Angewandte Chemie International EditionVolume 60, Issue 13 p. 6833-6854 Graphical AbstractFree Access Graphical Abstract: Angew. Chem. Int. Ed. 13/2021 First published: 16 March 2021 https://doi.org/10.1002/anie.202181311AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Volume60, Issue13March 22, 2021Pages 6833-6854 RelatedInformation

The structure of multiply twinned particles (MTPs) provides an example of how specific crystallographic features dictate the geometric shape of finite-sized crystals. The formation of MTPs during colloidal synthesis can occur through at least two different pathways: 1) growth from multiply twinned seeds or 2) the stepwise formation of new twin boundaries on single-crystalline seeds (either by particle overgrowth or multiparticle attachment). By utilizing in situ transmission electron microscopy, recent studies have provided real-time evidence for both pathways. Looking forward, the knowledge of specific evolution pathways that occur under a given synthetic condition will aid in the design of robust MTP syntheses. More importantly, further studies pertaining to the structural evolution and energetics of nanoparticles are needed to provide a complete understanding of MTP formation pathways.

The potential implementation of extreme ultraviolet (EUV) lithography into next generation device processing is bringing urgency to identify resist materials that optimize EUV lithographic performance. Inorganic/organic hybrid nanoparticles or clusters constitute a promising new class of materials, with high EUV sensitivity from the core and tunable chemistry through the coordinating ligands. Development of a thorough mechanistic understanding of the solubility switching reactions in these materials is an essential first step toward their implementation in patterning applications but remains challenging due to the complexity of their structures, limitations in EUV sources, and lack of rigorous in situ characterization. Here we report a mechanistic investigation of the solubility switching reactions in hybrid clusters comprised of a small HfOx core capped with a methacrylic acid ligand shell (HfMAA). We show that EUV-induced reactions can be studied by performing in situ IR spectroscopy of electronirradiated films using a variable energy electron gun. Combining additional ex situ metrology, we track the chemical evolution of the material at each stage of a typical resist processing sequence. For instance, we find that a crosslinking reaction initiated by decarboxylation of the methacrylate ligands under electron irradiation constitutes the main solubility switching mechanism, although there are also chemical changes imparted by a typical the post application bake (PAB) step alone. Lastly, synchrotronbased IR microspectroscopy measurements of EUV-irradiated HfMAA films enable a comparison of reactions induced by EUV vs electron beam irradiation of the same resist material, yielding important insight into the use of electron beam irradiation as an experimental model for EUV exposure. Introduction As density scaling drives electronic device components toward smaller sizes, high resolution and precision patterning techniques are playing an increasingly vital role. Extreme ultraviolet (EUV) lithography is anticipated to achieve the dimensions required by the next generation nanoelectronic devices owing to the much smaller photon wavelength (13.5 nm). 2 Implementation of EUV lithography to adapt to the comparatively high photon energy naturally requires the development of compatible photoresist materials with properties tailored to optimize the response to EUV irradiation. The current state-of-the-art photoresist materials for optical lithography, chemically amplified resists (CARs), are composed of a polymer matrix, a photoacid generator (PAG), and an acid quencher, and in some cases an acid generation promoter. CARs are sensitized for UV wavelengths through an amplification mechanism that is initiated when PAGs become activated by UV photon absorption, leading to the generation of acids that disrupt the polymer network. While CARs have many appealing characteristics including a well-developed processing infrastructure, they also possess several drawbacks that limit the achievable patterning resolution. For example, CARs are susceptible to pattern collapse as features become smaller, and appreciable acid diffusion can hinder obtainable resolution. In addition, inhomogeneous distributions of the CAR components can further deteriorate resolution and lead to improper development of patterned features. 8 To circumvent the challenges posed by conventional resist materials, efforts have recently begun focusing on a new class of materials, hybrid inorganic-organic clusters, composed of an inorganic metal or metal oxide core or nanoparticle capped with organic ligands. These systems, first developed at Cornell University, possess properties that are inherently amenable to EUV patterning, such as comparatively strong EUV absorption from its inorganic core, high etch resistance, and the ability to tailor their chemistry through tunable ligand terminations. One such system that has demonstrated high sensitivity for negative-tone patterning consists of a hafnium oxide core capped with methacrylic acid (MAA) ligands (HfMAA). Along with their Zr-based analog (ZrMAA), HfMAA resists have demonstrated high-resolution patterning with EUV doses as low as 4.2 mJ/cm. These results have generated an intense interest in the underlying EUV-resist interaction and solubility-switching mechanism of HfMAA and related systems. The first studies of the patterning mechanism in these materials reached a number of important conclusions; first, the particle size following UV exposure increases from 3-4 nm to sizes approaching 1 m, indicating coalescence of the nanoparticles. A similar aggregation effect was observed in resist films subjected to a soft-baking step. Second, the interpretation of ex situ IR spectra ruled out a photochemical crosslinking mechanism. 23 Third, CO2 was detected in unpublished work as a byproduct of the reaction in outgassing experiments, and the dissociation of ligands was further supported by analysis of ex situ IR and core-level photoemission spectra of resist films subjected to comparatively high EUV doses (50-100 mJ/cm). Taken together, a model emerged whereby solubility switching was attributed mostly to ligand exchange resulting from decomposition of the native ligands in alcohol developers. While consistent with initial data from various ex situ metrology, these studies could not derive an atomic scale description of this mechanism for lack of in situ characterization. Furthermore, Castellanos and coworkers reported IR, XPS and grazing incidence x-ray scattering measurements on HfZrand Ti-methacrylate clusters and proposed an alternate mechanism where solubility switching was characterized by a mechanism initiated by decarboxylation and subsequent crosslinking of the clusters. Consequently, comprehensive in situ spectroscopic measurements would be useful to gain insight into the underlying mechanisms of these resists and enable operando quantification, particularly in the low-to-intermediate dose regime. Such measurements could address important observations of previous studies, such as the evolution of CO2 and the apparent lack of consumption of unsaturated hydrocarbon following irradiation, and ultimately contribute to the optimization of HfMAA and related systems industrial-scale implementation. Such detailed studies are needed to provide insight into the underlying mechanisms of HfMAA and related systems. Part of the difficulty in unraveling inorganic resist mechanisms lies in the complexity of the EUVresist interaction. Absorption of EUV photons can result in the generation of photoelectrons from valence and shallow core levels. Chemistry resulting from EUV photon absorption can be induced through one of several pathways; for example, photoemission can lead to the creation of unstable radical cations that can decompose into smaller fragments and induce reactions. In addition, energetic electrons can also result in the emission of lower energy secondary electrons through inelastic scattering processes, which can in turn foster crosslinking reactions that affect solubility. An additional challenge in studying these interactions is the lack of EUV light sources; the only options for generation of 92 eV photons are synchrotron radiation or large plasma sources that can be challenging to interface with in situ metrology. One approach to study the processes that take place following EUV photon absorption is to irradiate the thin resist film with a beam of energetic electrons 26-29 to probe the effects of both primary photoelectrons and secondary electrons. For example, an incident beam of 90 eV electrons can serve both to probe the effect of energetic primary photoelectrons generated following EUV photon absorption, as well as to eject electrons from shallow core levels through impact ionization, just as in the case of EUV photons. In the present work, we have demonstrated that this approach can be combined with in situ probes to track the evolution of resist chemistry under well-controlled conditions to mimic the reactions taking place during EUV irradiation. Specifically, we focus on understanding electron-induced solubility switching mechanisms of HfMAA resists, following each step of a typical industrial processing sequence: a post-application baking (PAB) step used following deposition to remove excess solvent, electron irradiation (in lieu of EUV irradiation), a post exposure baking (PEB) step, and finally development in isopropanol (IPA). The evolution of HfMAA at each stage is studied by in situ infrared (IR) absorption measurements as a function of electron-beam irradiation or thermal annealing, coupled with ex situ IR and x-ray photoemission measurements. The chemical specificity provided by IR spectra, supplemented by ab initio calculations, enables direct quantitative evaluation of the reactions that result from the electron-resist interaction. From these studies, we first find that the typical PAB step used to remove excess solvent results in condensation reactions involving extrinsic hydroxyl groups incorporated into the clusters, which in turn affects solubility. Next, analysis of IR spectra collected upon electron-beam irradiation show that, besides further condensation reactions induced by electrons, an additional mechanism involving crosslinking of neighboring clusters dominates the solubility switching. Additionally, we have evaluated the use of electron-beam irradiation as a model for EUV exposure to probe materials irradiated with EUV synchrotron radiation with IR microspectroscopy. Detailed crosslinking pathways leading to solubility switching mechanism are uncovered from spectral quantification and comparison to first principles models, providing a starting point for material optimization and a novel approach to the evaluation of inorganic resist mechanisms.

The potential implementation of extreme ultraviolet (EUV) lithography into next generation device processing is bringing urgency to identify resist materials that optimize EUV lithographic performance. Inorganic/organic hybrid nanoparticles or clusters constitute a promising new class of materials, with high EUV sensitivity from the core and tunable chemistry through the coordinating ligands. Development of a thorough mechanistic understanding of the solubility switching reactions in these materials is an essential first step toward their implementation in patterning applications but remains challenging due to the complexity of their structures, limitations in EUV sources, and lack of rigorous in situ characterization. Here, we report a mechanistic investigation of the solubility switching reactions in hybrid clusters comprising a small HfOx core capped with a methacrylic acid ligand shell (HfMAA). We show that EUV-induced reactions can be studied by performing in situ infrared (IR) spectroscopy of electron-irradiated films using a variable energy electron gun. Combining additional ex situ metrology, we track the chemical evolution of the material at each stage of a typical resist processing sequence. For instance, we find that a cross linking reaction initiated by decarboxylation of the methacrylate ligands under electron irradiation constitutes the main solubility switching mechanism, although there are also chemical changes imparted by a typical post application bake (PAB) step alone. Lastly, synchrotron-based IR microspectroscopy measurements of EUV-irradiated HfMAA films enable a comparison of reactions induced by EUV vs electron beam irradiation of the same resist material, yielding important insight into the use of electron beam irradiation as an experimental model for EUV exposure.

Managing deposition of multilayered nanocrystal quantum dot (NQD) thin films is crucial for future photonic devices to maximize solar energy extraction efficiency. Solution based NQD deposition methods require additional protection to achieve a discrete layered structure and to prevent optical degradation during processing. An attractive method to passivate and protect NQD films is overcoating with metal oxides, usually grown using atomic layer deposition (ALD). However, a significant quenching of NQD photoluminescence (PL) is typically observed after encapsulation, hindering performance and applicability. Here, we demonstrate a modified gas-phase deposition technique that fully passivates NQD assemblies and, in contrast to standard ALD, maintains PL properties. Combined in situ FTIR and ex situ XPS measurements reveal that upon Al2O3 deposition by ALD, the metal precursor trimethylaluminum (TMA) interacts with oleic acid-capped CdSe-CdS-ZnS core-shell-shell NQDs by reorganizing the ligands and replacing Zn atoms with Al. This modification leads to PL quenching, particularly severe at elevated temperatures (similar to 100 degrees C). In contrast, simultaneous exposures of both precursors (TMA and water) lead to metal oxide deposition from gas-phase reactions taking place in the immediate vicinity of the NQD surface, without affecting the chemical nature of the NQDs. Contrary to ALD, this technique retains and even improves NQDs' photoluminescence, observed as increased PL intensities and longer lifetimes.

The chemical structure and thermal reactivity of recently discovered inorganic-organic hybrid resist materials are characterized using a combination of in situ and ex situ infrared (IR) spectroscopy and x-ray photoemission spectroscopy (XPS). The materials are comprised of a small HfOx core capped with methacrylic acid ligands that form a combined hybrid cluster, HfMAA. The observed IR modes are consistent with the calculated modes predicted from the previously determined x-ray crystal structure of the HfMAA-12 cluster, but also contain extrinsic hydroxyl groups. We find that the water content of the films is dependent on the concentration of excess ligand added to the solution. The effect of environment used during post-application baking (PAB) is studied and correlated to changes in solubility of the films. In doing so, we find that hydroxylation of the clusters results in formation of additional Hf-O-Hf linkages upon heating, which in turn impacts the solubility of the films.

Wet chemical processes can lead to oxide-free, hydrogen-terminated silicon surfaces with controlled morphology. These H-terminated Si surfaces are stable in air, but their stability is an issue for subsequent applications. This article examines several methods to activate H-passivated Si surfaces for atomic layer deposition (ALD), either by grafting organic layers with tailored end group or by replacing Si–H with more active groups, such as Si–OH, without oxidizing the silicon surfaces. For instance, hydrosilylation is used to graft monolayers with carboxylic acid end groups with remarkably good electrical properties of the Si/monolayer interface. Examples of both metal oxide and metal films deposited by ALD are presented. Alternatively, methanol is shown to be effective in replacing some Si–H bonds with methoxy groups, from which Si–OH can be obtained to foster the ALD of metal oxides. This article illustrates how spectroscopic techniques make it possible to derive a mechanistic understanding of the surface chemistry involved in these processes.

High absorption metal and main group oxide nanoclusters have recently become the subject of intense investigation due to their potential as alternatives to conventional organic chemically amplified photoresists for negative tone patterning in the extreme ultraviolet (EUV) range. It is believed that the low energy secondary electrons generated from photoionization of the nanoclusters are responsible for the chemistry that occurs following irradiation. Despite their potential, little is known about many aspects of their function as resist materials; in particular, the role of high absorption elements in the absorption/photoionization process, the role of electrons of different energies, the formation of radicals and chemically reactive species and the overall structural changes that occur at each stage of processing. Thus, the precise atomic mechanism underlying solubility switching upon irradiation remains unknown. We have investigated one such class of materials: nanoclusters based on a hafnium oxide core decorated with methacrylic acid ligands (HfMAA) , using a combination of in situ infrared (IR) spectroscopy and x-ray photoemission spectroscopy (XPS). Thin films (10-30 nm) of the HfMAA nanoclusters deposited onto SiO2 from solution are characterized by XPS and IR spectroscopy in conjunction with density functional calculations. The mechanisms of solubility switching following EUV exposure are investigated using irradiation with low energy electrons to emulate the photoexcited carriers generated in the EUV absorption process. Using in situ IR spectroscopy, we are able to monitor the structural evolution of the nanoclusters at each stage of processing, beginning from the initial deposition to post application bake (PAB), electron irradiation, post exposure bake (PEB) and development. Monochromatic electron beams with energies ranging from 5 eV up to the typical EUV wavelength of 92 eV enable a correlation between the role of secondary electron energy with chemistry and functional character. In conjunction with XPS analysis, these measurements detail the structural evolution of the films under typical processing conditions, providing insight into the solubility switching mechanism of this emerging class of inorganic photoresists.

Integration of colloidal nanocrystal quantum dots (NQDs) with strongly absorbing semiconductors offers the possibility of developing optoelectronic and photonic devices with new functionalities. We examine the process of energy transfer (ET) from photoactive CdSe/ZnS core/shell NQDs into lead-halide perovskite polycrystalline films as a function of distance from the perovskite surface using time-resolved photoluminescence (TRPL) spectroscopy. We demonstrate near-field electromagnetic coupling between vastly dissimilar excitation in two materials that can reach an efficiency of 99% at room temperature. Our experimental results, combined with electrodynamics modeling, reveal the leading role of non-radiative ET at close distances, augmented by the waveguide emission coupling and light reabsorption at separations >10 nm. These results open the way to combining materials with different dimensionalities to achieve novel nanoscale architectures with improved photovoltaic and light emitting functionalities.

Two-dimensional transition metal dichalcogenides (TMDCs) like MoS 2 are promising candidates for various optoelectronic applications. The typical photoluminescence (PL) of monolayer MoS 2 is however known to suffer very low quantum yields. We demonstrate a 10-fold increase of MoS 2 excitonic PL enabled by nonradiative energy transfer (NRET) from adjacent nanocrystal quantum dot (NQD) films. The understanding of this effect is facilitated by our application of transient absorption (TA) spectroscopy to monitor the energy influx into the monolayer MoS 2 in the process of ET from photoexcited CdSe/ZnS nanocrystals. In contrast to PL spectroscopy, TA can detect even non-emissive excitons, and we register an order of magnitude enhancement of the MoS 2 excitonic TA signatures in hybrids with NQDs. The appearance of ET-induced nanosecond-scale kinetics in TA features is consistent with PL dynamics of energy-accepting MoS 2 and PL quenching data of the energy-donating NQDs. The observed enhancement is attributed to the reduction of recombination losses for excitons gradually transferred into MoS 2 under quasi-resonant conditions as compared with their direct photoproduction. The TA and PL data clearly illustrate the efficacy of MoS 2 and likely other TMDC materials as energy acceptors and the possibility of their practical utilization in NRET-coupled hybrid nanostructures.

Observation of energy transfer (ET) from multiexcitonic (MX) complexes in nanocrystal quantum dots (NQDs) has been severely restricted due to efficient nonradiative Auger recombination leading to very low MX emission quantum yields. Here we employed "giant" CdSe/CdS NQDs with suppressed Auger recombination to study ET of biexcitons (BX) and charged excitons (trions) into Si substrate. Photoluminescence (PL) measurements of (sub)monolayers of gNQDs controllably assembled on various interacting surfaces and augmented by single gNQD's imaging reveal appearance of BX spectral signatures and progressive acceleration of PL lifetimes of all excitonic species on Si substrates. From statistical analysis of a large number of PL lifetime traces, representative exciton, trion and BX ET efficiencies are measured as ∼75%, 55% and 45% respectively. Detailed analysis of the MX's radiative rates demonstrate the crucial role of the radiative (waveguide) ET in maintaining high overall transfer efficiency despite the prevalent Auger recombination. Our observations point towards practical utilization of MX-bearing nanocrystals in future optoelectronics architectures.

The initial stages of cobalt metal growth by atomic layer deposition are described using the precursors bis(1,4-di-tert-butyl-1,3-diazadienyl)cobalt and formic acid. Ruthenium, platinum, copper, Si(100), Si-H, SiO2, and carbon-doped oxide substrates were used with a growth temperature of 180 °C. On platinum and copper, plots of thickness versus number of growth cycles were linear between 25 and 250 cycles, with growth rates of 0.98 Å/cycle. By contrast, growth on ruthenium showed a delay of up to 250 cycles before a normal growth rate was obtained. No films were observed after 25 and 50 cycles. Between 100 and 150 cycles, a rapid growth rate of ∼1.6 Å/cycle was observed, which suggests that a chemical vapor deposition-like growth occurs until the ruthenium surface is covered with ∼10 nm of cobalt metal. Atomic force microscopy showed smooth, continuous cobalt metal films on platinum after 150 cycles, with an rms surface roughness of 0.6 nm. Films grown on copper gave rms surface roughnesses of 1.1-2.4 nm after 150 cycles. Films grown on ruthenium, platinum, and copper showed resistivities of <20 μΩ cm after 250 cycles and had values close to those of the uncoated substrates at ≤150 cycles. X-ray photoelectron spectroscopy of films grown with 150 cycles on a platinum substrate showed surface oxidation of the cobalt, with cobalt metal underneath. Analogous analysis of a film grown with 150 cycles on a copper substrate showed cobalt oxide throughout the film. No film growth was observed after 1000 cycles on Si(100), Si-H, and carbon-doped oxide substrates. Growth on thermal SiO2 substrates gave ∼35 nm thick layers of cobalt(ii) formate after ≥500 cycles. Inherently selective deposition of cobalt on metallic substrates over Si(100), Si-H, and carbon-doped oxide was observed from 160 °C to 200 °C. Particle deposition occurred on carbon-doped oxide substrates at 220 °C.

of exciton decay in a controlled stratifi ed environment. This agreement attests to the viability of the sequential deposition technique to fabricate thick, designer NQD solids. In particular, our analysis of PL from multilayer structures indicates effi cient energy transfer (ET) from NQDs into the underlying Si substrate, which is important for hybrid ET-based photovoltaic devices. [ 16–18 ] Successful excitonic sensitization of crystalline Si via NRET or radiative (RET) energy transfer [ 19,20 ] may enable development of novel thin-fi lm hybrid solar cells. [ 18 ]

Ultrafast transient pump-probe measurements of thin CH3NH3PbI3 perovskite films over a wide spectral range from 350 to 800 nm reveal a family of photoinduced bleach (PB) and absorption (PA) features unequivocally pointing to the fundamentally multiband character of the underlying electronic structure. Excitation pump-energy dependent kinetics of three long-lived PB peaks at 1.65, 2.55, and 3.15 eV along with a broad PA band shows the involvement of band-edge thermalized carriers in all transitions and at least four, possibly more, electronic bands. The evolution of the transient signatures is described in terms of the redistribution of the conserved oscillator strength of the whole system. The multiband perspective opens up different directions for understanding and controlling photoexcitations in hybrid perovskites.

We report the observation of a large enhancement of the wavelength-dependent photocurrent in ultrathin silicon nanomembranes (SiNM) decorated with colloidal CdSe/ZnS nanocrystal quantum dots (NQDs). Back-gated, field-effect transistor structures based on 75 nm-thick SiNMs are functionalized with self-assembled monolayers (SAMs) preventing surface oxidation and minimizing the surface defect densities. NQDs are drop cast on the active region of the device and the photocurrent is measured as a function of the excitation wavelength across the NQD absorption region. Photocurrent enhancement on the order of several hundred nA's is observed for NQD/SAM/SiNM devices compared to reference SAM/SiNM structures, with the device peak response closely correlated to the NQD absorption peak. We propose light-induced gating of the surface electrostatic potential and forward self-biasing of the FET channel as the two key mechanisms leading to the large photocurrent increase. Our findings open the possibility of employing silicon-nanocrystal hybrid structures for light sensing applications.

The classical SiO2/Si interface, which is the basis of integrated circuit technology, is prepared by thermal oxidation followed by high temperature (>800 °C) annealing. Here we show that an interface synthesized between titanium dioxide (TiO2) and hydrogen-terminated silicon (H:Si) is a highly efficient solar cell heterojunction that can be prepared under typical laboratory conditions from a simple organometallic precursor. A thin film of TiO2 is grown on the surface of H:Si through a sequence of vapor deposition of titanium tetra(tert-butoxide) (1) and heating to 100 °C. The TiO2 film serves as a hole-blocking layer in a TiO2/Si heterojunction solar cell. Further heating to 250 °C and then treating with a dilute solution of 1 yields a hole surface recombination velocity of 16 cm/s, which is comparable to the best values reported for the classical SiO2/Si interface. The outstanding performance of this heterojunction is attributed to Si-O-Ti bonding at the TiO2/Si interface, which was probed by angle-resolved X-ray photoelectron spectroscopy. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) showed that Si-H bonds remain even after annealing at 250 °C. The ease and scalability of the synthetic route employed and the quality of the interface it provides suggest that this surface chemistry has the potential to enable fundamentally new, efficient silicon solar cell devices.

Two solution-based strategies for the preparation of partially fluorinated Si(111) surfaces from H/Si(111) were investigated using a combination of scanning tunneling microscopy, X-ray photoemission spectroscopy, infrared spectroscopy, and kinetic Monte Carlo simulations. Direct fluorination of H/Si(111) with HF (aq) produced atomically flat surfaces with 11% fluorination. A two-step reaction that first methoxylated the surface by reaction in methanol and then converted the methoxy termination to F termination by reaction in HF (aq) produced atomically rough, fissured surfaces with 24% fluorination. The atomic-scale roughness was induced by the methoxylation reaction. Methanol was shown to react with H/Si(111) surfaces through two parallel mechanisms: an etching reaction and a methoxylation reaction. The methoxylation reaction locally inhibited or frustrated the etching reaction, leading to the development of a characteristic fissured morphology. The H and F atoms on the fluorinated surface were imaged with atomic resolution, and no evidence of the previously proposed nanopatterning mechanism was observed