Fabrication, Transfer, and Transport Properties of Monolayered Freestanding Nanoparticle Sheets

Abstract

Nanoparticles, often referred to as 'artifi cial atoms', can be arranged into 1D, [ 1 , 2 ] 2D, [3][4][5][6] and 3D assemblies, [ 7 , 8 ] in analogy to the formation of crystals by atoms and molecules in nature. These artifi cial solids exhibit unique collective optical, [ 9 ] electrical, [10][11][12] and magnetic properties, [ 13 ] which stem from the novel electronic properties of the individual nanoparticles and the coupling between neighboring building blocks. [ 14 ] Motivated by fundamental and practical interests, researchers have developed various techniques to assemble 2D nanoparticle arrays and investigated their properties. [ 9 , 12 , 15 ] Freestanding 2D nanoparticle arrays not only offer a new type of ultrathin, robust, and fl exible material, but also enable the study of substrate-free optical and electrical properties. [ 16 ] Thus far, freestanding nanoparticle membranes have been fabricated by embedding them in polymers, [ 17 , 18 ] crosslinking, [ 19 , 20 ] sintering, [ 21 ] layer-by-layer assembly, [ 22 ] and electrophoretic deposition. [ 23 ] These nanoparticle membranes, however, either lack the packing order or lose the intrinsic features of nanoparticle superlattices. Impressive progress has been made by two groups. Jaeger and co-workers self-assembled freestanding, close-packed nanoparticle arrays over prefabricated holes. [ 24 , 25 ] Interactions between ligand molecules rendered the fi lms freestanding without the need for polymer encapsulation or ligand crosslinking. This method is also applicable to binary nanocrystal superlattices. [ 26 ] Luo and co-workers demonstrated another strategy to fabricate freestanding nanoparticle superlattices, using DNA molecules as ligands. [ 27 ] Both groups characterized the mechanical properties of as-prepared freestanding nanoparticle arrays. Despite these advances, measurement of the substrate-free transport properties of freestanding nanoparticle superlattices requires techniques to manipulate them and integrate them into solid-state devices, although it remains a great experimental challenge.Herein, we present a feasible route for the preparation, transfer, and electrical measurements of monolayered freestanding nanoparticle sheets. Under a gas fl ow, freestanding nanoparticle arrays can be transferred from microgrids onto arbitrary solid substrates. We measured the electronic transport properties of nanoparticle sheets suspended over trenches, and found the scaling exponent to be signifi cantly different from that of substrate-supported nanoparticle arrays prepared by the same technique. Our method enables the investigation of the intrinsic transport properties of artifi cial assemblies and paves the way towards device applications of freestanding nanoparticle arrays. Figure 1 a shows a schematic representation of the homemade experimental setup used to prepare monolayered freestanding nanoparticle sheets over a microgrid. For the convenience of transmission electron microscopy (TEM) characterization, we used TEM grids. In our confi guration...

Rapid One‐Pot Preparation of Large Freestanding Nanoparticle‐Polymer Films

“…There have been some attempts to develop methods to transferring them from liquid phase into freestanding films. Several research groups have shown that by drying MeLLFs over holey substrates micrometer‐sized freestanding arrays can be created . To transform MeLLFs into larger freestanding cm 2 scale films, up to now the most effective approach was to sinter the particles but this gave a final product which was too weak even to withstand direct characterization under electron microscopy .…”

Mechanically Strong, Optically Transparent, Giant Metal Superlattice Nanomembranes From Ultrathin Gold Nanowires

“…Lipids and polymers are classic materials to be used in natural and artificial membrane systems, respectively. In contrast, recently there is burgeoning interest in constructing new class of membranes from nanoscale optoelectronic building blocks 1–11. In particular, it is possible to fabricate free‐standing periodic nanoparticle arrays (superlattices) which behave as mechanically strong nanomembranes with Young's moduli of several GPa 3, 6, 11.…”

“…Remarkably, both optoelectronic and mechanical properties of such superlattice membranes are tunable by varying materials types, adjusting ligand length, and regulating lattice structures, etc. Hence, engineering superlattice nanomembranes constitutes an exciting route to integrate unique properties of optoelectronic nanomaterials with striking mechanical robustness and flexibility into one tailorable multifunctional system, leading to lightweight metamaterials and devices with new functions for novel applications in optoelectronics,9, 16, 17 electrocatalysis,18 and ultrafiltration 19…”

Comparison of the Mechanical Properties of Self-Assembled Langmuir Monolayers of Nanoparticles and Phospholipids

“…Like amphiphilic molecules, these capped nanoparticles self-assemble into 2D lattices at the air-water interface. [3][4][5][6][7][8][9][10][11][12][13][14] Nanoparticles exhibit remarkable physical properties including quantum confinement, 15 plasmon resonance 9,[16][17][18][19] and superparamagnetism 5,20 Thin films composed of nanoparticles have shown potential applications in biosensors 16,19,21 , high sensitivity resonators 14 , filtration devices 22 , magnetoresistive devices 10 , and flexible electronics. [23][24][25] Langmuir monolayers in particular have remarkable mechanical properties as two-dimensional materials that can extend into the third dimension.…”