how different anisotropic single-Dirac-cones can be constructed in a Bi_1–xSb_x thin film for different applications or research purposes. For predicting anisotropic single-Dirac-cones, we have developed an iterative-two-dimensional-two-band model to get a consistent inverse-effective-mass-tensor and band gap, which can be used in a general two-dimensional system that has a nonparabolic dispersion relation as in the Bi_1–xSb_x thin film system.

Flexible Electronics: Imbricate Scales as a Design Construct for Microsystem Technologies

1. Seok Kim¹, 
2. Yewang Su², 
3. Agustin Mihi³,
4. Seungwoo Lee⁴, 
5. Zhuangjian Liu⁵, 
6. Tanmay K. Bhandakkar¹, 
7. Jian Wu⁶, 
8. Joseph B. Geddes III³, 
9. Harley T. Johnson¹, 
10. Yongwei Zhang⁵, 
11. Jung-Ki Park⁴, 
12. Paul V. Braun³,
13. Yonggang Huang², 
14. John A. Rogers^7,*

Article first published online: 19 MAR 2012

DOI: 10.1002/smll.201290038

The cover picture shows a composite image of two colorized scanning electron microscopy images. One illustrates an overlapping assembly of microscale silicon, photonic, and plasmonic plates on an elastomeric substrate. This imbricate layout is often found in nature–the scales of snake skin or a butterfly wing, but never in man-made microsystems. Appealing attributes include fault-tolerant, multi-functional capabilities, in layouts that can provide mechanical stretchability even with full, 100% areal coverages of rigid plates. The other image shows a silicon plate on the surface of a stamp with a microtip geometry designed for manufacturing this type of system. For more information, please read the Full Paper “Imbricate Scales as a Design Construct for Microsystem Technologies” by J. A. Rogers and co-workers, beginning on page 901. The paper describes fundamental and applied aspects of the associated materials science and mechanical engineering.

Direct Gravure Printing of Silicon Nanowires Using Entropic Attraction Forces

1. Jungmok Seo¹, 
2. Hyonik Lee¹, 
3. Seulah Lee¹,
4. Tae Il Lee², 
5. Jae-Min Myoung², 
6. Taeyoon Lee^1,*

Article first published online: 19 MAR 2012

DOI: 10.1002/smll.201102367The development of a method for large-scale printing of nanowire (NW) arrays onto a desired substrate is crucial for fabricating high-performance NW-based electronics. Here, the alignment of highly ordered and dense silicon (Si) NW arrays at anisotropically etched micro-engraved structures is demonstrated using a simple evaporation process. During evaporation, entropic attraction combined with the internal flow of the NW solution induced the alignment of NWs at the corners of pre-defined structures, and the assembly characteristics of the NWs were highly dependent on the polarity of the NW solutions. After complete evaporation, the aligned NW arrays are subsequently transferred onto a flexible substrate with 95% selectivity using a direct gravure printing technique. As a proof-of-concept, flexible back-gated NW field-effect transistors (FETs) are fabricated. The fabricated FETs have an effective hole mobility of 17.1 cm²·V⁻¹·s⁻¹ and an on/off ratio of ∼2.6 × 10⁵.

Insight into the 3D Architecture and Quasicrystal Symmetry of Multilayer Nanorod Assemblies from Moiré Interference Patterns

Ajay Singh†‡, Calum Dickinson†, and Kevin M. Ryan†‡*

^† Materials and Surface Science Institute and Department of Chemical and Environmental Sciences,University of Limerick, Limerick, Ireland

^‡ SFI-Strategic Research Cluster in Solar Energy Research, University of Limerick, Limerick, Ireland

ACS Nano, Article ASAP

DOI: 10.1021/nn300331x

Publication Date (Web): March 12, 2012

Copyright © 2012 American Chemical Society

Vertical nanorod assembly over three dimensions is shown to result in the formation of Moiré interference patterns arising from rotational offsets between respective monolayer sheets. Six distinct patterns are observed in HRTEM and angular dark-field STEM (DF-STEM) images, allowing the exact angle of rotation to be determined from their respective size and repeat order. At large rotation angles approaching 30°, the aperiodicity in the structure of the nanorod supercrystals becomes apparent, resulting in 12-fold ordering characteristics of a quasicrystal. The rotational offsets are further elucidated from Fourier transform and small angle electron diffraction, allowing interpretation of several multilayers when combined with DF-STEM and SEM. Pattern formation owing to angular rotation is differentiated from those occurring from a lateral shift, providing an important insight into the complex multilayered structures in assembled rods that may have an impact on their collective electronic or photonic properties. We also show how random tetrapods when present at low concentrations in colloidal nanorod solutions act as termination points for 2D sheet crystallization, impacting the size and shape of the resultant assemblies. The occurrence of Moiré patterns in rod assemblies demonstrates the extraordinary order achievable in their assembly and offers a nondestructive technique to precisely map the placement of each nanorod in this important nanoarchitecture.

Dynamic Electrostatic Lithography: Multiscale On-Demand Patterning on Large-Area Curved Surfaces

1. Qiming Wang, 
2. Mukarram Tahir, 
3. Jianfeng Zang, 
4. Xuanhe Zhao^*

Article first published online: 15 MAR 2012

DOI: 10.1002/adma.201200272

Dynamic electrostatic lithography is invented to dynamically generate various patterns on large-area and curved polymer surfaces under the control of electrical voltages. The shape of the pattern can be tuned from random creases and craters to aligned creases, craters and lines, and the size of the pattern from millimeters to sub-micrometers.

Engineering of Micro- and Nanostructured Surfaces with Anisotropic Geometries and Properties

1. . Tawfick¹, 
2. Michael De Volder^1,2,
3. Davor Copic¹, 
4. Sei Jin Park¹, 
5. C. Ryan Oliver¹, 
6. Erik S. Polsen¹, 
7. Megan J. Roberts¹,
8. A. John Hart^1,*

Article first published online: 6 MAR 2012

DOI: 10.1002/adma.201103796

1. Widespread approaches to fabricate surfaces with robust micro- and nanostructured topographies have been stimulated by opportunities to enhance interface performance by combining physical and chemical effects. In particular, arrays of asymmetric surface features, such as arrays of grooves, inclined pillars, and helical protrusions, have been shown to impart unique anisotropy in properties including wetting, adhesion, thermal and/or electrical conductivity, optical activity, and capability to direct cell growth. These properties are of wide interest for applications including energy conversion, microelectronics, chemical and biological sensing, and bioengineering. However, fabrication of asymmetric surface features often pushes the limits of traditional etching and deposition techniques, making it challenging to produce the desired surfaces in a scalable and cost-effective manner. We review and classify approaches to fabricate arrays of asymmetric 2D and 3D surface features, in polymers, metals, and ceramics. Analytical and empirical relationships among geometries, materials, and surface properties are discussed, especially in the context of the applications mentioned above. Further, opportunities for new fabrication methods that combine lithography with principles of self-assembly are identified, aiming to establish design principles for fabrication of arbitrary 3D surface textures over large areas