The manufacturing of devices from methylammonium-based perovskites asks for reliable and scalable processing. As solvent engineering is not the option of choice to obtain homogeneous layers on large areas, our idea is to 'upgrade' a non-perfect pristine layer by recrystallization in a thermal imprint step (called 'planar hot pressing') and thus to reduce the demands on the layer formation itself. Recently, imprint has proven both its capability to improve the crystal size of perovskite layers and its usability for large area manufacturing. We start with methylammonium lead bromide layers obtained from a conventional solution-based process. Acetate is used as a competitive lead source; even under perfect conditions the resulting perovskite layer then will contain side-products due to layer formation besides the desired perovskite. Based on the physical properties of the materials involved we discuss the impact of the temperature on the status of the layer both during soft-bake and during thermal imprint. By using a special imprint technique called 'hot loading' we are able to visualize the upgrade of the layer with time, namely a growth of the grains and an accumulation of the side-products at the grain boundaries. By means of a subsequent vacuum exposition we reveal the presence of non-perovskite components with a simple inspection of the morphology of the layer; all experiments are supported by X-ray and electron diffraction measurements. Besides degradation, we discuss recrystallization and propose post-crystallization to explain the experimental results. This physical approach towards perovskite layers with large grains by post-processing is a key step towards large-area preparation of high-quality layers for device manufacturing.

The quality and the stability of devices prepared from polycrystalline layers of organic–inorganic perovskites highly depend on the grain sizes prevailing. Tuning of the grain size is either done during layer preparation or in a post-processing step. Our investigation refers to thermal imprint as the post-processing step to induce grain growth in perovskite layers, offering the additional benefit of providing a flat surface for multi-layer devices. The material studied is MAPbBr 3 ; we investigate grain growth at a pressure of 100 bar and temperatures of up to 150 °C, a temperature range where the pressurized stamp is beneficial to avoid thermal degradation. Grain coarsening develops in a self-similar way, featuring a log-normal grain size distribution; categories like ‘normal’ or ‘secondary’ growth are less applicable as the layers feature a preferential orientation already before imprint-induced grain growth. The experiments are simulated with a capillary-based growth law; the respective parameters are determined experimentally, with an activation energy of Q ≈ 0.3 eV. It turns out that with imprint as well the main parameter relevant to grain growth is temperature; to induce grain growth in MAPbBr 3 within a reasonable processing time a temperature of 120 °C and beyond is advised. An analysis of the mechanical situation during imprint indicates a dominance of thermal stress. The minimization of elastic energy and surface energy together favours the development of grains with (100)-orientation in MaPbBr 3 layers. Furthermore, the experiments indicate that the purity of the materials used for layer preparation is a major factor to achieve large grains; however, a diligent and always similar preparation of the layer is equally important as it defines the pureness of the resulting perovskite layer, intimately connected with its capability to grow. The results are not only of interest to assess the potential of a layer with respect to grain growth when specific temperatures and times are chosen; they also help to rate the long-term stability of a layer under temperature loading, e.g. during the operation of a device.

Our study refers to the highly stretchable elastomer PDMS (polydimethylsiloxane), a material used with a wide range of applications. Its basic mechanical properties can be tuned, e.g., by varying the curing conditions; moreover, its surface properties can be tuned by modification techniques. We modified our PDMS by irradiating the samples with an excimer lamp at 172 nm. Such a treatment hardens the elastomer at the surface, and it becomes silicalike; the sample changes to a “quasi” two-layer system with a graded interface. When such samples are stretched, surface cracks occur beyond a critical strain. The increase of crack length with increasing strain is evaluated by means of video screenshots. The impact of the curing conditions is addressed by analyzing samples prepared at different cross-linking temperatures, resulting in differing bulk properties but similar surface properties. Crack length and crack velocity are evaluated with each sample based on single randomly chosen cracks. The results are discussed on the basis of theoretical concepts for channeling cracks in multilayer systems with polymeric substrates. Typically, with applications, random cracks should develop at high strain only and, if present, should propagate slowly along the surface but not into the depth of the sample. Our investigation shows that the mechanical material properties of the substrate are vital with respect to such stable cracking, rather than the surface properties. In particular, the curing conditions chosen for the substrate are essential to reduce cracking, a fact less regarded with applications so far.

When nanoimprint serves as a lithography process, it is most attractive for the ability to overcome the typical residual layer remaining without the need for etching. Then, ‘partial cavity filling’ is an efficient strategy to provide a negligible residual layer. However, this strategy requires an adequate choice of the initial layer thickness to work without defects. To promote the application of this strategy we provide a ‘guiding chart’ for initial layer choice. Due to volume conservation of the imprint polymer this guiding chart has to consider the geometric parameters of the stamp, where the polymer fills the cavities only up to a certain height, building a meniscus at its top. Furthermore, defects that may develop during the imprint due to some instability of the polymer within the cavity have to be avoided; with nanoimprint, the main instabilities are caused by van der Waals forces, temperature gradients, and electrostatic fields. Moreover, practical aspects such as a minimum polymer height required for a subsequent etching of the substrate come into play. With periodic stamp structures the guiding chart provided will indicate a window for defect-free processing considering all these limitations. As some of the relevant factors are system-specific, the user has to construct his own guiding chart in praxis, tailor-made to his particular imprint situation. To facilitate this task, all theoretical results required are presented in a graphical form, so that the quantities required can simply be read from these graphs. By means of examples, the implications of the guiding chart with respect to the choice of the initial layer are discussed with typical imprint scenarios, nanoimprint at room temperature, at elevated temperature, and under electrostatic forces. With periodic structures, the guiding chart represents a powerful and straightforward tool to avoid defects in praxis, without in-depth knowledge of the underlying physics.

The transfer of systems from nature to technical schemes is still a challenge as the fabrication of such complex systems in a cost efficient way while keeping the intended technical functionality requires appropriate materials and fabrication techniques. Here the multi-scale hierarchy of the gecko foot was transferred into membrane-actuated gecko-inspired adhesion elements (AE) to study the adhesion to surfaces. Therefore, a three level hierarchical polydimethylsiloxane (PDMS) structure comprising a membrane for actuating and the micron-patterned AE were mould from a microtechnical fabricated master into PDMS by thermal and UV curing. The micron-patterned AE consist of dot or line micron patterns on top of cuboid posts with 15 µm height that representing the second hierarchical level. For master fabrication lithographic masking, wet and dry etching as well as SU8 3D lithography was applied. The adhesion of such AE on smooth, rough and chemically modified glass surfaces was measured by determining the shear force for several different AE designs and PDMS materials. A slip of the micron sized PDMS AEs at the adhesion test sample surface was measured. This slip of the PDMS elements being in contact to the surface can enhance the accepted shear force and the measured anisotropy of the shear force for linear AEs. The results found can contribute to the optimization of PDMS AE for technical surfaces with chemically and topographically inhomogeneous properties.

The nanopatterning of surfaces and thin films with pattern dimensions of less than 100 nm is challenging for laser processing in particular in the case of large-area, low-cost fabrication. Self-assembly processes, however, provide a mechanism of pattern generation in this dimensional range offering an alternative fabrication method. The current work focuses on high-temperature, short-time laser annealing of PS-b-PMMA block copolymer (PS-b-PMMA BCP: poly(styrene-block-methyl methacrylate)) films on fused silica samples to achieve self-assembly into vertical lamellas with periods of approximately 50 nm. BCP samples were irradiated with a focussed CO2-laser beam for studying the influence of the laser power P-L and the scanning speed vs on the lamellae formation in BCP films. The formation of lamellae is observed in the centre of the laser track at sufficient laser irradiation (PL: 2 to 15 W, vs: 1 to 250 mms(-1)). With increasing laser irradiation, first the quality of the lamellas improves to a certain point but thereafter a partial degradation of the BCP and dewetting of the BCP film occurs. The partial degradation of the PMMA micro-phase of the ordered BCP results in a local self-developing process reducing the processing steps for nanopattern formation. The experimental results on laser-induced local self-assembly with irradiation times below 0.1 s are discussed in relation to laser-induced temperature field simulations. Combining self-assembly capabilities of BCP with local heating by laser beams can provide a tool for direct writing of hierarchical nano-/microscale patterns that is useful for various applications mimicking bio-inspired structures.

Recent work on methyl ammonium based halide perovskites has demonstrated their capability as active materials for lasers [1] or solar cells [2]. However, solution-processed polycrystalline layers, which are obtained by spin coating, are rough and may exhibit structural defects, such as holes. It could be demonstrated that thermal nanoimprint is well suited to improve the quality of these layers. The perovskite layers are flattened, holes are less prominent and especially the crystal grains have grown. Most commonly, the quality of the layers is characterized for control purposes and with respect to the application envisaged (e.g. by XRD, PL, ASE). The physics underlying the crystal grain growth during the imprint process has not been studied yet [3].

Cesium lead halide perovskites are of interest for light-emitting diodes and lasers. So far, thin-films of CsPbX 3 have typically afforded very low photoluminescence quantum yields (PL-QY < 20%) and amplified spontaneous emission (ASE) only at cryogenic temperatures, as defect related nonradiative recombination dominated at room temperature (RT). There is a current belief that, for efficient light emission from lead halide perovskites at RT, the charge carriers/excitons need to be confined on the nanometer scale, like in CsPbX 3 nanoparticles (NPs). Here, thin films of cesium lead bromide, which show a high PL-QY of 68% and low-threshold ASE at RT, are presented. As-deposited layers are recrystallized by thermal imprint, which results in continuous films (100% coverage of the substrate), composed of large crystals with micrometer lateral extension. Using these layers, the first cesium lead bromide thin-film distributed feedback and vertical cavity surface emitting lasers with ultralow threshold at RT that do not rely on the use of NPs are demonstrated. It is foreseen that these results will have a broader impact beyond perovskite lasers and will advise a revision of the paradigm that efficient light emission from CsPbX 3 perovskites can only be achieved with NPs.

Guiding of the phase separation of a block copolymer (BCP) by an electric field perpendicular to the substrate is investigated in order to obtain vertical structures that can provide a mask for subsequent etching. Because of practical aspects, the substrate is bare Si without any neutral brush and the process time is limited to 1 h. A polystyrene-block polymethylmethacrylate lamellar material is employed in the study. For a unique guiding of the lamellar phase, an ordering mechanism orthogonal to the electric field is introduced by the interaction with the stamp in a thermal nanoimprint process. The naturally low surface energy of the stamp shall induce the formation of lamellae along the sidewalls of linear cavities. In order to fully utilize these two ordering mechanisms, the stamp sidewalls and the electric field, the imprint process is conducted in such a way that no residual layer remains below the stamp structures and the whole BCP is accumulated inside the cavities which are just partly filled. The electrically-assisted imprint process is studied analytically, considering the capacitive effects due to the local electric field in the cavity and in particular in the BCP. In addition, a numerical simulation is performed for the actual experimental conditions to compute the electric vector field in the BCP. In this way, an extensive understanding of the situation is gained which is the basis for choosing optimal experimental conditions for electrically-assisted thermal nanoimprint. Furthermore, the ambiguity of the electric field in a thermal nanoimprint process with partly filled cavities is addressed. The field shall induce vertical phase separation but, due to instabilities, it also may induce capillary bridges that represent replication defects. An improvement of the vertical phase separation by applying an electric field as high as 25 V/μm could be identified under specific experimental conditions. However, the guiding effect within the cavities and thus the long-range order of the lamellae remained limited. This may be due to a field strength too low in the BCP; in the present configuration, higher field strengths are prohibited by an electrical breakthrough.

The copy of structures in the same tone as the original asks at least for a double replication. Each replication generation will suffer from shrinkage of the replication material used, due to curing or thermal contraction. The impact of shrinkage is addressed in a basic study by simulation and experiment. The main replication materials investigated are OrmoStamp and SU-8. Presently, the preparation of anisotropic adhesion elements with hierarchical structures is investigated. The structures are micrometer-sized isolated pads with nanometer-sized self-aligned ripples (laser-induced periodic surface structures) on top. The initial structures are available as isolated photoresist patterns on Si. The double replication of this polymeric master raises questions with respect to its chemical and mechanical stability. The authors report an optimized replication process with an OrmoStamp intermediate template and a final replication in an elastomer. The anisotropic character of the so prepared hierarchical elastomeric adhesion elements is demonstrated by measurement.

Surface-hardened polydimethylsiloxane (PDMS) with random cracks is studied by means of double replication. The PDMS samples are prepared under different curing conditions, resulting in a different Young's modulus of the bulk. To modify the surface, an excimer lamp at 172 nm is used. The samples are stretched uniaxially until the first cracks appear. As an evaluation under strain is hard or rather impossible, the idea is to replicate the cracked sample in a curable material and to characterize the cracks by inspection of the replica. To protect the sample from mechanical loading, this replication is done by molding in OrmoStamp on glass; these replicas are used for optical inspection to determine the crack spacing. As a characterization of the depth and width of the cracks is highly facilitated when cleaving is enabled, a second replication is performed into a thin layer of SU-8 on Si; these second replica are analyzed by secondary electron microscopy of cross sections. They provide a realistic picture of the crack shape. It is found that the curing conditions affect the crack shape; a U-shape occurs with a low bulk modulus, whereas a V-shape occurs with a high bulk modulus. The parameters width, depth, and spacing are largely unaffected by the curing conditions. This work provides a background to understand the behavior of random cracks, which is, e.g., useful to design a system with controlled cracks that remain stable. An important finding is that as soon as cracking occurs at a certain strain, already a number of cracks develop, yet without any further stretching. The cracks behave independently from neighboring cracks. Upon further stretching, new cracks develop, and the crack width and depth remain similar. Therefore, these random cracks are stable, and sample failure does not occur up to a strain of at least 40%.

Flexible composite stamps are commonly used in low pressure and low temperature processes, e.g., UV-assisted nanoimprint, as they provide a good conformal contact between stamp and substrate. The composite stamps investigated here consist of two layers, a thin hard top layer to enable stable nanometre-scaled structures with high aspect ratio and a soft backplane to ensure conformal contact. Stamps with two different material combinations were investigated, OrmoStamp /PDMS (polydimethylsiloxane) and h-PUA/s-PUA (polyurethane acrylate). The stability of both composite stamps was tested under harsh imprint conditions in a process at elevated temperature and pressure. Temperature and pressure loading results in strain in the top layer and may lead to break when the tensile strength is exceeded. To vary the stress level in the top layer, two different stamp designs were investigated, one with a thin backplane and one with a thicker backplane. The experimental results clearly show that a thin stamp (low stress level) is more stable than a thick stamp. Moreover, composite stamps are also suitable for a thermal imprint process when temperature and pressure remain limited.

Halide perovskites are currently of interest for a variety of optoelectronic applications. While, typical wet-chemical preparation techniques afford relatively rough polycrystalline layers, we have recently demonstrated that thermal imprint is a powerful post-deposition processing tool that affords extremely smooth perovskite thin-films with crystals that extend over tens of microns laterally.[1,2] A comparative study of optical, morphological and thermal properties (e.g. thermal conductivity) reveals some striking similarity of pressed MAPbX3 thin-films and their single crystalline analogues.[3] More recently, we successfully used thermal imprint also for entirely inorganic halide perovskite materials, such as CsPbBr3. While as-deposited CsPbBr3 layers are typically discontinuous and rough with a large number of pinholes, thermal imprint at relatively low temperature and pressure (150°C, 100 bar) will be shown to turn them into dense, smooth and pinhole-free thin films, which show substantially enhanced luminescence quantum yield and in contrast to pristine CsPbBr3 layers even enable room-temperature amplified spontaneous emission (ASE). Perovskite thin films patterned by thermal nanoimprint with photonic resonator structures will be shown to afford hybrid and entirely inorganic distributed feedback lasers, with ultra-low lasing thresholds.[2,4] [1] A. Mayer et al. J. Vac. Sci. & Techol. B 2017, 35, 06G803. [2] N. Pourdavoud et al. Adv. Mater. Technol. 2018, 3, 1700253. [3] R. Heiderhoff et al. J. Phys. Chem. C 2017, 121, 28306. [4] N. Pourdavoud et al. Adv. Mater. 2017, 29, 1605003.

Hybrid perovskite semiconductors hold great promise as low‐cost, yet high performance gain media for lasers. Distributed feedback (DFB) resonator structures are a key to unlock low laser threshold levels, which are essential on the way to the first electrically operated perovskite laser diode. Here, the first DFB lasers based on methylammonium lead bromide (MAPbBr 3 ) thin films, with a linear photonic grating imprinted into the MAPbBr 3 active layers is presented. High‐Q Bragg resonator gratings with a periodicity of 300 nm are directly patterned by thermal nanoimprinting into thin films of MAPbBr 3 at a temperature as low as 100 °C. A notable effect of the imprinting process is a substantial flattening of the initially very rough polycrystalline perovskite layers to layers consisting of large crystals on the order of tens of microns with a surface roughness of 0.6 nm. The smooth surface affords a significantly lowered threshold for the onset of amplified spontaneous emission due to reduced scattering. In optically pumped DFB laser structures, very low lasing thresholds of 3.4 µJ cm −2 are achieved. It is foreseen that these results will influence research on perovskite‐based optoelectronic devices beyond lasers, e.g., light emitting diodes and solar cells.

Perovskites have high potential for future electronic devices, in particular, in the field of opto-electronics. However, the electronic and optic properties of these materials highly depend on the morphology and thus on the preparation; in particular, highly crystalline layers with large crystals and without pinholes are required. Here, nanoimprint is used to improve the morphology of such layers in a thermal imprint step. Two types of material are investigated, MAPbI3 and MAPbBr3, with MA being methylammonium, CH3NH3+. The perovskite layers are prepared from solution, and the crystal size of the domains is substantially increased by imprinting them at temperatures of 100–150 °C. Although imprint is performed under atmospheric conditions which, in general, enhances the degradation, the stamp that covers the layer under elevated temperature is able to protect the perovskite largely from decomposition. Comparing imprinting experiments with pure annealing at a similar temperature and time proves this. Furthermore, imprint is capable of patterning the surface of the perovskite layers; lines and spaces of 150 nm width were reproducibly obtained under imprint at 150 °C. Moreover, a through-layer patterning is possible by using the partial cavity filling approach. Although not yet optimized, this simple way to define isolated perovskite patterns within a layer simply by thermal nanoimprint is of impact for the preparation of devices, as patterning of perovskite layers by conventional techniques is limited.

Photonic nanostructures are created in organo-metal halide perovskites by thermal nanoimprint lithography at a temperature of 100 °C. The imprinted layers are significantly smoothened compared to the initially rough, polycrystalline layers and the impact of surface defects is substantially mitigated upon imprint. As a case study, 2D photonic crystals are shown to afford lasing with ultralow lasing thresholds at room temperature.

Thermal management in devices like solar cells, light-emitting diodes, and lasers based on hybrid halide perovskite thin films is expected to be of paramount importance for optimal performance and reliability. As of yet, experimental data of thermal properties of non-iodine-based hybrid halide perovskites is very scarce. Here the thermal conductivity of methylammonium lead 15 halide perovskite (CH3NH3PbX3 X = I, Br, and CO single crystals and thin films is analyzed by scanning near-field thermal 75 microscopy. The thermal conductivity of CH3NH3PbX3 single crystals with X=I, Br, and Cl is found to be 0.34 +/- 0.12, 0.44 +/- 0.08, and 0.50 +/- 0.05 W/(mK) at room temperature, respectively. Strikingly, similar thermal conductivities are determined for the corresponding thin-film samples. The thermal conductivity of MAPbI(3) in the cubic phase (T > 55 degrees C) increases to (1.1 +/- 0.1) W/(mK). In addition, the temperature dependence of the thermal conductivities and of thermal expansion coefficients of MAPbI(3) around the phase transition from the tetragonal to cubic phase is presented.

The directed self-assembly (DSA) of block copolymers (BCP) has attracted high interest for the definition of nanostructures in an almost self-forming way when adequate boundary conditions are given. At present, grapho- and chemo-epitaxy are the workhorses but they require precisely patterned substrates to serve as the guiding pattern. Nanoimprint may replace this laborious pre-patterning of each substrate by employing an adequate stamp that can be used multiple times, inducing the guided DSA from the top of the film. Here, the DSA of BCPs is revisited in view of the specific nanoimprint situation. As a consequence, the BCP layer is imprinted in a partial cavity-filling mode, using a stamp of sufficient height provided with a conventional anti-sticking layer; substrate pre-treatment is minimized or rather avoided. Even with a highly preferential substrate it is possible to obtain vertical lamellae that are largely oriented in parallel to the stamp edges in PS-b-PMMA (polystyrene-block-polymethyl methacrylate) already after 3min of imprint. The vertical lamellae are at least 70nm high, freestanding on the substrate. Though optimization is required the results indicate the high potential of nanoimprint to simplify the DSA of BCPs for technical applications, also beyond Si technology.

Replica stamps fabricated of cross-linked materials are commonly used for nanoimprint lithography. Their mechanical properties depend on the preparation conditions and on the configuration used for curing as well. Typically, curing of a thin layer goes hand in hand with a shrinkage in the vertical direction. In a curing situation where this shrinkage is hindered, the cross-link density is reduced, resulting in a lower modulus. With respect to this issue, three cross-linkable materials are investigated, sylgard-polydimethylsiloxane, ultraviolet-polydimethylsiloxane, and OrmoStamp. The modulus is determined from the deflection of two-layer samples under temperature loading. Curing is performed under varying processing conditions; moreover, the impact of hindered shrinkage is addressed by experiments holding the layer thickness fixed during curing, a situation typical with automated stamp replication. The hindered shrinkage results in a modulus reduced by about 20%. The results obtained are vital for reproducible and comparable stamp preparation, beyond a single laboratory. A thorough and complete specification of replica stamp preparation does not only require information on the conventional processing parameters; it requires information on the curing configuration as well.

The recently re-discovered class of organometal-halide perovskites hold great promise for solar cells, LEDs and lasers.[1] Today, their potential has not been fully unlocked partially because of the lack of suitable nano-patterning techniques, which are mandatory to create resonator structures, waveguides etc. with a maximum level of precision directly into perovskite layers. Their chemical and thermal instability prevents the use of established wet-chemical patterning techniques.[2] In contrast to conventional inorganic semiconductors, crystal binding in these perovskites includes significant contributions of van der Waals interactions among the halide atoms and Hydrogen bonding.[3] The formation enthalpy per unit cell is only about 0.1eV in MAPbI3.[4] Here, we take advantage of the “soft-matter properties” of organo-metal halide perovskites and demonstrate that photonic nano-structures can be prepared by direct thermal nano-imprint lithography in MAPbI3 and MAPbBr3 at relatively low temperatures (<150°C). The resulting periodic patterns provide distributed feedback resonators, which afford lasing in MAPbI3 with ultra-low threshold levels on the order of 1 μJ/cm2.[5] Moreover, our results also state the first DFB lasers based on MAPbBr3. We will discuss the applicability of thermal imprinting for perovskite solar cells and LEDs. [1] B. R. Sutherland et al. Nat Photon 2016, 10, 295. [2] D. Lyashenko et al. physica status solidi (a) 2017, 214, 10.1002/pssa.201600302. [3] D. A. Egger et al. Journal of Physical Chemistry Letters 2014, 5, 2728. [4] A. Buin et al. Nano Lett 2014, 14, 6281. [5] N. Pourdavoud et al. Adv Mater 2017, 10.1002/adma.201605003.