Soft Matter Nanotechnology E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 796

Veröffentlichungsjahr: 2015

Beschreibung

Using the well-honed tools of nanotechnology, this book presents breakthrough results in soft matter research, benefitting from the synergies between the chemistry, physics, biology, materials science, and engineering communities. The team of international authors delves beyond mere structure-making and places the emphasis firmly on imparting functionality to soft nanomaterials with a focus on devices and applications. Alongside reviewing the current level of knowledge, they also put forward novel ideas to foster research and development in such expanding fields as nanobiotechnology and nanomedicine. As such, the book covers DNA-induced nanoparticle assembly, nanostructured substrates for circulating tumor cell capturing, and organic nano field effect transistors, as well as advanced dynamic gels and self-healing electronic nanodevices. With its interdisciplinary approach this book gives readers a complete picture of nanotechnology with soft matter.

Leseprobe

CONTENTS

Cover

Related Titles

Title Page

Copyright

List of Contributors

Preface

Chapter 1: Chemical Reactions for the Synthesis of Organic Nanomaterials on Surfaces

1.1 Introduction

1.2 Alkane Polymerization

1.3 Azide–Alkyne Cycloaddition

1.4 Glaser Coupling

1.5 Decarboxylative Polymerization of Acids

1.6 Conclusions

Acknowledgments

References

Chapter 2: Self-Assembly of Organic Molecules into Nanostructures

2.1 Introduction

2.2 Classification of Nanostructures

2.3 General Self-Assembly Method for the Construction of Nanostructures

2.4 Molecular Design and Building Blocks

2.5 Functions of Some Typical Nanostructures

2.6 Conclusions and Outlook

References

Chapter 3: Supramolecular Nanotechnology: Soft Assembly of Hard Nanomaterials

3.1 Introduction

3.2 Soft Cell-Like Structures with Hard Nanomaterials

3.3 For Hierarchical Assembly: LbL and Others

3.4 Summary

Acknowledgments

References

Chapter 4: Nanoparticles: Important Tools to Overcome the Blood–Brain Barrier and Their Use for Brain Imaging

4.1 Introduction

4.2 Physiology of the Blood–Brain Barrier

4.3 Definition and Type of Nanoparticles and Nanocarriers for Brain Uptake

4.4 Nanoparticles and Imaging

4.5 Conclusion and Outlook

Acknowledgment

References

Chapter 5: Organic Nanophotonics: Controllable Assembly of Optofunctional Molecules toward Low-Dimensional Materials with Desired Photonic Properties

5.1 Introduction

5.2 From Molecules to Assembly

5.3 From Assembly to Structures

5.4 From Structures to Photonic Properties

5.5 Conclusions

Acknowledgments

References

Chapter 6: Functional Lipid Assemblies by Dip-Pen Nanolithography and Polymer Pen Lithography

6.1 Introduction

6.2 Techniques and Methods

6.3 Ink Transfer Models

6.4 Applications

6.5 Conclusions

Acknowledgments

References

Chapter 7: PEG-Based Antigen-Presenting Cell Surrogates for Immunological Applications

7.1 Introduction

7.2 Elastic Nanopatterned and Specifically Biofunctionalized 2D PEG-DA Hydrogels: General Properties

7.3 Nanostructured PEG-DA Hydrogel Beads: General Properties

7.4 Nanostructured and Specifically Biofunctionalized Droplets of Water-in-Oil Emulsion: General Properties

7.5 Summary and Outlook for the Future

Acknowledgments

References

Chapter 8: Soft Matter Assembly for Atomically Precise Fabrication of Solid Oxide

8.1 Introduction

8.2 The Ultimate Goal of Nanotechnology: Atomically Precise Fabrication

8.3 Soft Mater Assembly for Atomically Precise Oxide Layers

8.4 Soft Matter Assembly for Atomically Precise Oxide Dots

8.5 Summary for the Future Works

References

Chapter 9: Conductive Polymer Nanostructures

9.1 Introduction

9.2 Solution-Based Synthesis of Conducting Polymer Nanostructures

9.3 Substrate-Based Fabrication of Conducting Polymer Nanostructures

9.4 Electrospinning Technique of Conducting Polymer

9.5 Summary and Outlook

References

Chapter 10: DNA-Induced Nanoparticle Assembly

10.1 Introduction

10.2 DNA as a Template Material

10.3 DNA as Ligand

10.4 Applications

10.5 Summary

References

Chapter 11: Nanostructured Substrates for Circulating Tumor Cell Capturing

11.1 Introduction

11.2 Nanostructured Substrates for CTC Capturing

11.3 Nanostructured Substrates for Other Cells Capturing

11.4 Conclusions and Perspectives

References

Chapter 12: Organic Nano Field-Effect Transistor

12.1 Introduction

12.2 The Fabrication of Organic Semiconductor Nanostructures

12.3 Device Structures of Organic Nano Field-Effect Transistor

12.4 The Preparation of Organic Nano Field-Effect Transistor

12.5 Properties of Organic Nanoscale Field-Effect Transistor

12.6 Application of Organic Nano-FETs

12.7 Summary and Outlook

References

Chapter 13: Advanced Dynamic Gels

13.1 Introduction

13.2 Gels in Nature

13.3 Characterization of VEGs

13.4 Redox-Responsive VEGs

13.5 pH-Responsive VEGs

13.6 Temperature-Responsive VEGs

13.7 Photoresponsive VEGs

13.8 Applications

13.9 Conclusions

13.10 Theory

References

Chapter 14: Micro/Nanocrystal Conversion beyond Inorganic Nanostructures

14.1 Introduction

14.2 Micro/Nanostructure Conversion through Charge Transfer Complex Formation

14.3 Micro/Nanostructure Conversion through Ion and Ligand Exchange

14.4 Micro/Nanostructure Conversion through Reduction

14.5 Micro/Nanostructure Conversion through Photoinduced Reaction

14.6 Micro/Nanostructure Conversion through Thermal-induced Reaction

14.7 Properties and Applications

14.8 Summary and Outlook

References

Chapter 15: Self-Healing Electronic Nanodevices

15.1 Introduction

15.2 Self-Healing Materials

15.3 Self-Healing Electrical Conductors

15.4 Self-Healing in Energy Storage Devices

15.5 Self-Healing Electronic Skin

15.6 Conclusive Remarks and Outlook

References

Index

EULA

List of Tables

Table 2.1

Table 7.1

Table 7.2

Table 7.3

Table 12.1

List of Illustrations

Figure 1.1

Figure 1.2

Figure 1.3

Figure 1.4

Figure 1.5

Figure 1.6

Figure 1.7

Figure 1.8

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 2.10

Figure 2.11

Figure 2.12

Figure 2.13

Figure 2.14

Figure 2.15

Figure 2.16

Figure 2.17

Figure 2.18

Figure 2.19

Figure 2.20

Figure 2.21

Figure 2.22

Figure 2.23

Figure 2.24

Figure 2.25

Figure 2.26

Figure 2.27

Figure 2.28

Figure 2.29

Figure 2.30

Figure 2.31

Figure 2.32

Figure 2.33

Figure 2.34

Figure 2.35

Figure 2.36

Figure 2.37

Figure 2.38

Figure 2.39

Figure 2.40

Figure 2.41

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figure 3.6

Figure 3.7

Figure 3.8

Figure 3.9

Figure 3.10

Figure 3.11

Figure 4.1

Figure 4.2

Figure 4.3

Figure 5.1

Figure 5.2

Figure 5.3

Figure 5.4

Figure 5.5

Figure 5.6

Figure 5.7

Figure 5.8

Figure 5.9

Figure 5.10

Figure 5.11

Figure 5.12

Figure 5.13

Figure 5.14

Figure 6.1

Figure 6.2

Figure 6.3

Figure 6.4

Figure 6.5

Figure 6.6

Figure 6.7

Figure 6.8

Figure 6.9

Figure 6.10

Figure 6.11

Figure 6.12

Figure 6.13

Figure 6.14

Figure 6.15

Figure 6.16

Figure 7.1

Figure 7.2

Figure 7.3

Figure 7.4

Figure 7.5

Figure 7.6

Figure 7.7

Figure 7.8

Figure 7.9

Figure 7.10

Figure 7.11

Figure 7.12

Figure 7.13

Figure 7.14

Figure 7.15

Figure 7.16

Figure 7.17

Figure 7.18

Figure 8.1

Figure 8.2

Figure 8.3

Figure 8.4

Figure 8.5

Figure 8.6

Figure 8.7

Figure 9.1

Figure 9.2

Figure 9.3

Figure 9.4

Figure 9.5

Figure 9.6

Figure 9.7

Figure 9.8

Figure 9.9

Figure 9.10

Figure 9.11

Figure 9.12

Figure 9.13

Figure 9.14

Figure 9.15

Figure 9.16

Figure 9.17

Figure 10.1

Figure 10.2

Figure 10.3

Figure 10.4

Figure 10.5

Figure 10.6

Figure 10.7

Figure 10.8

Figure 10.9

Figure 10.10

Figure 10.11

Figure 10.12

Figure 10.13

Figure 10.14

Figure 10.15

Figure 10.16

Figure 10.17

Figure 10.18

Figure 10.19

Figure 11.1

Figure 11.2

Figure 11.3

Figure 11.4

Figure 11.5

Figure 11.6

Figure 11.7

Figure 12.1

Figure 12.2

Figure 12.3

Figure 12.4

Figure 12.5

Figure 12.6

Figure 12.7

Figure 12.8

Figure 12.9

Figure 12.10

Figure 12.11

Figure 12.12

Figure 12.13

Figure 12.14

Figure 12.15

Figure 12.16

Figure 12.17

Figure 12.18

Figure 12.19

Figure 12.20

Figure 12.21

Figure 12.22

Figure 12.23

Figure 12.24

Figure 12.25

Figure 12.26

Figure 12.27

Figure 12.28

Figure 12.29

Figure 12.30

Figure 12.31

Figure 12.32

Figure 12.33

Figure 12.34

Figure 12.35

Figure 13.1

Figure 13.2

Figure 13.3

Figure 13.4

Figure 13.5

Figure 13.6

Figure 13.7

Figure 13.8

Figure 13.9

Figure 13.10

Figure 13.11

Figure 14.1

Figure 14.2

Figure 14.3

Figure 14.4

Figure 14.5

Figure 14.6

Figure 14.7

Figure 14.8

Figure 14.9

Figure 14.10

Figure 14.11

Figure 14.12

Figure 15.1

Figure 15.2

Figure 15.3

Figure 15.4

Figure 15.5

Figure 15.6

Figure 15.7

Figure 15.8

Figure 15.9

Figure 15.10

Figure 15.11

Guide

Cover

Table of Contents

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Chapter 1

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Soft Matter Nanotechnology

From Structure to Function

Edited by Xiaodong Chen and Harald Fuchs

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List of Contributors

Oscar Díaz Arado

Westfälische Wilhelms-Universität Münster

Physikalisches Institut

Wilhelm-Klemmstrasse 10

48149 Münster

Germany

Katsuhiko Ariga

National Institute for Materials Science (NIMS)

World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA)

1-1 Namiki

Tsukuba 305-0044

Japan

Anne Buchkremer

RWTH Aachen University

Institute of Inorganic Chemistry

Landoltweg 1

52056 Aachen

Germany

Bevita K. Chandran

Nanyang Technological University

School of Materials Science and Engineering

50 Nanyang Avenue

Singapore 639798

Singapore

Xiaodong Chen

Nanyang Technological University

School of Materials Science and Engineering

50 Nanyang Avenue

Singapore 639798

Singapore

Lifeng Chi

Soochow University

Institute of Functional Nano & Soft Materials (FUNSOM)

Ren-Ai street, 199

Suzhou, 215123 Jiangsu

China

Bin Dong

Soochow University

Institute of Functional Nano & Soft Materials (FUNSOM)

Ren-Ai street, 199

Suzhou, 215123 Jiangsu

China

Harald Fuchs

Karlsruhe Institute of Technology (KIT)

Karlsruhe Nano Micro Facility (KNMF)

Institute of Nanotechnology (INT)

KIT Campus North

76021 Karlsruhe

Germany

and

Westfälische Wilhelms-Universität Münster

Physical Institute and Center for Nanotechnology (CeNTech)

48149 Münster

Germany

Hans-Joachim Galla

University of Münster

Institute for Biochemistry

Wilhelm Klemm Str.2

Münster 48149

Germany

and

Chinese Academy of Science Beijing

Institute of Chemistry

Bei Yi Jie 2, Zhong Guan Cun

Beijing 100190

China

Hong-Ying Gao

Westfälische Wilhelms-Universität Münster

Physikalisches Institut

Wilhelm-Klemmstrasse 10

48149 Münster

Germany

Mingyuan Gao

Chinese Academy of Science Beijing

Institute of Chemistry

Bei Yi Jie 2, Zhong Guan Cun

Beijing 100190

China

Carsten Hentschel

Soochow University

Institute of Functional Nano & Soft Materials (FUNSOM)

Ren-Ai street, 199

Suzhou, 215123 Jiangsu

China

Jonathan P. Hill

National Institute for Materials Science (NIMS)

World Premier International (WPI)

Research Center for Materials

Nanoarchitectonics (MANA) 1-1 Namiki

Tsukuba 305-0044

Japan

Michael Hirtz

Karlsruhe Institute of Technology (KIT)

Karlsruhe Nano Micro Facility (KNMF)

Institute of Nanotechnology (INT)

KIT Campus North

76021 Karlsruhe

Germany

Wenping Hu

Institute of Chemistry, Chinese Academy of Sciences

Beijing National Laboratory for Molecular Sciences (BNLMS)

Zhongguancun North First Street 2

Beijing 100190

China

Jan-Willi Janiesch

Max Planck Institute for Intelligent Systems

Department of New Materials and Biosystems

Heisenbergstr.3

70569 Stuttgart

Germany

and

University of Heidelberg

Department of Biophysical Chemistry

Im Neuenheimer Feld 253

69120 Heidelberg

Germany

Qingmin Ji

National Institute for Materials Science (NIMS)

World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA)

1-2 Namiki

Tsukuba 305-0044

Japan

Lin Jiang

Soochow University

Institute of Functional Nano & Soft Materials (FUNSOM)

Ren-Ai street, 199

Suzhou, 215123 Jiangsu

China

Li Junbo

Nanyang Technological University

School of Materials Science and Engineering

50 Nanyang Avenue

Singapore 639798

Singapore

Gerri Kannenberg

Max Planck Institute for Intelligent Systems

Department of New Materials and Biosystems

Heisenbergstr.3

70569 Stuttgart

Germany

and

University of Heidelberg

Department of Biophysical Chemistry

Im Neuenheimer Feld 253

69120 Heidelberg

Germany

Hongliang Liu

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences

Laboratory of Bio-Inspired Smart Interface Science

29 Zhongguancun East Road, Haidian District

Beijing 100190

China

Minghua Liu

Institute of Chemistry, Chinese Academy of Sciences

CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics

Beijing National Laboratory for Molecular Science (BNLMS)

Zhongguancun North First Street 2

Beijing 100190

China

Kai Lv

Institute of Chemistry, Chinese Academy of Sciences

CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics

Beijing National Laboratory for Molecular Science (BNLMS)

Beijing 100190

Zhongguancun North First Street 2

China

Jovana Mati

Max Planck Institute for Intelligent Systems

Department of New Materials and Biosystems

Heisenbergstr.3

70569 Stuttgart

Germany

and

University of Heidelberg

Department of Biophysical Chemistry

Im Neuenheimer Feld 253

69120 Heidelberg

Germany

Jingxin Meng

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences

Laboratory of Bio-Inspired Smart Interface Science

29 Zhongguancun East Road, Haidian District

Beijing 100190

China

Harry Mönig

Westfälische Wilhelms-Universität Münster

Physikalisches Institut

Wilhelm-Klemmstrasse 10

48149 Münster

Germany

Masanobu Naito

National Institute for Materials Science

Environment and Energy Materials Division

Environmental Remediation Materials Unit

Catalytic Materials Group

1-1 Namiki

Tsukuba, Ibaraki 305-0044

Japan

Ilia Platzman

Max Planck Institute for Intelligent Systems

Department of New Materials and Biosystems

Heisenbergstr.3

70569 Stuttgart

Germany

and

University of Heidelberg

Department of Biophysical Chemistry

Im Neuenheimer Feld 253

69120 Heidelberg

Germany

Ruirui Qiao

Chinese Academy of Science Beijing

Institute of Chemistry

Bei Yi Jie 2, Zhong Guan Cun

Beijing 100190

China

Long Qin

Institute of Chemistry, Chinese Academy of Sciences

CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics

Beijing National Laboratory for Molecular Science (BNLMS)

Zhongguancun North First Street 2

Beijing 100190

China

Norifusa Satoh

National Institute for Materials Science (NIMS)

Environment and Energy Materials Division

1-1 Namiki

Tsukuba, Ibaraki 305-0044

Japan

Sylwia Sekula-Neuner

Karlsruhe Institute of Technology (KIT)

Karlsruhe Nano Micro Facility (KNMF)

Institute of Nanotechnology (INT)

KIT Campus North

76021 Karlsruhe

Germany

Zhaocun Shen

Institute of Chemistry, Chinese Academy of Sciences

CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics

Beijing National Laboratory for Molecular Science (BNLMS)

Zhongguancun North First Street 2

Beijing 100190

China

Rekha G. Shrestha

National Institute for Materials Science

Environment and Energy Materials Division

Environmental Remediation Materials Unit

Catalytic Materials Group

1-1 Namiki

Tsukuba, Ibaraki 305-0044

Japan

Ulrich Simon

RWTH Aachen University

Institute of Inorganic Chemistry

Landoltweg 1

52056 Aachen

Germany

Joachim P. Spatz

Max Planck Institute for Intelligent Systems

Department of New Materials and Biosystems

Heisenbergstr.3

70569 Stuttgart

Germany

and

University of Heidelberg

Department of Biophysical Chemistry

Im Neuenheimer Feld 253

69120 Heidelberg

Germany

Ainhoa Urtizberea

Karlsruhe Institute of Technology (KIT)

Karlsruhe Nano Micro Facility (KNMF)

Institute of Nanotechnology (INT)

KIT Campus North

76021 Karlsruhe

Germany

Shutao Wang

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences

Laboratory of Bio-Inspired Smart Interface Science

29 Zhongguancun East Road, Haidian District

Beijing 100190

China

Jiansheng Wu

Nanyang Technological University

School of Materials Science and Engineering

50 Nanyang Avenue

Singapore 639798

Singapore

Yongli Yan

Institute of Chemistry, Chinese Academy of Sciences

CAS Key Laboratory of Photochemistry

Beijing National Laboratory for olecular Sciences (BNLMS)

Zhongguancun North First Street 2

Beijing 100190

China

Li Zhang

Nanyang Technological University

School of Materials Science and Engineering

50 Nanyang Avenue

Singapore 639798

Singapore

Qichun Zhang

Nanyang Technological University

School of Materials Science and Engineering

50 Nanyang Avenue

Singapore 639798

Singapore

Yong Sheng Zhao

Institute of Chemistry, Chinese Academy of Sciences

CAS Key Laboratory of Photochemistry

Beijing National Laboratory for olecular Sciences (BNLMS)

Zhongguancun North First Street 2

Beijing 100190

China

Yonggang Zhen

Institute of Chemistry, Chinese Academy of Sciences

Beijing National Laboratory for Molecular Sciences (BNLMS)

Zhongguancun North First Street 2

Beijing 100190

China

Preface

In Nature, nanoscale components of a cell are organized into highly dynamic hierarchical structures and in a way that we call life. This self-organization mainly includes modular design, fault tolerance, and the ability to self-repair damage and even to reproduce itself. The precise and spatiotemporal dynamic arrangement of the individual building blocks causes the biological functionality. This inspires us to develop a controlled mastery of natural functioning in synthetic, biomimetic nanosystems. However, we are still at the infant stages of learning how to design and master the control of soft nanoscale structures with specific properties. Both technical and basic thermodynamic processes need to be understood by learning from Nature before higher degrees of complexity and functions such as self-replication and self-healing can be targeted.

Soft matter nanotechnology is a multidisciplinary area of chemistry, physics, biology, medicine, and materials science. It essentially deals with matter such as amphiphiles, liquid crystals, colloids, polymers, and a number of hierarchically ordered biological molecular systems, using the well-honed tools of nanotechnology. This book reflects some representative examples and advanced fruitful synergies between soft matter science and nanotechnology. Bringing together 15 contributions from many leading scientists, it covers the following aspects of soft matter nanotechnology: (1) structuring soft-materials at the nanoscale; (2) fabrication of complex or composite nanomaterials; (3) nanotechnology tools to study soft nanostructures, (4) nano–bio interface, and (5) applications of soft nanomaterials.

We hope that this book will provide stimulating concepts on the design, fabrication, and exploitation of soft nanomaterials. New soft nanoscale materials and structures, responsive and active soft nanomaterials and devices, functional organic nanoelectronics, integrated nano–bio interface, and energy-related issues are several key areas to be addressed in this book. In addition, recent methodological aspects such as nanoanalytical methods are also displayed.

We are extremely grateful to the authors for their high-quality contributions to this collection. We would also like to thank the production team of Wiley-VCH, in particular Dr. Waltraud Wüst, Dr. Martin Preuss, and Dr. Gudrun Walter, for continuous and dedicated help during the production of this book.

Xiaodong Chen Harald Fuchs

Singapore Münster March 2015

1Chemical Reactions for the Synthesis of Organic Nanomaterials on Surfaces

Hong-Ying Gao, Oscar Díaz Arado, Harry Mönig, and Harald Fuchs

1.1 Introduction

The bottom-up growth of covalently connected organic networks is a fascinating approach for the development of new functional nanomaterials with tunable mechanical and optoelectronic properties [1–5]. Hereby, organic molecules are the building blocks, which can be deposited on a substrate by evaporation under ultrahigh vacuum (UHV) conditions or by solution growth in liquid environments. By the proper design of the precursor molecules and by controlling the parameters such as deposition rates, concentrations, and substrate temperature or irradiation, chemical reactions can be induced. This can lead to the formation of either conjugated polymers or 2D covalently connected networks [6–8]. The structural and optoelectronic properties of these products can be adjusted by the manifold possibilities of organic chemistry, which allows tailoring the reactive end groups, and the geometry as well as the electronic properties of the reactants. Following this idea, nanoscale devices ranging from simple wires to transistors, capacitors, or solar cells can be envisioned. Although recent years have shown significant progress in this field, there are only a few established on-surface reaction mechanisms that actually lead to covalent coupling directly at surfaces. Therefore, there are still considerable scientific challenges at play, making it a current hot topic in nanotechnology [2,5,6].

To study the chemical processes involved in the synthesis of such materials, experimental techniques with high lateral resolution are particularly desirable. Therefore, scanning probe microscopy and in particular scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM) have become the standard characterization tools providing direct insight into reaction mechanisms. These techniques offer topographic and electronic information of single molecules and on-surface chemical processes with high resolution [9–11], and the probe tip itself can be used to trigger chemical reactions [12–14] or to manipulate molecular species at surfaces [15,16]. Furthermore, both STM and NC-AFM experiments can be performed under different environmental conditions, such as in UHV, in liquids, or in gaseous environments, providing further versatility to the targeted experiments. Many studies also rely on complementary experimental techniques such as X-ray photoelectron spectroscopy (XPS) or temperature-programmed desorption (TPD). From the theory side, density functional theory (DFT) is applied to describe adsorption geometries and chemical processes that take place on various substrates.

In the following, the so far established on-surface reaction mechanisms are reviewed, which potentially can be utilized for the bottom-up growth of covalently connected organic networks. Other approaches where surfaces are exclusively involved to catalyze reactions and where the products do not stay on the surface are not considered in this chapter. Besides a general overview of the various reaction types, we present a compilation of the results obtained by our group at the University of Münster, which contributed to this exciting research field.

1.1.1 Ullmann Coupling

This particular reaction is the most studied one in the recent years. It involves the thermal activation of precursor molecules with halogenated moieties after or during deposition on a substrate, which, by the supply of heat, leads to the formation of C–C bonds between the radicalized reactants. This reaction has been performed mostly on metal substrates. However, recently it has been shown to also proceed on insulators [17], which represents a significant step forward toward potential applications of the Ullmann coupling mechanism. The halogen specie mostly studied is bromine [18–24], although successful covalent coupling after scission of terminal iodine [19,25–27] and chlorine [17,28] has also been reported. The difference in activation temperature for the different carbon–halogen species has allowed to hierarchically control the formation of two-dimensional networks [19,24]. Another interesting nanomaterial with functional electronic properties developed with this method is based on graphene nanoribbons. These are obtained by a two-step reaction sequence where after initial on-surface Ullmann coupling, a further annealing step at a higher temperature leads to cyclodehydrogenation of adjacent aromatic species [23,29]. In particular, graphene nanoribbons were also successfully grown on a stepped Au(788) surface, which form well-defined (111) terraces with a width of about 3.8 nm along the [01–1] direction (Figure 1.1). This allows to gain control over the orientation of the nanoribbons, which predominantly grow along the terraces. High-resolution direct and inverse photoemission experiments of occupied and unoccupied states allowed to determine the energetic position and momentum dispersion of electronic states, which showed a bandgap of several electronvolts for different types of graphene nanoribbons [23,29].

Figure 1.1 (a) STM image of spatially aligned graphene nanoribbons on a Au(788) surface. (b) After initial Ullmann coupling at a substrate temperature of 440–470 K, an additional heating step at 590 K results in a dehydrogenation process leading to the formation of the nanoribbons. (c) Statistical distribution of the ribbon length. (Reprinted figure with permission from [29]. Copyright (2012) by the American Physical Society.)

It has been demonstrated that in general it is possible to achieve covalent bonding between organic molecules and metal atoms on surfaces [11,30]. Therefore, there are also efforts to not directly bind the organic building blocks for supramolecular structures, but instead use metal atoms to mediate the coupling. Following this approach under UHV conditions, covalent bonding between tetracyanobenzene molecules and Mn atoms was accomplished to form Mn-phthalocyanine [31]. In another study, strong bonds between metal atoms and dehalogenated reactants have been shown to play an important role in the on-surface Ullmann coupling, stabilizing two-dimensional networks [24]. Furthermore, an on-surface chemical reaction between copper ions and quinone ligands at the liquid/solid interface of a Au(111) surface has been shown to successfully form two-dimensional metal–organic polymers [32].

The results on metal–organic materials lead to further developments, which use Ullmann coupling to form gold–organic hybrids, where chemical bonding between organic precursor molecules is mediated by bonding to a substrate gold atom. With this approach, the on-surface synthesis of gold–organic linear polymers by the dehalogenation of chloro-substituted perylene-3,4,9,10-tetracarboxylic acid bisimides (PBIs) was accomplished on the Au(111) and Au(100) surfaces [17,28]. Figure 1.2a shows an STM image after depositing the PBIs at room temperature on a Au(111) surface, leading to a self-assembled monolayer of the unreacted molecules. The STM contrast features a combination of two small bumps and a big rod corresponding to the short alkyl chains and the aromatic core of the PBI molecule, respectively. The bright spots on both sides of the core correspond to the twisted two chlorine atoms. Linear molecular chains were formed via depositing the precursor molecules on a 490 K preheated Au(111) surface at a deposition rate of about 0.1 monolayer per hour. Figure 1.2b is a representative STM image of the reaction products, in which polymers with a length of up to 70 nm can be observed. Interestingly, the orientation of the molecular chains was found mainly along 〈110〉 directions on the Au(111) surface. A high-resolution STM image of such a polymer is shown in Figure 1.2c, with its proposed chemical structure shown in Figure 1.2d. The measured periodic distance along the chain is (0.83 ± 0.04) nm, which matches well with the theoretical distance of 0.82 nm as determined by DFT calculations. Further experiments with the PBI deposited on the reconstructed Au(100) surface showed that on this surface the polymerization occurs exclusively along the [011] direction [28]. Further DFT calculations on a simplified model system confirmed that the reaction mechanism involves an intermediate state where a PBI radical generated from the homolytic C–Cl bond dissociation binds to a surface gold atom, which partially is pulled out from the surface to form a stable PBI–gold hybrid species.

Figure 1.2 Ullmann reaction to form gold–organic hybrids. (a) The self-assembly structures of tetrachloro-PBI at a Au(111) surface (10 nm × 10 nm). (b) Polymerized molecular chains of tetrachloro-PBI on a Au(111) surface (via depositing molecules onto the hot surface of a substrate at 490 K). (c) High-resolution STM image of a polymerized PBI–Au chain and (d) its proposed chemical structure. (Reproduced with permission from Wiley-VCH Verlag GmbH [28].)

1.1.2 Condensation Reactions

Condensation reactions are commonly referred to as dehydration synthesis, which involves the combination of two functional groups to form a unique covalently bound product, together with a small molecular by-product, following an addition–elimination mechanism. Several condensation reactions have been proven to be suitable candidates for on-surface synthesis and have received increased attention due to their reversibility by simply adding a solution with a certain concentration of the by-product [33,34]. Successful polyimide condensation [35,36] was accomplished on a Au(111) surface by reacting anhydrides and amines with H2O as a by-product, where the careful design of the reactants allowed to obtain both polymeric strands and porous networks. Moreover, by reacting aldehydes and amines on Au(111) [37,38], successful polyimine condensation was accomplished to produce 1D and 2D polymers on a substrate, together with H2O as a by-product. This process has also been successfully accomplished at a liquid–solid interface on highly oriented pyrolytic graphite (HOPG) [34]. Furthermore, with HCl as a by-product, two on-surface condensation reactions have been performed to successfully form 2D porous covalent networks: polyester condensation [39] by reacting alcohols and acyl chlorides on a Au(111) surface and polyamide condensation [40] by reacting acid chlorides with amines.

However, despite the versatility of these condensation processes, the on-surface condensation of boronic acids has received most of the attention to grow nanostructures on surfaces. This chemical reaction is a good example of how noncovalent interactions controlling self-assembly processes can be used to later initiate covalent coupling between adsorbed molecules on a substrate. Furthermore, given their capability to form reversible covalent complexes, boronic acids are extensively used in organic chemistry. Taking these factors into consideration, successful on-surface covalent coupling between boronic acids has been accomplished on a large variety of substrates, such as Ag(111) [41,42], Ag(100) [41,43], Cu(111) [41], at the liquid–solid interface on HOPG [33], and Au(111) [41,44]. These studies have demonstrated not only the reversibility of the on-surface reaction, but also that the pore size and quality of the 2D networks can be tailored through careful control over the kinetic parameters of the reaction and the design of the reactants. Moreover, successful attempts to combine the success of the condensation of boronic acids and the Ullmann coupling have been carried out on Au(111) [44]. By the reaction of a single reactant charged with boronic acid and bromine moieties, subsequent stepwise activation leads to an optimization of the growth mechanism of the 2D network on that substrate, with the polymerization yield reaching almost 100%.

1.2 Alkane Polymerization

To study the on-surface polymerization of alkanes, the Au(110) surface was found to be a suitable substrate [45]. After the treatment of several sputtering–annealing cycles, this surface reconstructs to form a missing row structure along the [1–10] direction. Acting as one-dimensional constraint, the reconstruction of the surface efficiently confines the diffusion of adsorbed molecules in the [1–10] direction within the atomic channels. In the first step, n-dotriacontane (C32H66) was used as a precursor molecule, which was deposited under UHV conditions onto the gold substrate held at room temperature. Shorter alkane chains can easily desorb, whereas the strong interaction of n-dotriacontane with the surface inhibits desorption at elevated temperatures. The monomers adsorb on the Au(110) surface exclusively along the atomic channels (Figure 1.3a). After subsequent annealing at 440 K, the alkane monomers were bonded end to end, forming long molecular chains within the atomic channels of the Au(110) substrate (Figure 1.3b). The successful polymerization was checked by controlled manipulation experiments with the STM tip, showing that sections of the reacted chains could be pulled out from the substrate grooves (Figure 1.3c) and providing further evidence that the products are covalently bound. It was found that most of the reacted polymers have a length of more than 200 nm. In TPD experiments, the desorption of H2 could be verified during the thermally activated polymerization pointing to a dehydrogenation polymerization process. This was further confirmed by testing the reaction with 1,4-di(eicosyl)benzene and eicosylbenzene monomers, which both have a phenylene moiety as a marker connecting the alkyl chains (for more details see Ref. [45]). Results from DFT calculations provide a reasonable mechanism for the activation of the monomers, which exclusively takes place at the terminal CH3 or penultimate CH2 groups. Furthermore, the DFT results confirm that the orientational constraint of the reconstructed Au(110) surface is a major factor for the reaction to proceed.

Figure 1.3 Dehydrogenative polymerization of n-dotriacontane (C32H66). (a) STM image of C32H66 monomers on the Au(110)-(1 × 2) surface (20 nm × 15 nm). (b) Parallel polyethylene chains after annealing up to 440 K for 30 min (50 nm × 50 nm; inset: 9 nm × 13 nm). (c) Several polyethylene chains partially released from the grooves by the STM tip manipulations (50 nm × 70 nm). (d) High-resolution STM image of Au atomic rows and polyethylene chain (6 nm × 2.5 nm). (e) Height profiles at the dotted lines in (d). (f) H2 signal detected by mass spectrometry during the annealing process (heating power: 1.2 A by 10.25 V). (Reprinted with permission from AAAS [45].)

1.3 Azide–Alkyne Cycloaddition

Among the many conceivable reactions that have potential to be performed as on-surface processes, azide–alkyne cycloadditions (often referred to as “click” reactions) are interesting candidates due to their strong covalently linked products and their low activation energies [46]. Such reactions are free of by-products and performed as solution-phase processes that have been widely used for the modification of surfaces and materials [47–49] as well as for the preparation of biologically active compounds in pharmaceutical research [50]. The most commonly used “click” reaction is the azide–alkyne 1,3-dipolar cycloaddition [51,52] leading to 1,4- and 1,5-triazoles. Whereas the classical thermal process (Huisgen azide–alkyne [3 + 2]-cycloaddition), which proceeds under harsh conditions, that is, high temperatures, generally delivers a mixture of the two regioisomers, the use of a copper catalyst provides 1,4-triazoles with high efficiency and regioselectivity under ambient conditions (CuAAC) [51].

Resembling the CuAAC mechanism in solution, a successful cycloaddition of alkynes and azides was recently accomplished on a Cu(111) surface with complete regioselectivity toward the formation of the corresponding 1,4-triazoles [53]. As for the solution-phase process, the high regioselectivity was discussed considering the involvement of a copper acetylide (C–H activation) and bonding of the alkyne group to the Cu(111) surface. However, a strong interaction between the Cu atoms of the surface and the azide moiety causes the degradation of the latter, limiting its reactivity considerably.

In an own recent study, STM experiments and DFT simulations were combined to investigate on-surface cycloaddition reactions of N-(4-azidophenyl)-4-ethynylbenzamide (AEB) monomers on a Au(111) surface under UHV conditions [54]. The design of the AEB monomers (Figure 1.4a) is a combination of two phenyl rings connected through an amide linker as backbone, allowing the monomers to be thermally deposited on the surface, where they can lay flat and diffuse, thus increasing the probability of the alkyne and azide groups to meet with each other and react. The selection of the Au(111) surface as a template is based on an attempt to diminish/prevent the degradation of the azides on the surface, thus enhancing the reactivity.

Figure 1.4 (a) Scheme of the proposed on-surface azide–alkyne cycloaddition reaction. Green circles enclose the triazole moieties. (b) STM images of AEB monomer deposition on Au(111) at room temperature. (i) Large coverage deposition (50 nm × 50 nm). (ii) Self-assembly of monomers and dimers (13 nm × 17 nm). (iii) Dimers and trimers formed as a result of the successful azide–alkyne “click” reaction (3.5 nm × 7 nm). Inset: Molecular structure of the 1,4-triazole dimer. (Adapted with permission from Ref. [54]. Copyright 2013, American Chemical Society.)

The deposition of the AEB molecules on the Au(111) surface was carried out by sublimation at a crucible temperature of 85 °C in UHV, while the surface was kept at room temperature. The covered surface usually showed a first layer mixture of reactants mixed with already reacted dimers as well as trimers, surrounded by a disordered molecular phase (Figure 1.4b(i)). Such phase could be ascribed to the azide end group degradation on Au(111) [53]. The quantitative assessment of the yield of the observed cycloaddition revealed that from the total of intact molecules found for four different deposition runs, about 29% reacted to dimers and about 8% to trimers. Around 63% were found as nonreacted monomers. It is interesting to note that the dimerization yield for the cycloaddition on Au(111) is considerably larger than the one observed on Cu(111) [53], which one would expect larger due to the potential catalytic activity of copper. To confirm the covalent linkage of the reactants, controlled manipulations with the STM tip were carried out on the dimers and the trimers. Triazoles are known for their robustness and their high torsional degree of freedom and such features are also displayed by the on-surface coupling products on Au(111).

For a successful on-surface reaction to proceed, two monomers have to diffuse toward each other so that the reactive alkyne and azide functionalities are close enough for successful cycloaddition. Therefore, the self-assembled structure obtained after initial deposition must somehow relate to the reactivity. In our experimental studies, we found the unreacted AEB monomers to be present in three different configurations: (i) nucleation along the step edges, which is in agreement with the alkyne group high affinity for highly reactive sites, such as step edges; (ii) agglomeration adjacent to molecular islands as a result of diffusion of the molecules on the Au(111) surface and self-assembly driven by van der Waals forces; most likely those observed monomers are unreacted remnants, which did not find a counterpart to react with (lower section of Figure 1.4b(ii)); and (iii) self-assembled arrangement of three monomers in a stable triangular shaped island (upper section of Figure 1.4b(ii)). We found this configuration quite frequently (∼27% of the monomers that remain intact after the deposition), being the only configuration where exclusively monomers self-assembled independently.

The inset in Figure 1.4b(iii) represents the molecular structure of a 1,4-triazole dimer. This structure matches very well the STM observations, since the alternative 1,5-regioisomer would present a significant tilt (L-shaped structure, see scheme in Figure 1.4a) instead of the observed linear structure. In these experiments, only the formation of the 1,4-regioisomer was observed. DFT calculations showed that the Au substrate (i) lowers the activation energy, which causes the reaction to proceed at room temperature, (ii) does not act as a catalyst but merely as a two-dimensional constraint, and (iii) steers the complete 1,4-regioselectivity of the [3 + 2] on-surface cycloaddition reaction by means of surface-induced steric effects preventing the formation of the 1,5-regioisomer. The fact that the regioselectivity of the azide–alkyne cycloaddition can be controlled by the surface constraint and the reactants' design provides an efficient method to develop well-defined nanostructures at surfaces, without the requirement of a catalyst or additional thermal activation.

1.4 Glaser Coupling

Another interesting coupling mechanism involves the halogen-free reaction of aryl alkynes (Glaser coupling), which was studied at gold and silver surfaces [55]. For initial investigations, the diethynyl-substituted π-system C38, as shown in Figure 1.5a, was studied. Figure 1.5b(i) shows an STM image of the C38 molecule, which forms a self-assembled structure dominated by Van der Waals interactions between the C6-alkane chains. After thermal annealing, the C38 molecules form a covalent bound nanostructure via reactions of the alkynes (Figure 1.5b(ii)). However, due to various possible reaction pathways, several side products were identified: Glaser coupling (Figure 1.5c(i)), formal hydroalkynylation of the terminal alkyne functionality either at the α-position or at the β-position (Figure 1.5c(ii) and (iii)), dieyne products (Figure 1.5c(iv)), enediyne moiety (Figure 1.5c(v)), and diyne polycyclotrimerization (Figure 1.5c(vi)). To further study the catalytic role of the metal surface, the relative frequency of occurrence of the reaction pathways (i)–(vi) was determined on the Au(111) and Ag(111) surfaces. As in some cases the reactions (i) and (ii), as well as (iv) and (v), were undistinguishable, they were combined in this analysis. However, it is important to note that the Glaser reaction (i) is the main reaction type within the (i)/(ii) pair of the statistical analysis. It was found that the reaction pathways (i)/(ii) and (iv)/(v) occurred with frequencies of (26 ± 3) and (50 ± 3.6)% on the Au(111) surface, while frequencies of (64.3 ± 8.1) and (19.7 ± 5.7)% were found on the Ag(111) surface, respectively. Thus, Glaser coupling seems to be more efficient on Ag(111) than on Au(111) surfaces, while the side reactions (iv)/(v) reverse.

Figure 1.5 Glaser coupling of the C38 molecule. (a) Molecular chemical structure. (b) High-resolution STM image of C38 precursors at a Au(111) surface (i: 6 nm × 6 nm), as well as high-resolution image of C38 oligomers after thermal annealing at 123–140 °C (ii: 25 nm × 25 nm). (c) Rationalization of the different observed covalent bonding types in on-surface oligomerization by STM images (all with 6 nm × 6 nm) and the corresponding chemical structures. (d) Corresponding statistical analysis for the distribution of the observed products on Au(111) and Ag(111) surfaces, which clearly prove that the Ag(111) surface works more efficiently toward the Glaser coupling. (Redrawn with permission from Wiley-VCH Verlag GmbH [55].)

To further improve the selectivity toward Glaser coupling, dimers of 1,4-diethynylbenzene having no alkane chains on its side (C20, Figure 1.6a(i)) were tested as alternative reactants. Deposited on the Au(111) surface, they formed ordered (Figure 1.6a(ii)) as well as disordered self-assembled structures [55]. After annealing, branched covalent bound nanostructures can be observed (Figure 1.6a(iii)). In a similar statistical analysis as for the C38 case, Glaser coupling events occurred at a frequency of (43.3 ± 4.3)% on the Au(111) surface. The selectivity toward this reaction type could be further improved by performing the experiment on the Ag(111) surface, where a frequency of (67 ± 3.8)% could be achieved. At the same time, the side reactions (iv)/(v) were slightly reduced to (40.8 ± 4.1)% on the Au(111) surface and (21.6 ± 3.6)% on the Ag(111) surface.

Figure 1.6 Glaser coupling of C20 and C22 molecules. (a) Chemical structure of the C20 molecule (i) and its high-resolution STM images before (ii: 6 nm × 6 nm) and after (iii: 25 nm × 25 nm) the Glaser coupling reaction on the Au(111) substrate. (b) Chemical structure of the C22 molecule (i) and its high-resolution STM images before (ii: 6 nm × 6 nm) and after (ii: 25 nm × 25 nm) reaction on the Au(111) substrate. (c) Summary of the statistical analysis for the distribution of the observed products on the different substrates allowing the comparison of the catalytic abilities of Au(111) and Ag(111) surfaces toward Glaser coupling, side cross-coupling, and the influences of chemical structures from C38 to C20 and C22. ((a) Adapted with permission from Ref. [56]. Copyright 2013, American Chemical Society. (b) Redrawn with permission from Wiley-VCH Verlag GmbH [55].)

By further optimization of the precursor molecules, the side reactions of alkynes could be completely suppressed. Figure 1.6b(i) shows the structure of the resulting C22 molecule with an ortho-substituent next to the alkyne functionality, which suppresses side reactions due to steric reasons. As shown in Figure 1.6b(ii), STM images of the self-assembled C22 molecules on Au(111) match its chemical structure very well. After reaction, C22 molecules were mainly connected one by one through Glaser coupling (Figure 1.6b(iii)). The length of the polymers could be increased up to 59 monomers when Ag(111) was used as a substrate. The statistical analysis revealed that the Glaser coupling was optimized up to (92.1 ± 5.1)% on Au(111) and (96.4 ± 1.7)% on Ag(111). At the same time, the side reactions (iv)/(v) were completely suppressed.

Figure 1.6c shows a summary of the statistical frequencies for the different tested reactants C38, C20, and C22. As indicated by the yellow arrow, the ortho-substituent next to the alkyne works very efficiently toward chemical selectivity of Glaser coupling in alkyne reactions. Another important point is that the Ag(111) surface has a stronger catalytic effect toward Glaser coupling than the Au(111) surface, where the latter leads to more side reactions. To understand the different catalytic effects of on-surface Glaser coupling on metal surfaces, DFT simulations were conducted [56]. The results suggest a model with two possible reaction pathways: C–C coupling via direct C–H activation and C–C coupling via alkynyl activation by π-complex formation. On both Au and Ag surfaces, the alkyne direct C–H activation was found to be a high-energy pathway (1.64 and 1.85 eV on Au and Ag, respectively), which is very unlikely to occur in experiments. The alternative low-energy route (0.79 and 0.9 eV on Au and Ag, respectively) comprises interaction of the alkyne functionality with the surface and direct C–C bond formation as the rate-determining step.

In a subsequent study on the (111) surfaces of Ag, Au, and Cu, photoinduced effects on the Glaser coupling were investigated by exposing self-assembled monolayers of the C22 molecule to UV irradiation [57]. As shown in Figure 1.7a, the formation of aryl alkyne dimers at the Ag(111) surface could be induced after UV irradiation for 67 h at a wavelength of 375 nm. This photochemical aryl alkyne dimerization occurred also on Cu(111), but with a significantly reduced efficiency compared with Ag(111). On the gold surface, no photoinduced effects could be observed at all, emphasizing the catalytic role of the substrate also for the photoinduced approach of the Glaser coupling. Short aryl alkyne oligomers such as trimers and tetramers could be observed on the Ag(111) surface as shown in Figure 1.7b. The statistical analysis of the oligomer length distribution shows that the main products of photochemical reaction of the C22 molecules are the dimers with an abundance of (87.4 ± 2.1)%, which significantly differs from the products of the thermally induced C22 reactions on Ag(111). In comparison with the thermal process, the major advantage of the photochemical on-surface reaction at room temperature is that the self-assembly of the reactants after initial deposition may be restored, allowing spatiotemporal control of the C–C bond formation. The DFT simulations indicate why the Ag surface is more efficient than Au. This is because the gold surface interacts stronger with the alkynyl groups resulting in a reduced mobility of the molecules on the substrate and a longer trapping of C–C coupled intermediates to branching reactions.

Figure 1.7 Photochemical Glaser coupling of the C22 molecule after UV irradiation (67 h) on Ag(111). (a) STM image of C22 dimers (17 nm × 17 nm), with a high-resolution image shown in the inset (5 nm × 5 nm). (b) STM image of short C22 oligomers (20 nm × 20 nm), with an inset image (8.4 nm × 8.4 nm). (c) The corresponding statistical analysis for the oligomer distribution. (Adapted with permission from Ref. [57]. Copyright 2014, American Chemical Society.)

1.5 Decarboxylative Polymerization of Acids

The metal-catalyzed polymerization of 2,6-naphthalenedicarboxylic acid (NDCA) is another important recent achievement in the research filed [58]. The C–C coupling of NDCAs was found to occur via a three-step process, which is visualized in the scheme in Figure 1.8a: step 1 is a dehydrogenation process to provide the corresponding metal carboxylate; in step 2, a decarboxylative process leads to the formation of polymeric bisnaphthyl-Cu as an intermediate species; and step 3 finally leads to C–C coupling forming poly-2,6-naphthalnene chains via decarboxylation. First of all, it was found that this reaction seems generally not to proceed on the Au(111) surface. Switching to the Ag(111) surface, the interaction of the carboxyl group with the substrate could be increased and covalent coupling of NDCAs occurred after thermal annealing to 156 °C. However, due to the desorption of NDCA during annealing, the efficiency toward poly-bisnaphthyl-Ag intermediates as well as the oligo-2,6-naphthalene was rather low. Therefore, Cu(111) was tested as substrate for the reaction, as it shows the strongest interaction with carboxyl groups compared with the Au(111) or Ag(111) surfaces. Expectedly, a hydrogen-bonded self-assembly structure (one of the different phases is shown in Figure 1.8b(i)), dehydrogenated product (Figure 1.8b(ii)), bisnaphthyl-Cu species (Figure 1.8b(iii)), and final polynaphthalenes (Figure 1.8b(iv)) were obtained with high efficiency. Moreover, the differences of Cu(111), Cu(100), and Cu(110) surfaces were compared toward such decarboxylative polymerization. Surprisingly, the following reactivity order was observed: Cu(111) > Cu(100) > Cu(110), which likely relates to the ability of metal atoms being pulled out of the surface. The experiments also revealed that the organometallic polymers (bisnaphthalene-Cu species) on the Cu(100) surface were grown only in [001] and [010] directions, while organometallic polymers on the Cu(111) surface grow in [110], [011], and [101] directions. This substrate dependence indicates that the metal atoms part of the organometallic polymers should have a specific interaction with the metal surfaces.

Figure 1.8 Metal-catalyzed on-surface polymerization of NDCA. (a) Schematic illustration of the reaction pathway. (b) STM images for the polymerization of NDCA on Cu(111) from unreacted precursor molecules (i: 6 nm × 6 nm), to their dehydrogenated state (ii: 10 nm × 10 nm), via the bisnaphthyl-Cu intermediates (iii: 4.24 nm × 4.24 nm), and finally to poly-2,6-naphthalene (iv: 4.2 nm × 4.2 nm). (Adapted with permission from Ref. [58]. Copyright 2014, American Chemical Society.)

1.6 Conclusions

An interesting feature of on-surface chemical reactions is the possibility of targeted conjugation in more than one direction on a substrate, extending 1D polymerization to the growth of 2D organic networks. Thereby, on-surface reactions performed under UHV conditions benefit from the lack of solvents required, which allows us to gain insight into chemical processes under well-defined conditions. The recent achievements in this research field allow to identify some important prerequisites for a successful covalent coupling, which are summarized in the following.

An important aspect is the flat adsorption geometry of the reactants, where the adsorption strength is crucial to avoid desorption when the system is supplied with additional energy by thermal annealing and/or light illumination. In particular, it has been observed to impact directly the ordering of the products after the on-surface coupling [55,59,60]. Too strongly adsorbed molecules might inhibit a reaction by reduced diffusion, which is vital for the reactive moieties to meet in the desired geometry on a surface. Therefore, reduced diffusion can lead to poor reactivity and stereocontrol [55,59,60]. For experiments under UHV conditions, the design of the reactants should enable a straightforward deposition, which in most cases is done by thermal sublimation. Potential issues could be the decomposition of the molecules or reactive end groups due to the elevated sublimation temperatures in the crucible. For the proper design of the reactants, steric effects on the surface should be considered, providing further control on the reaction mechanism, for example, in view of regioselectivity of the anticipated bond formation. Another important aspect is the choice of the substrate. As shown earlier, a surface can act as both a two-dimensional spatial constraint [54] and an active catalytic agent in a specific reaction mechanism [28,45]. Therefore, a simple transfer of reaction mechanisms from solution-phase chemistry to on-surface reactions is not straightforward. Furthermore, surface orientation, reconstructions, or well-oriented stepped surfaces can help to provide directional alignment of the products [23,28,29,45]. Finally, reaction by-products are usually detrimental for the properties of a highly defined nanomaterial and can hinder the studied process or even suppress it [26].

Given the remarkable achievements of the last few years in the field of on-surface reactions with organic molecules, a promising approach for the development of novel nanomaterials with tailored optoelectronic properties and potential applications in devices emerged. The current major challenges are to determine the critical parameters to control reaction pathways and material properties in the two-dimensional environment of a surface. However, future work should also focus on extending the existing tool kit of on-surface chemistry by finding additional reaction pathways.

Acknowledgments

The authors acknowledge support by the Deutsche Forschungsgemeinschaft (DFG) through the Collaborative Research Center SFB 858, project B2, and TRR 61, projects B3 and B7.

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