Understanding On-Surface Synthesis Approaches Using AFM

Since Feynman's renowned lecture in 1959, controllable construction of nanomaterials with atomic resolution has been one of the dominant ideas of nanotechnology. He explained that ‘there’s plenty of room at the bottom.’¹ This article explains some of the current applications of Asylum AFMs in on-surface synthesis.

How is Nanofabrication Conducted?

Nanofabrication is characteristically conducted in two ways: “top-down” or “bottom-up.” “Top-down” defines processes that begin with a base material, then, eradicate and adapt it to create the chosen nanoscale structure. A commonly used example of this is conventional silicon microfabrication.

This procedure involves a silicon wafer that is selectively masked and etched.² However, top-down synthesis is usually a slow process, with larger expenses, and there is difficulty controlling it with atomic resolution.³

“Bottom-up” describes the synthesis methods in which molecules and architectures are constructed by building particular bonds between molecular building blocks. One route to bottom-up synthesis is molecular self-assembly.

Techniques of this kind can offer precise control at atomic resolution. For instance, in 2017, research was published in Scientific Reports and outlined the self-assembly of a protein nanotriangle. This was powered by interactions between the protein coils, alike to the interactions that are the root of naturally-occurring proteins to fold into complex 3D structures (see Figure 1).⁴

Figure 1. Atomic force microscopy (AFM) produced on an Asylum Research Cypher AFM image of a protein nanotriangle. Produced under ambient conditions. Image credit: Scientific Reports.⁴

Currently, applications of “bottom-up” nanofabrication by self-assembly have had limitations due to the decreased stability of self-assembled structures. There is a method to construct stable nanostructures with strong interactions, like covalent or coordinated bonds, through on-surface synthesis.⁵

On-surface Synthesis: An Introduction

Over the last decade, much emphasis has been put on on-surface synthesis in much nanofabrication research. Luckily, this has resulted in a plethora of developments in the field. The method of on-surface synthesis involves molecular building blocks adsorbed to a metallic, semiconducting, or insulating surface in order to produce nanostructures that are atomically precise. Additionally, on-surface synthesis leads to a hopeful avenue for learning about the dynamics of reactions in situ.^5,6,7

On-surface synthesis can involve two main things: self-assembly on a surface, or traditional chemical reactions, like polymerization, on a surface. Until now, numerous reactions have effectively been conducted using on-surface synthesis. This has included classical organic reactions, polymer synthesis, and 2D and 3D molecular array construction.^5,6,7

Detecting On-surface Synthesis Reactions and Products

For on-surface synthesis that is successful, it is required for researchers to analyze adsorbed molecules and surfaces with atomic resolution. The most commonly used analytical techniques for on-surface synthesis are atomic force microscopy (AFM) and scanning tunneling microscopy (STM).

Together, these techniques produce nanoscale images of surfaces by scanning an ultra-sharp tip across the sample. Firstly, AFM traces the topography by observing the weak intermolecular forces between the tip and sample, while instead, the STM monitors a low tunneling current between the tip and sample.⁸

Early research in the field of on-surface synthesis focused primarily on synthesis on metal or graphite surfaces. More recent research has tried to use substitute surfaces including insulators, like boron nitrate, consequently needing the use of AFM. Unfortunately, STM can usually only be used on conducting surfaces.⁹

Typically, molecular resolution can be achieved using high-vacuum AFM. Though, more recent developments in ambient AFM technology, together with new hardware and software, now allow structures to be resolved with atomic resolution using ambient conditions. This simplifies the characterization and enables AFM to be conducted in situ.^5,10

On-surface Synthesis: Applications of Ambient AFM

AFM of On-surface Self-assembly

Researchers can approve the molecular arrangement of adsorbed molecules through monitoring on-surface self-assembly with AFM. For instance, Professor Beton directed a group of researchers and scientists at the University of Nottingham, and they used AFM to observe self-assembly of porphyrin derivatives on boron nitride. This allowed them to work out the spacing of the adsorbed molecules (see Figure 2).⁹

Figure 2. AFM image showing a square lattice with 2.24 ± 0.05 nm spacings of Porphyrin 2D arrays on hexagonal boron nitride in air. Image credit: P. Beton, Univ. Nottingham.¹¹

Additional work, as seen below, by the same group summarizes the use of black phosphorous for on-surface synthesis. This surface is usually unstable under atmospheric conditions. AFM provided pictures of self-assembled supramolecular networks and structural evidence about the original black phosphorus surface (Figure 3).¹²

Figure 3. AFM images of melamine cyanurate deposited on black phosphorous. It was produced using an Asylum Research Cypher AFM in tapping mode under ambient conditions. Image credit: Nature Communications.¹²

On-surface Nanofabrication: The Use of Chemical Reactions in Scaffolds

The use of on-surface synthesis in order to weave a polymer-based ‘textile’ material has been recently investigated by Zhengbang Wang et al., and was published in Nature Communications.¹³ The scaffolds were in the form of surface-mounted metal-organic frameworks (SURMOFs), comprising of systematically arranged monomers.

Applied to a cross-linking catalyst, the monomers in the active SURMOF layers produced polymer threads. Once the metal ions were detached, a layer of interwoven polymer threads remained. AFM was used to define the structure of the textile product and the structure of the individual polymer strands.¹³

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Figure 4. Schematic diagram of on-surface synthesis of molecular textile. Image credit: Nature Communications.¹³

On-surface Synthesis: AFM in Three Dimensions

A large proportion of the research concerning on-surface synthesis focusses on the synthesis of two-dimensional supramolecular arrays. However, recent work was published in Nature Chemistry, and highlights the ability of on-surface synthesis to escape the confines of two dimensions.

This published work involved sequential deposition of 2D arrays that formed a heterostructure. It included ordered layers that were made stable using hydrogen bonding. Heterostructures with resolution high enough could recognize the relative placement of the molecules in the network using AFM imaging.

Consequently, this work signifies one of the first times AFM has been used under ambient conditions to image 3D networks with resolution high enough to provide structural information with added detail.¹⁴  

Asylum Research AFMs: What Can They Provide?

The remarkable research outlined previously all used AFMs from Asylum Research. While several AFMs can achieve atomic resolution, the only AFMs that make achieving high-resolution easy and routine are the Cypher AFMs from Asylum Research.

This is no secret however; Cypher AFMs use unrivaled mechanical stability, remarkably low drift and reduced noise electronics to deliver superior images. Moreover, Cypher AFMs deliver quick scanning, simple operation, and a small lab footprint. They are perfect for investigating on-surface synthesis and consistently deliver excellent results.^11,15

References and Further Reading

1. ‘There’s plenty of room at the bottom’ — Feynman R, Caltech Engineering and Science, 1960.
2. ‘Chemical routes to top-down nanofabrication’ – H-D. Yu, M. D. Regulacio, E. Yea and M-Y. Han, Chemistry Society Reviews 14 6006-6018, (2013). https://doi.org/10.1039/C3CS60113G
3. ‘Top Down and Bottom Up Construction NanoFabrication Techniques Defined’ https://www.azonano.com/article.aspx?ArticleID=1835
4. ‘Modular assembly of a protein nanotriangle using orthogonally interacting coiled coils’ — W.M Park, M. Bedewy, K.K Berggren, A.E Keating, Scientific Reports, 7, 10577 (2017). https://doi.org/10.1038/s41598-017-10918-6
5. ‘Frontiers of on-surface synthesis: From principles to applications’ — Q. Shen, H.Y Gao, H. Fuchs, NanoToday, 13, 77-96 (2017). https://doi.org/10.1016/j.nantod.2017.02.007
6. ‘On-Surface Synthesis’ — A. Gourdon, Springer, (2016). https://doi.org/10.1007/978-3-319-26600-8
7. ‘On-Surface Synthesis II’ — D.G de Oteyza, C. Rogero, Springer, (2018). https://doi.org/10.1007/978-3-319-75810-7
8. ‘Surface Analysis with STM and AFM: Experimental and Theoretical Aspects of Image Analysis’ — S.N. Magonov, M.H. Whangbo, John Wiley & Sons, (2007). https://doi.org/10.1002/9783527615117
9. ‘van der Waals-Induced Chromatic Shifts in Hydrogen-Bonded Two-Dimensional Porphyrin Arrays on Boron Nitride’ — V.V. Korolkov, S.A. Svatek, A. Summerfield, J. Kerfoot, L. Yang, T. Taniguchi, K. Watanabe, N.R. Champness, N.A Besley, P.H. Beton, ACS Nano, 9(10), 10347-10355, (2015). https://doi.org/10.1021/acsnano.5b04443
10. ‘Advances in atomic force microscopy’ — F.J Giessibl, Reviews of Modern Physics, 75(3), 949 (2003). https://doi.org/10.1103/RevModPhys.75.949
11. Cypher AFMs. Accessed 25/10/2018: https://afm.oxinst.com/
12. ‘Supramolecular networks stabilise and functionalise black phosphorus’ — V.V. Korolkov, I.G. Timokhin, R. Haubrichs, E.F. Smith, L. Yang, S. Yang, N.R. Champness, M. Schröder, P.H. Beton, Nature Communications, 8, 1385, (2017). https://doi.org/10.1038/s41467-017-01797-6
13. ‘Molecular weaving via surface-templated epitaxy of crystalline coordination networks.’ — Z. Wang, A. Błaszczyk, O. Fuhr, S. Heissler, C. Wöll, M. Mayor, Nature Communications, 8 2017. https://doi.org/10.1038/ncomms14442
14. ‘Supramolecular heterostructures formed by sequential epitaxial deposition of two-dimensional hydrogen-bonded arrays’ — V.V. Korolkov, M. Baldoni, K. Watanabe, T. Taniguchi, E. Besley, P.H. Beton, Nature Chemistry, 9, 1191-1197, (2017). https://doi.org/10.1038/nchem.2824
15. ‘Asylum Research Cypher Family of AFMs’. Accessed 25/10/2018:  https://afm.oxinst.com/products/cypher-afm-systems/

This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.

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