Transfer printing represents a set of techniques for deterministic assembly of micro-and nanomaterials into spatially organized, functional arrangements with two and three-dimensional layouts. Such processes provide versatile routes not only to test structures and vehicles for scientific studies but also to high-performance, heterogeneously integrated functional systems, including those in flexible electronics, three-dimensional and/or curvilinear optoelectronics, and bio-integrated sensing and therapeutic devices. This article summarizes recent advances in a variety of transfer printing techniques, ranging from the mechanics and materials aspects that govern their operation to engineering features of their use in systems with varying levels of complexity. A concluding section presents perspectives on opportunities for basic and applied research, and on emerging use of these methods in high throughput, industrial-scale manufacturing.

Transfer printing by kinetically switchable adhesion to an elastomeric stamp shows promise as a powerful micromanufacturing method to pickup microstructures and microdevices from the donor substrate and to print them to the receiving substrate. This can be viewed as the competing fracture of two interfaces. This paper examines the mechanics of competing fracture in a model transfer printing system composed of three laminates: an elastic substrate, an elastic thin film, and a viscoelastic member (stamp). As the system is peeled apart, either the interface between the substrate and thin film fails or the interface between the thin film and the stamp fails. The speed-dependent nature of the film/stamp interface leads to the prediction of a critical separation velocity above which separation occurs between the film and the substrate (i.e., pickup) and below which separation occurs between the film and the stamp (i.e., printing). Experiments verify this prediction using films of gold adhered to glass, and the theoretical treatment extends to consider the competing fracture as it applies to discrete micro-objects. Temperature plays an important role in kinetically controlled transfer printing with its influences, making it advantageous to pickup printable objects at the reduced temperatures and to print them at the elevated ones.

applications. Some notable examples of techniques motivated by topics attracting signifi cant attention in current research include nanoimprint lithography, [ 1 , 2 ] nanosphere lithography, [ 3 , 4 ] scanning probe lithography techniques, [ 5 , 6 ] and advanced forms of soft lithography, [7][8][9][10] including interference lithography with elastomeric contact masks. [11][12][13] Additionally, the realization of structures with triangular crosssections, such as cones and prisms, would enable applications in microfl uidic lab-ona-chip devices, [ 14 , 15 ] optical components, antirefl ective coatings, [ 16 , 17 ] self-cleaning surfaces with tuned contact angles, [18][19][20] surface enhanced Raman spectroscopy (SERS) sensing, [ 21 , 22 ] and probe-based patterning techniques. [ 23 , 24 ] A number of useful fabrication techniques have been developed to achieve this geometrical attribute, most commonly relying on etching to defi ne the patterns. The most commonly used method uses KOH-based anisotropic wet etching of Si(100) patterned with photoresist lines, to etch along the < 110 > direction and thus form an array of linear pits with isosceles triangle shaped cross-sections. [25][26][27][28] The feature pitch is dictated by the precision of the alignment of the photoresist features The use of a decal transfer lithography technique to fabricate elastomeric stamps with triangular cross-sections, specifi cally triangular prisms and cones, is described. These stamps are used in demonstrations for several prototypical optical applications, including the fabrication of multiheight 3D photoresist patterns with near zero-width features using near-fi eld phase shift lithography, fabrication of periodic porous polymer structures by maskless proximity fi eld nanopatterning, embossing thin-fi lm antirefl ection coatings for improved device performance, and effi cient fabrication of substrates for surface-enhanced Raman spectroscopic sensing. The applications illustrate the utility of the triangular poly(dimethylsiloxane) decals for a wide variety of optics-centric applications, particularly those that exploit the ability of the designed geometries and materials combinations to manipulate light-matter interactions in a predictable and controllable manner.

This paper describes soft lithography methods that expand current fabrication capabilities by enabling high‐throughput patterning on nonplanar substrates. These techniques exploit optically dense elastomeric mask elements embedded in a transparent poly(dimethylsiloxane) (PDMS) matrix by vacuum‐assisted microfluidic patterning, UV–ozone‐mediated irreversible sealing, and chemical etching. These protocols provide highly flexible photomasks exhibiting either positive‐ or negative‐image contrasts, which serve as amplitude masks for large‐area photolithographic patterning on a variety of curved (and planar) surfaces. When patterning on cylindrical surfaces, the developed masks do not experience significant pattern distortions. For substrates with 3D curvatures/geometries, however, the PDMS mask must undergo relatively large strains in order to make conformal contact. The new methods described in this report provide planar masks that can be patterned to compliantly compensate for both the displacements and distortions of features that result from stretching the mask to span the 3D geometry. To demonstrate this, a distortion‐corrected grid pattern mask was fabricated and used in conjunction with a homemade inflation device to pattern an electrode mesh on a glass hemisphere with predictable registration and distortion compensation. The showcased mask fabrication processes are compatible with a broad range of substrates, illustrating the potential for development of complex lithographic patterns for a variety of applications in the realm of curved electronics (i.e., synthetic retinal implants and curved LED arrays) and wide field‐of‐view optics.

The widely used steady-state energy release rate G = F/w is extended to accotmt for the elastic energy of deformed compliant stamps, e.g., low-modulus poly(dimethyl siloxane) (PDMS). An analytical expression for the energy release rate is obtained to quantify inteifacial adhesion strength in tape peeling tests, and to analyze the dynamics of kineticaiiy controlled transfer printing. The critical delamination velocity to separate retrieval and printing is related to the critical energy release rate and the tensile stiffness of the stamp. Experimental results validate the analytical expression established by the mechanics model.

A method for fabricating chemical gradients on planar and nonplanar substrates using grayscale lithography is reported. Compliant grayscale amplitude masks are fabricated using a vacuum‐assisted microfluidic filling protocol that employs dilutions of a carbon‐black‐containing polydimethylsiloxane emulsion (bPDMS) within traditional clear PDMS (cPDMS) to create planar, fully self‐supporting mask elements. The mask is then placed over a surface functionalized with a hydrophobic coumarin‐based photocleavable monolayer, which exposes a polar group upon irradiation. The mask serves to modulate the intensity of incident UV light, thereby controlling the density of molecules cleaved. The resulting molecular‐level grayscale patterns are characterized by condensation microscopy and imaging mode time‐of‐flight secondary‐ion mass spectrometry (ToF‐SIMS). Due to the inherent flexibility of this technique, the photofuse as well as the gradient patterns can be designed for a wide range of applications; in this paper two proof‐of‐concept demonstrations are shown. The first utilizes the ability to control the resulting contact angle of the surface for the fabrication of a passive pressure‐sensitive microfluidic gating system. The second is a model surface modification process that utilizes the functional groups deprotected during the photocleavage to pattern the deposition of moieties with complementary chemistry. The spatial layout, resolution, and concentration of these covalently linked molecules follow the gradient pattern created by the grayscale mask during exposure. The programmable chemical gradient fabrication scheme presented in this work allows explicit engineering of both surface properties that dictate nonspecific interactions (surface energy, charge, etc.) and functional chemistry necessary for covalent bonding.

As integrated circuit interconnect dimensions continue to shrink and signaling frequencies increase, interconnect performance degrades. The performance degradation is due to several factors such as power consumption, cross-talk, and signal attenuation. On-chip optical interconnects are a potential solution to these scaling issues because they offer the promise of providing higher bandwidth.In this paper, progress on the major on-chip optical building blocks will be reviewed. It will be shown that significant advances have been made in the design and fabrication of waveguides, detectors, and couplers. However, major challenges in high speed electrical to optical conversion and signaling remain.

This paper demonstrates the application of a modified Levich equation for chemical systems with varying viscosity. A commonly used technique to analyze rotating disc electrode (RDE) experiments is to fit the data to the Levich equation assuming a constant effective diffusion coefficient which may be valid for conditions where the viscosity does not vary significantly (less than an order of magnitude). However, most diffusion coefficient models (e.g. Stokes-Einstein) show an inverse relationship with viscosity which consequently indicates that a constant effective diffusion coefficient may result in poorer model-to-data agreement. Here, data are presented for a series of RDE experiments for the electrodissolution of Cu in phosphoric acid, water and glycerin based baths. Viscosity changes of greater than one order of magnitude allow for testing the assumption of a constant effective diffusion coefficient. The collected data, as well as data published elsewhere, can be explained by a modified Levich equation which takes into account the viscosity dependence of the diffusion coefficient. List of SymbolsC A Concentration of A in solution (mol l -1 ) D Diffusion coefficient (m 2 s -1 ) D A Effective diffusion coefficient of A (m 2 s -1 ) D AB Mutual diffusivity at infinite dilution of A in B (m 2 s -1 ) D AB Mutual diffusivity (m 2 s -1 ) F Faraday's constant (C mol -1 ) I lim Limiting current per unit area (A m -2 ) k Boltzman's constant (J K -1 ) m Molality of solute (mol (kg of solvent) -1 ) n Ionic charge r Effective radius (m) s Solvent coordination number T Absolute temperature (K) V Molar volume (m 3 mol -1 ) c ± Mean ionic activity coefficient of solute l Absolute viscosity (cP) m Kinematic viscosity (m 2 s -1 ) u B Association factor for solvent B for Wilke-Chang equation w B Parachor parameter for component B for Tyn-Calus equation xRotational speed (rad s -1 )