Ligand-Free Direct Optical Lithography of Bare Colloidal Nanocrystals via Photo-Oxidation of Surface Ions with Porosity Control

Abstract

Microscale patterning of colloidal nanocrystal (NC) films is important for their integration in devices. Here, we introduce the direct optical patterning of all-inorganic NCs without the use of additional photosensitive ligands or additives. We determined that photoexposure of ligand-stripped, “bare” NCs in air significantly reduces their solubility in polar solvents due to photo-oxidation of surface ions. Doses as low as 20 mJ/cm2 could be used; the only obvious criterion for material selection is that the NCs need to have significant absorption at the irradiation wavelength. However, transparent NCs can still be patterned by mixing them with suitably absorbing NCs. This approach enabled the patterning of bare ZnSe, CdSe, ZnS, InP, CeO2, CdSe/CdS, and CdSe/ZnS NCs as well as mixtures of ZrO2 or HfO2 NCs with ZnSe NCs. Optical, X-ray photoelectron, and infrared spectroscopies show that solubility loss results from desorption of bound solvent due to photo-oxidation of surface ions. We also demonstrate two approaches, compatible with our patterning method, for modulating the porosity and refractive index of NC films. Block copolymer templating decreases the film density, and thus the refractive index, by introducing mesoporosity. Alternatively, hot isostatic pressing increases the packing density and refractive index of NC layers. For example, the packing fraction of a ZnS NC film can be increased from 0.51 to 0.87 upon hot isostatic pressing at 450 °C and 15 000 psi. Our findings demonstrate that direct lithography by photo-oxidation of bare NC surfaces is an accessible patterning method for facilitating the exploration of more complex NC device architectures while eliminating the influence of bulky or insulating surfactants.

Direct photocatalytic patterning of colloidal emissive nanomaterials

“…In direct optical lithography, patterns are formed by photoinduced chemical reactions of photosensitive ligands or additives, which result in a solubility change of nanomaterials [e.g., metal oxide (16), PeNCs (17), and QDs (12)] at light-exposed regions without the need for a polymeric photoresist. Various photosensitive motifs and photochemical reactions such as azide (17)(18)(19)(20)(21), benzophenone (22), cinnamoyl (23), photo-acid generation (12), oxime sulfonate (13), thiol-ene (24)(25)(26)(27), photo-amine generation (28), oxide bridging (16), photo-oxidation (29), and alkene cross-linking (30) have been explored for direct optical patterning of colloidal emissive nanocrystals. However, the lithography process often induces substantial surface damage to the deposited emissive nanomaterials, thereby degrading their optical properties such as PLQY (13,17).…”

A Generalized Approach to Photon Avalanche Upconversion in Luminescent Nanocrystals

“…Meanwhile, the protective NaYF 4 outer shell (Figure S1 shows elemental mapping of Gd 3+ and Y 3+ ions) was used to reduce surface quenching and prevent excitation energy spill-out from the Gd 3+ network. , As synthesized, the Eu 3+ -activated ANPs were observed to be a pure Na­(Y/Gd)­F 4 β-phase (Figure S2) and less than 25 nm in diameter, determined by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. High crystallinity, small size, and narrow size distribution (<5%) ideally position these ANPs for biological or nanoscale patterning applications. − …”

Stimuli-Responsive Surface Ligands for Direct Lithography of Functional Inorganic Nanomaterials

“…42 Recently, we also showed that bare, absorbing NCs such as ZnSe and ZnS NCs can be patterned directly without having to add any other photosensitive component. 41 After subjecting the films to a hot isostatic pressing (HIP), a refractive index of up to 2.22 (at 633 nm) was obtained which corresponded to a packing fraction of 0.87. On the other hand, the addition of block copolymer micelles enabled the fabrication of NC patterns with lower inorganic fractions and increased porosity, thus allowing patterning of low-k dielectric components valuable for high-frequency electronic components and antireflective coatings.…”

“…Copyright 2021 American Chemical Society. (F) Adapted with permission from ref . Copyright 2022 American Chemical Society.…”

“…In our group, we also recently found that the photoinduced surface oxidation of "bare" semiconductor nanocrystals leads to the desorption of surface DMF ligands, leading to reduction in dispersibility. 41…”