An Epoxy Photoresist Modified by Luminescent Nanocrystals for the Fabrication of 3D High‐Aspect‐Ratio Microstructures

An epoxy‐based negative‐tone photoresist, which is known as a suitable material for high‐aspect‐ratio surface micromachining, is functionalized with red‐light‐emitting CdSe@ZnS nanocrystals (NCs). The proper selection of a common solvent for the NCs and the resist is found to be critical for the efficient incorporation of the NCs in the epoxy matrix. The NC‐modified resist can be patterned by standard UV lithography down to micrometer‐scale resolution, and high‐aspect‐ratio structures have been successfully fabricated on a 100 mm scaled wafer. The “as‐fabricated”, 3D, epoxy‐based surface microstructures show the characteristic luminescent properties of the embedded NCs, as verified by fluorescence microscopy. This issue demonstrates that the NC emission properties can be conveniently conveyed into the polymer matrix without deteriorating the lithographic performance of the latter. The dimensions, the resolution, and the surface morphology of the NC‐modified‐epoxy microstructures exhibit only minor deviations with respect to that of the unmodified reference material, as examined by means of microscopic and metrologic investigations. The proposed approach of the incorporation of emitting and non‐bleachable NCs into a photoresist opens novel routes for surface patterning of integrated microsystems with inherent photonic functionality at the micro‐ and nanometer‐scale for light sensing and emitting applications.     

Inside Front Cover: Inkjet Printing of Luminescent CdTe Nanocrystal–Polymer Composites (Adv. Funct. Mater. 1/2007)

Schubert and co‐workers have performed a detailed investigation on ink‐jet printing of well‐defined dots of luminescent CdTe nanocrystals (NCs) embedded in a poly(vinyl alcohol) matrix, as reported on p. 23, and subsequently made studies of their morphology and photoluminescence. The inside cover shows a photograph of an ink‐jet‐printed combinatorial library of differently sized CdTe NCs emitting at different wavelengths, and a 3D profilometer image of an array of printed dots. Inkjet printing is used to produce well‐defined patterns of dots (with diameters of ca. 120 μm) that are composed of luminescent CdTe nanocrystals (NCs) embedded within a poly(vinylalcohol) (PVA) matrix. Addition of ethylene glycol (1–2 vol %) to the aqueous solution of CdTe NCs suppresses the well‐known ring‐formation effect in inkjet printing leading to exceptionally uniform dots. Atomic force microscopy characterization reveals that in the CdTe NC films the particle–particle interaction could be prevented using inert PVA as a matrix. Combinatorial libraries of CdTe NC–PVA composites with variable NC sizes and polymer/NC ratios are prepared using inkjet printing. These libraries are subsequently characterized using a UV/fluorescence plate reader to determine their luminescent properties. Energy transfer from green‐light‐emitting to red‐light‐emitting CdTe NCs in the composite containing green‐ (2.6 nm diameter) and red‐emitting (3.5 nm diameter) NCs are demonstrated.     

Inkjet Printing of Luminescent CdTe Nanocrystal–Polymer Composites

Inkjet printing is used to produce well‐defined patterns of dots (with diameters of ca. 120 μm) that are composed of luminescent CdTe nanocrystals (NCs) embedded within a poly(vinylalcohol) (PVA) matrix. Addition of ethylene glycol (1–2 vol %) to the aqueous solution of CdTe NCs suppresses the well‐known ring‐formation effect in inkjet printing leading to exceptionally uniform dots. Atomic force microscopy characterization reveals that in the CdTe NC films the particle–particle interaction could be prevented using inert PVA as a matrix. Combinatorial libraries of CdTe NC–PVA composites with variable NC sizes and polymer/NC ratios are prepared using inkjet printing. These libraries are subsequently characterized using a UV/fluorescence plate reader to determine their luminescent properties. Energy transfer from green‐light‐emitting to red‐light‐emitting CdTe NCs in the composite containing green‐ (2.6 nm diameter) and red‐emitting (3.5 nm diameter) NCs are demonstrated.     

Soft‐Contact Optical Lithography Using Transparent Elastomeric Stamps and Application to Nanopatterned Organic Light‐Emitting Devices

Conventional photolithography uses rigid photomasks of fused quartz and high‐purity silica glass plates covered with patterned microstructures of an opaque material. We introduce new, transparent, elastomeric molds (or stamps) of poly(dimethylsiloxane) (PDMS) that can be employed as photomasks to produce the same resist pattern as the pattern of the recessed (or non‐contact) regions of the stamps, in contrast to other reports in the literature^[1] of using PDMS masks to generate edge patterns. The exposure dose of the non‐contact regions with the photoresist through the PDMS is lower than that of the contact regions. Therefore, we employ a difference in the effective exposure dose between the contact and the non‐contact regions through the PDMS stamp to generate the same pattern as the PDMS photomask. The photomasking capability of the PDMS stamps, which is similar to rigid photomasks in conventional photolithography, widens the application boundaries of soft‐contact optical lithography and makes the photolithography process and equipment very simple. This soft‐contact optical lithography process can be widely used to perform photolithography on flexible substrates, avoiding metal or resist cracks, as it uses soft, conformable, intimate contact with the photoresist without any external pressure. To this end, we demonstrate soft‐contact optical lithography on a gold‐coated PDMS substrate and utilized the patterned Au/PDMS substrate with feature sizes into the nanometer regime as a top electrode in organic light‐emitting diodes that are formed by soft‐contact lamination.     

Soft Nanopatterning on Light‐Emitting Inorganic–Organic Composites

In this work we demonstrate the nanopatterning of nanocomposites made by luminescent zinc oxide nanoparticles and light‐emitting conjugated polymers by means of soft molding lithography. Vertical nanofluidics is exploited to overcome the polymer transport difficulties intrinsic in materials incorporating nanocrystals, and the rheology, fluorescence, absolute quantum yield, and emission directionality of the nanostructured composites are investigated. We study the effect of patterned gratings on the directionality of light emitted from the nanocomposites, finding evidence of the enhancement of forward emitted light, due to the printed wavelength‐scale periodicity. These results open new possibilities for the realization of nanopatterned devices based on hybrid organic‐inorganic systems.     

Simple Holographic Patterning for High‐Aspect‐Ratio Three‐Dimensional Nanostructures with Large Coverage Area

Using the vertical standing wave phenomena commonly regarded as a deterrent in holographic lithography, multifaceted three‐dimensional (3D) nanostructures are fabricated on polymeric photoresist materials using a simple two‐beam interferometer. Large‐area 3D nanostructures with high aspect ratios (greater than 10) are readily produced using this methodology, including grating, pillar and pore patterns. Furthermore, manipulation of the lithography process conditions results in unique sidewall profiles of the nanostructures. Such 3D holographic control even produces highly porous polymer membranes composed of 3D interconnected pore networks, which resembles the 3D photonic crystal compound nanostructures that were previously attainable only with limited pattern coverage area using complex multibeam holographic lithography processes. Such well‐tailored high‐aspect‐ratio 3D nanostructures with large pattern coverage area further enable the fabrication of novel nanostructures for functionalized materials via various additive and subtractive pattern transfer techniques such as etching, deposition, and molding. In particular, direct molding followed by thermal decomposition process leads to the synthesis of hierarchical titanium oxide nanostructures of tunable 3D geometry, which would be of great significance in applications of photonic crystals, photovoltaic solar cells, and photocatalyst in water decontamination.     

High Color Rendering Index Hybrid III‐Nitride/Nanocrystals White Light‐Emitting Diodes

An excellent hybrid III‐nitride/nanocrystal nanohole light‐emitting diode (h‐LED) has been developed utilizing nonradiative resonant energy transfer (NRET) between violet/blue emitting InGaN/GaN multiple quantum wells (MQWs) and various wavelength emitting nanocrystals (NCs) as color‐conversion mediums. InGaN/GaN MQWs are fabricated into nanoholes by soft nanoimprint lithography to minimize the separation between MQWs and NCs. A significant reduction in the decay lifetime of excitons in the MQWs of the hybrid structure has been observed as a result of the NRET from the nitride emitter to NCs. The NRET efficiency of the hybrid structures is obtained from the decay curves, as high as 80%. Moreover, a modified Förster formulation has exhibited that the exciton coupling distance in the hybrid structures is less than the Förster's radius, demonstrating a strong coupling between MQWs and NCs. Finally, based on a systemic optimization for white emission indexes, a series of hybrid ternary complementary color h‐LEDs have been demonstrated with a high color rendering index, up to 82, covering the white light emission at different correlated color temperatures ranging from 2629 to 6636 K, corresponding to warm white, natural white, and cold white.     

Nanostructured Polymer Brushes by UV‐Assisted Imprint Lithography and Surface‐Initiated Polymerization for Biological Functions

Functional polymer brush nanostructures are obtained by combining step‐and‐flash imprint lithography (SFIL) with controlled, surface‐initiated polymerization (CSIP). Patterning is achieved at length scales such that the smallest elements have dimensions in the sub‐100 nm range. The patterns exhibit different shapes, including lines and pillars, over large surface areas. The platforms obtained are used to selectively immobilize functional biomacromolecules. Acrylate‐based polymer resist films patterned by SFIL are first used for the selective immobilization of ATRP silane‐based initiators, which are coupled to unprotected domains of silicon substrates. These selectively deposited initiators are then utilized in the controlled radical SIP of poly(ethylene glycol)methacrylates (PEGMA). Nanostructured brush surfaces are then obtained by removal of the resist material. The areas previously protected by the SFIL resist are passivated by inert, PEG‐based silane monolayers following resist removal. PEGMA brush nanostructures are finally functionalized with biotin units in order to provide selective attachment points for streptavidin proteins. Atomic force microscopy and fluorescence spectroscopy confirm the successful immobilization of streptavidin molecules on the polymer grafts. Finally, it is demontrated that this fabrication method allows the immobilization of a few tens of protein chains attached selectively to brush nanostructures, which are surrounded by nonfouling PEG‐functionalized areas.     

Novel Patterning of Gold Using Spin‐Coatable Gold Electron‐Beam Resist

Conventional lithography methods of gold patterning are based on deposition and lift‐off or deposition and etching. In this letter, we demonstrate a novel method of gold patterning using spin‐coatable gold electron‐beam resist which is functionalized gold nanocrystals with amine ligands. Amine‐stabilized gold electron beam resist exhibits good sensitivity, 3.0 mC/cm², compared to that of thiol‐stabilized gold electron beam resists. The proposed method reduces the number of processing steps and provides greater freedom in the patterning of complex nanostructures.     

Conductivity of SU‐8 Thin Films through Atomic Force Microscopy Nano‐Patterning

Processing flexibility and good mechanical properties are the two major reasons for SU‐8 extensive applicability in the micro‐fabrication of devices. In order to expand its usability down to the nanoscale, conductivity of ultra‐thin SU‐8 layers as well as its patterning by AFM are explored. By performing local electrical measurements outstanding insulating properties and a dielectric strength 100 times larger than that of SiO₂ are shown. It is also demonstrated that the resist can be nano‐patterned using AFM, obtaining minimum dimensions below 40nm and that it can be combined with parallel lithographic methods like UV‐lithography. The concurrence of excellent insulating properties and nanometer‐scale patternability enables a valuable new approach for the fabrication of nanodevices. As a proof of principle, nano‐electrode arrays for electrochemical measurements which show radial diffusion and no overlap between different diffusion layers are fabricated. This indicates the potential of the developed technique for the nanofabrication of devices.     

CsPbBr3 Nanocrystal Films: Deviations from Bulk Vibrational and Optoelectronic Properties

Metal‐halide perovskites (MHP) are highly promising semiconductors for light‐emitting and photovoltaic applications. The colloidal synthesis of nanocrystals (NCs) is an effective approach for obtaining nearly defect‐free MHP that can be processed into inks for low‐cost, high‐performance device fabrication. However, disentangling the effects of surface ligands, morphology, and boundaries on charge‐carrier transport in thin films fabricated with these high‐quality NCs is inherently difficult. To overcome this fundamental challenge, terahertz (THz) spectroscopy is employed to optically probe the photoconductivity of CsPbBr₃ NC films. The vibrational and optoelectronic properties of the NCs are compared with those of the corresponding bulk polycrystalline perovskite and significant deviations are found. Charge‐carrier mobilities and recombination rates are demonstrated to vary significantly with the NC size. Such dependences derive from the localized nature of charge carriers within NCs, with local mobilities dominating over interparticle transport. It is further shown that the colloidally synthesized NCs have distinct vibrational properties with respect to the bulk perovskite, exhibiting blue‐shifted optical phonon modes with enhanced THz absorption strength that also manifest as strong modulations in the THz photoconductivity spectra. Such fundamental insights into NC versus bulk properties will guide the optimization of nanocrystalline perovskite thin films for optoelectronic applications.     

Chemically amplified resists: Chemistry and processes

Continued advances in mocroelectronic device fabrication are trying the limits of conventional lithographic techniques. In particular, conventional photoresist materials are not appropriate for use with the new technologies that will be necessary for sub-0.5 μm lithography. One approach to the desing of new resist chemistries involves the concept of chemical amplification, where one photochemical event can lead to a cascade of subsequent reactions that effect a change in solubility of the parent material. The most well-known chemically amplified resists utilise photchemically generated acid to catalyse crosslinking or deprotection reactions. This paper reviews the acid generator, crosslinking, deprotection and depolymerisation chemistries that have been evaluated for chemical amplification resist processes. Additionaly, process characteristics and resist performance relative to the process environment are addressed.     

Patterning Semiconductor Nanocrystals in Polymer Films

This paper reports a method of patterning semiconductor nanocrystals (NCs) in a polymer film. By combining lithographic concept with polymerization‐induced phase separation in a NC‐monomer mixture, we produced topographic and lateral compositional patterns in NC‐polymer films. The quality of the patterns was controlled by the competition between the rate of polymerization‐induced phase separation and the change in viscosity of the polymerizing mixture.     

α,ω‐Dithiol Oligo(phenylene vinylene)s for the Preparation of High‐Quality π‐Conjugated Self‐Assembled Monolayers and Nanoparticle‐ Functionalized Electrodes

While thioacetate‐terminated oligo(phenylene vinylene)s (OPVs) have been synthesized and employed in applications involving the formation of metal–molecule–metal junctions, the synthesis and application of potentially more versatile α,ω‐dithiol OPVs have not previously been described. Here, a thiomethyl‐precursor route to the synthesis of α,ω‐dithiol OPVs is reported and their ability to form well‐ordered self‐assembled monolayers (SAMs) without the addition of exogenous deprotection reagents is described. α,ω‐Dithiol OPV monolayers exhibit thicknesses consistent with molecular length and are nearly defect‐free, as assayed by electrochemical measurements. To demonstrate the ease with which SAMs containing these bifunctional OPVs can, in contrast to thioacetate functionalized OPVs, be further functionalized with materials other than gold, we have modified them in a single step with a sub‐monolayer of cadmium selenide nanocrystals (NCs). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirm that these NC‐modified films are both smooth and uniform over the largest areas investigated (> 10 μm²) and no evidence of NC aggregation is observed. To evaluate the electrochemical response of these metal–molecule–semiconductor assemblies we have fabricated NC‐modified OPV SAMs with ferrocene‐coated NCs. Variable‐frequency alternating current voltammetry indicates that electron transfer in these assemblies is much more rapid than in analogous structures formed using simple alkane dithiols. It thus appears that α,ω‐dithiol OPVs are well suited for the formation of high‐quality conducting SAMs for the functionalization of gold and other surfaces.     

Chloride and Indium‐Chloride‐Complex Inorganic Ligands for Efficient Stabilization of Nanocrystals in Solution and Doping of Nanocrystal Solids

Here, the surface functionalization of CdSe and CdSe/CdS core/shell nanocrystals (NCs) with compact chloride and indium‐chloride‐complex ligands is reported. The ligands provide not only short interparticle distances but additionally control doping and passivation of surface trap states, leading to enhanced electronic coupling in NC‐based arrays. The solids based on these NCs show an excellent electronic transport behavior after heat treatment at the relatively low temperature of 190 °C. Indeed, the indium‐chlorido‐capped 4.5 nm CdSe NC based thin‐film field‐effect transistor reaches a saturation mobility of μ = 4.1 cm² (V s)^?1 accompanied by a low hysteresis, while retaining the typical features of strongly quantum confined semiconductor NCs. The capping with chloride ions preserves the high photoluminescence quantum yield ( ≈ 66%) of CdSe/CdS core/shell NCs even when the CdS shell is relatively thin (six monolayers). The simplicity of the chemical incorporation of chlorine and indium species via solution ligand exchange, the efficient electronic passivation of the NC surface, as well as their high stability as dispersions make these materials especially attractive for wide‐area solution‐processable fabrication of NC‐based devices.     

Role of PbSe Structural Stabilization in Photovoltaic Cells

Semiconductor nanocrystals are promising materials for printed optoelectronic devices, but their high surface areas are susceptible to forming defects that hinder charge carrier transport. Furthermore, correlation of chalcogenide nanocrystal (NC) material properties with solar cell operation is not straightforward due to the disorder often induced into NC films during processing. Here, an improvement in long‐range ordering of PbSe NCs symmetry that results from halide surface passivation is described, and the effects on chemical, optical, and photovoltaic device properties are investigated. Notably, this passivation method leads to a nanometer‐scale rearrangement of PbSe NCs during ligand exchange, improving the long‐range ordering of nanocrystal symmetry entirely with inorganic surface chemistry. Solar cells constructed with a variety of architectures show varying improvement and suggest that triplet formation and ionization, rather than carrier transport, is the limiting factor in singlet fission solar cells. Compared to existing protocols, our synthesis leads to PbSe nanocrystals with surface‐bound chloride ions, reduced sub‐bandgap absorption and robust materials and devices that retain performance characteristics many hours longer than their unpassivated counterparts.     

The Impact of Carrier Transport Confinement on the Energy Transfer Between InGaN/GaN Quantum‐Well Nanorods and Colloidal Nanocrystals

The energy transfer (ET) between InGaN/GaN multiple‐quantum‐well (MQW) nanorods (NRs) and semiconductor nanocrystals (NCs) for efficient color conversion is studied. An exceptional contribution of carrier transport confinement to the ET mechanisms is observed in the proximal side‐wall coupling system, which consists of InGaN/GaN NRs and CdSe NCs. Under relatively low or high excitation, the ET rate shows different carrier‐density dependence, resulting from different electron‐hole configurations, i.e., bound excitons and free carriers. In the localized exciton regime, the ET rate decreases when increasing temperature from 20 K to 200 K. However, in the free‐carrier regime, the ET rate varies insignificantly in the same temperature range. The temperature dependence in this NR‐NC coupling system is different from that in the previously studied planar MQW‐NC coupling system. It is suggested that the carrier transport confinement in NRs is a major factor for these divergences. The highly efficient ET with efficiency up to 80% shows a promising potential of using such NR‐NC coupled structures for ET‐pumped, NC‐based, light‐emitting devices.     

Poly(N‐isopropylacrylamide)‐Clay Nanocomposite Hydrogels with Responsive Bending Property as Temperature‐Controlled Manipulators

Novel poly(N‐isopropylacrylamide)‐clay (PNIPAM‐clay) nanocomposite (NC) hydrogels with both excellent responsive bending and elastic properties are developed as temperature‐controlled manipulators. The PNIPAM‐clay NC structure provides the hydrogel with excellent mechanical property, and the thermoresponsive bending property of the PNIPAM‐clay NC hydrogel is achieved by designing an asymmetrical distribution of nanoclays across the hydrogel thickness. The hydrogel is simply fabricated by a two‐step photo polymerization. The thermoresponsive bending property of the PNIPAM‐clay NC hydrogel is resulted from the unequal forces generated by the thermoinduced asynchronous shrinkage of hydrogel layers with different clay contents. The thermoresponsive bending direction and degree of the PNIPAM‐clay NC hydrogel can be adjusted by controlling the thickness ratio of the hydrogel layers with different clay contents. The prepared PNIPAM‐clay NC hydrogels exhibit rapid, reversible, and repeatable thermoresponsive bending/unbending characteristics upon heating and cooling. The proposed PNIPAM‐clay NC hydrogels with excellent responsive bending property are demonstrated as temperature‐controlled manipulators for various applications including encapsulation, capture, and transportation of targeted objects. They are highly attractive material candidates for stimuli‐responsive “smart” soft robots in myriad fields such as manipulators, grippers, and cantilever sensors.     

High Hole Mobility in Long‐Range Ordered 2D Lead Sulfide Nanocrystal Monolayer Films

Coupling between colloidal semiconductor nanocrystals (NCs) with long‐range order is critical for designing advanced nanostructures with controlled energy flow and charge carrier transport. Herein, under the premise of keeping long‐range order in 2D NC monolayer, its native organic ligands are exchanged with halogen ions in situ at the liquid–air interface to enhance the coupling between NCs. Further treatments on the films with dimethyl sulfoxide, methanol, or their mixture effectively improve carrier mobility of the devices. The devices show repeatable enhanced p‐type transport behavior with hole mobility of up to 0.224 ± 0.069 cm² V^?1 s^?1, the highest value reported for lead sulfide NC solids without annealing treatment. Thanks to accurate control over the surface of NCs as well as the structure of NC film, the ordered NC monolayer film of high hole mobility suggests great potentials for making reliable high performance devices.     

Bandgap Engineering in OH‐Functionalized Silicon Nanocrystals: Interplay between Surface Functionalization and Quantum Confinement

In this work, a systematic first‐principles study of the quasi‐band structure of silicon nanocrystals (Si‐NCs) is provided, focusing on bandgap engineering by combining quantum confinement of the electronic states with OH surface‐functionalization. A mapping between the bandgap, Si‐NC diameter, and the degree of hydroxide coverage is provided, which can be used as a guideline for bandgap engineering. Complementary to first‐principles calculations, the photoluminescence (PL) wavelength of Si‐NCs in the quantum‐confinement regime is measured with well‐defined diameters between 1 and 4 nm. The Si‐NCs are prepared by means of a microplasma technique, which allows a surfactant‐free engineering of the Si‐NCs surface with OH groups. The microplasma treatment technique allows us to gradually change the degree of OH coverage, enabling us, in turn, to gradually shift the emitted light in the PL spectra by up to 100 nm to longer wavelengths. The first‐principles calculations are consistent with the experimentally observed dependence of the wavelengths on the OH coverage and show that the PL redshift is determined by the charge transfer between the Si‐NC and the functional groups, while on the other hand surface strain plays only a minor part.