Synthesis of linked carbon monolayers: films, balloons, tubes, and pleated sheets

Abstract

Because of their potential for use in advanced electronic, nanomechanical, and other applications, large two-dimensional, carbon-rich networks have become an important target to the scientific community. Current methods for the synthesis of these materials have many limitations including lack of molecular-level control and poor diversity. Here, we present a method for the synthesis of two-dimensional carbon nanomaterials synthesized by Mo- and Cu-catalyzed cross-linking of alkyne-containing self-assembled monolayers on SiO(2) and Si(3)N(4). When deposited and cross-linked on flat surfaces, spheres, cylinders, or textured substrates, monolayers take the form of these templates and retain their structure on template removal. These nanomaterials can also be transferred from surface to surface and suspended over cavities without tearing. This approach to the synthesis of monolayer carbon networks greatly expands the chemistry, morphology, and size of carbon films accessible for analysis and device applications.

Fig. 1.

Schematic illustration of linked monolayer formation. (A) Self-assembled monolayers (SAMs) are synthesized on a substrate, then cross-linked to form linked monolayers. This approach is applied to the formation of 3D topologies (i), structured sheets (ii), and membranes (iii). The box above iii provides an idealized view of the chemical structure for a linked monomer network formed from monomer 3. (B) Chemical structures of monomers 1–3.

Fig. 2.

Spectroscopic data and AFMs of SAMs, linked monolayers, and transferred monolayer membranes. (A) Absorption spectra of a SAM (blue), a linked monolayer (red), from 1 on quartz. (B and C) Tapping-mode AFM of a SAM and a linked monolayer, respectively, from 1 patterned into stripes by photolithography and oxygen-reactive ion etching. The averaged line cuts show a step height of 1.1 nm for the SAM and 1.4 nm for the linked monolayer. (D) Absorption spectra of a SAM (blue), a linked monolayer (red), and a transferred monolayer membrane (green) from 2 on quartz. (E) Fluorescence spectra (λ^ex = 275 nm, Left; λ^ex = 370 nm, Right) of a SAM (blue curves), a linked monolayer (red curves), and an etched monolayer membrane from 2 (green curve) on 330-nm-diameter SiO₂ spheres. Peak heights were normalized to the 315-nm emission of the SAM from 2. (F) Raman spectra of a SAM (blue), linked monolayer (red), and transferred monolayer membranes on a Si wafer with 300 nm thermal SiO₂ (green) from 2.

Fig. 3.

Characterization of transferred monolayer membranes. (A) AFM images of a ribbon of a monolayer membrane from 2 (width = 500 nm) on a prestrained PDMS substrate before (Upper) and after (Lower) releasing the prestrain. (B) Image of a sample of monolayer membrane from 2 transferred to a substrate of Au(200 nm)/Cr(5 nm)/SiO₂(300 nm)/Si after wet etching the Au with a ferricyanide solution in a circular region (dashed line) surrounding the membrane. The etched areas appear as dark blue; the unetched gold region in the center corresponds to the transferred monolayer membranes. (Inset) An optical micrograph of a silicon substrate that supports a linked monolayer, collected after exposure to KOH etchant. The arrow indicates an etched pit corresponding to a pinhole defect in the monolayer. (C) Reflection mode optical micrograph of a monolayer membrane from 2 transferred to a Si wafer with a 300-nm thermal SiO₂ layer. (Inset) High-magnification micrograph of folds in the film. (D) Reflectance optical micrograph superimposed on tapping-mode AFM image of a monolayer membrane from 1 transferred to a Si wafer with a 300-nm thermal SiO₂ layer. (E) Tapping-mode AFM image and line-cut height profile showing a monolayer membrane from 1 with regions of 1.8- and 3.3-nm height, consistent with folding on transfer to a Si wafer with a 300-nm thermal SiO₂ layer. (F) Low-magnification TEM image of a monolayer membrane from 1 transferred to a holey carbon-coated grid. The arrow points toward a small defect in the film created by the transfer process. (Inset) An electron diffraction pattern with rings at 1.1 Å and 2 Å, but otherwise no significant in-plane ordering. (G) High-magnification TEM image of a monolayer membrane from 1 transferred to holey carbon-coated grid showing a film largely free of pinhole defects.

Fig. 4.

Images of “unusual” monolayer membrane structures. (A and B) SEM and AFM images, respectively, of a membrane from 1 transferred onto a substrate with a square array of cylindrical holes (diameters ≈440 nm and depths ≈400 nm) to form “drumhead” structures. Red arrows point to the same region of the film that is suspended over the edge of a hole. (C and D) AFM and high-resolution AFM images, respectively, of a monolayer membrane from 1 grown on a substrate similar to that in A, but with relief depths of ≈35 nm, and then transferred to a flat Si wafer with a 300-nm thermal SiO₂ layer. (C Inset Upper Left) Power spectrum of the AFM image, indicating a well defined periodicity consistent with that of the growth substrate. (C Inset Lower Right) Illustration of a “pleated sheet.” (E and F) TEM images and diameter distributions of a monolayer membrane from 1 deposited on SiO₂ spheres imaged on a holey carbon-coated grid before (E) and after (F) HF vapor etching of the SiO₂. Insets are illustrations of the imaged structures. (G–I) Time-resolved reflection mode optical micrographs of a tubular membrane from 3 filled with HF/water immediately after HF vapor etching of optical fiber, after 20 min open to the air, and after complete drying, respectively. Insets are illustrations of the imaged structures. (J) AFM image of this collapsed tube. The line scan corresponds to an average over the area indicated by the rectangle.