Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology PDF

Chem. Rev. 2005, 105, 1103−1169 1103 Self-Assembled Monolayers of Thiolates on Metals as a Form of...

Chem. Rev. 2005, 105, 1103−1169 1103 Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology J. Christopher Love,† Lara A. Estroff,† Jennah K. Kriebel,† Ralph G. Nuzzo,*,‡ and George M. Whitesides*,† Department of Chemistry and the Fredrick Seitz Materials Research Laboratory, University of Illinois−Urbana−Champaign, Urbana, Illinois 61801 and Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138 Received July 19, 2004 Contents 3.4.1. Defects Caused by Variations in the 1121 Surface of the Substrate 1. Introduction 1104 3.4.2. Reconstruction of the Surface during 1121 1.1. What Is Nanoscience? 1104 Assembly 1.2. Surfaces and Interfaces in Nanoscience 1106 3.4.3. Composition of SAMs 1121 1.3. SAMs and Organic Surfaces 1106 3.4.4. Structural Dynamics of SAMs Induce 1121 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 1.4. SAMs as Components of Nanoscience and 1106 Defects Nanotechnology 4. Removing SAMs from Surfaces 1122 1.5. Scope and Organization of the Review 1106 4.1. Electrochemical Desorption of SAMs 1122 2. Preparation of SAMs 1108 4.2. Displacement of SAMs by Exchange 1122 Downloaded via BROCK UNIV on October 23, 2024 at 16:42:33 (UTC). 2.1. Types of Substrates 1108 4.3. Photooxidation of SAMs. 1123 2.1.1. Preparation of Thin Metal Films as 1108 Substrates for SAMs 5. Tailoring the Composition and Structure of SAMs 1123 2.1.2. Other Substrates for SAMs 1110 5.1. Why Modify SAMs after Formation? 1123 2.1.3. Why Is Gold the Standard? 1111 5.2. Strategies for Covalent Coupling on SAMs 1124 2.2. Protocols for Preparing SAMs from 1111 5.2.1. Direct Reactions with Exposed Functional 1124 Organosulfur Precursors Groups 2.2.1. Adsorption of Alkanethiols from Solution 1111 5.2.2. Activation of Surfaces for Reactions 1125 2.2.2. Adsorption of Disulfides and Sulfides from 1113 5.2.3. Reactions that Break Covalent Bonds 1126 Solution 5.2.4. Surface-Initiated Polymerizations 1126 2.2.3. “Mixed” SAMs 1113 5.2.5. How Does the Structure of the SAM 1126 2.2.4. Adsorption from Gas Phase 1114 Influence Reactivity on Surfaces? 3. Characterization of SAMs: Structure, Assembly, 1114 5.3. Noncovalent Modifications 1127 and Defects 5.3.1. Nonspecific Adsorption of Molecules from 1127 3.1. Nature of the Metal−SAM Interface 1114 Solution onto SAMs 3.1.1. Thermodynamic Analysis of 1115 5.3.2. Fusion of Vesicles on SAMs 1127 Gold−Thiolate Bonds 5.3.3. Selective Deposition onto SAMs 1128 3.1.2. Surface Structure of Thiolates on Gold 1115 5.3.4. Modifications via Molecular Recognition 1128 3.1.3. Surface Structure of Thiolates on 1116 6. SAMs as Surface Layers on Nanoparticles 1128 Palladium 6.1. Formation of Monolayer-Protected Clusters 1128 3.1.4. Surface Structure of Thiolates on Silver 1116 (MPCs) 3.1.5. Surface Structure of Thiolates on Copper 1117 6.1.1. Thiols Are a Special Subclass of 1129 3.2. Organization of the Organic Layer 1117 Surfactants 3.2.1. Single-Chain Model for Describing the 1117 6.1.2. Thiols Can Influence the Size and Shape 1129 Average Organization of the Organic of Nanoparticles Layer in SAMs 6.2. Strategies for Functionalizing Nanoparticles 1130 3.2.2. “Odd−Even” Effect for SAMs on Gold 1118 with Ligands 3.2.3. Multichain Unit Cells 1119 6.2.1. Formation of Nanoparticles in the 1130 3.2.4. Effect of the Organic Component on the 1119 Presence of Thiols Stability of the SAM 6.2.2. Ligand-Exchange Methods 1130 3.3. Mechanisms of Assembly 1119 6.2.3. Covalent Modification 1131 3.3.1. Assembly of SAMs from the Gas Phase 1119 6.3. Structure of SAMs on Highly Curved 1131 3.3.2. Assembly of SAMs from Solution 1121 Surfaces 3.4. Defects in SAMs 1121 6.3.1. Spectroscopic Evidence for SAM 1132 Structure on Nanoparticles * To whom correspondence should be addressed. R.G.N.: phone, 6.3.2. Evidence for the Structure of SAMs on 1132 217-244-0809; fax, 217-244-2278; e-mail: [email protected]. Nanoparticles based on Chemical G.M.W.: phone, (617) 495-9430; fax, (617) 495-9857; e-mail: Reactivity [email protected]. † Harvard University. 6.4. SAMs and the Packing of Nanocrystals into 1132 ‡ University of IllinoissUrbana-Champaign. Superlattices 10.1021/cr0300789 CCC: $53.50 © 2005 American Chemical Society Published on Web 03/25/2005 1104 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al. 7. Patterning SAMs In Plane 1133 7.1. Microcontact Printing 1134 7.1.1. Composition of Topographically Patterned 1134 Stamps 7.1.2. Methods for Wetting Stamps with Thiols 1135 7.1.3. Mechanism for Forming SAMs by Printing 1135 7.1.4. Structure of SAMs Formed by µCP 1136 7.1.5. Transfer of PDMS to the Surface during 1136 Printing 7.1.6. Fabrication of Nanostructures by µCP 1136 7.2. Photolithography or Particle Beam 1137 Lithography 7.2.1. Photolithography 1137 7.2.2. E-Beam and X-ray Lithography 1137 7.2.3. Atomic Beam Lithography 1138 J. Christopher Love received his B.S. degree in Chemistry from the 7.3. Other Methods for Patterning SAMs 1138 University of Virginia in 1999 and Ph.D. degree from Harvard University in 2004. Under the direction of Professor George M. Whitesides, his 7.3.1. Formation of Gradients 1138 doctoral thesis included studies on the surface chemistry of thiols on 7.3.2. Ink-Jet Printing 1138 palladium and fabrication of magnetic micro- and nanostructures. He 7.3.3. Topographically Directed Assembly 1138 currently is a postdoctoral research fellow in Hidde L. Ploegh’s laboratory 7.3.4. Orthogonal Self-Assembly 1139 at Harvard Medical School. His present research interests include nanotechnology, surface chemistry, self-assembly, microfabrication, im- 8. Applications of SAMs on Thin Metal Films 1139 munology, and cell biology. 8.1. SAMs as Etch Resists 1139 8.2. SAMs as Barriers to Electron Transport 1139 8.2.1. SAMs for Electrochemistry 1140 8.2.2. SAMs in Organic/Molecular Electronics 1141 8.3. SAMs as Substrates for Crystallization 1143 8.3.1. Oriented Nucleation of Crystals 1143 8.3.2. Alignment of Liquid Crystals 1145 8.4. SAMs for Biochemistry and Biology 1145 8.4.1. Designing SAMs To Be Model Biological 1146 Surfaces 8.4.2. SAMs for Cell Biology 1147 8.4.3. Structure−Property Considerations for 1148 SAMs Used in Biology 9. Applications of SAMs on Nanostructures 1150 9.1. Electrodeposited Metal Rods 1150 Lara A. Estroff is currently an NIH postdoctoral fellow in Professor George 9.2. Gold Nanopores as Selective Channels 1151 M. Whitesides’ laboratory at Harvard University working on understanding 9.3. Arrays of Metallic Nanostructures 1151 multivalency in the immune system. In 2003 she received her Ph.D. degree 9.3.1. Arrays of Gold Dots 1151 from Yale University for work done in Professor Andrew D. Hamilton’s laboratory on the design and synthesis of organic superstructures to control 9.3.2. Silver Tetrahedrons for Localized Surface 1152 the growth of inorganic crystals. As part of her graduate work, Lara spent Plasmon Resonance (LSPR) time at the Weizmann Institute for Science (Rehovot, Israel) working in 9.4. Metallic Shells 1152 the labs of Professors Lia Addadi and Steve Weiner. Before that she 9.4.1. Metallic Half-Shells 1152 received her B.A. degree in Chemistry from Swarthmore College, where 9.4.2. Gold−Silica Core−Shell Particles 1153 she worked in Professor Robert S. Paley’s laboratory. 9.5. Metal Nanoparticles and Quantized 1153 1. Introduction Double-Layer Charging 9.6. Functional Surfaces on Nanoparticles 1154 1.1. What Is Nanoscience? 9.6.1. Biocompatible Surfaces on Quantum Dots 1154 Nanoscience includes the study of objects and 9.6.2. Functionalized Magnetic Nanoparticles 1154 systems in which at least one dimension is 9.6.3. Nanoparticles for the Polyvalent Display 1154 1-100 nm. The objects studied in this range of sizes of Ligands are larger than atoms and small molecules but 10. Challenges and Opportunities for SAMs 1155 smaller than the structures typically produced for use 10.1. Rules for “Designing” Surfaces 1156 in microtechnologies (e.g., microelectronics, photon- 10.2. New Methods for Characterizing SAMs 1156 ics, MEMS, and microfluidics) by fabrication methods 10.3. New Systems of SAMs 1156 such as photolithography. The dimensions of these 10.4. SAMs with Different Physical Properties 1156 systems are often equal to, or smaller than, the 10.5. In-Plane Patterning 1156 characteristic length scales that define the physical 11. Outlook and Conclusions 1157 properties of materials. At these sizes, nanosystems can exhibit interesting and useful physical behaviors 12. Acknowledgments 1157 based on quantum phenomena (electron confine- 13. References 1157 ment,1 near-field optical effects,2 quantum entangle- Self-Assembled Monolayers of Thiolates Chemical Reviews, 2005, Vol. 105, No. 4 1105 Jennah Kriebel was born in Hawaii in 1976. She attended the University George M. Whitesides received his A.B. degree from Harvard University of Washington as an undergraduate and completed a thesis on microfluidic in 1960 and his Ph.D. degree from the California Institute of Technology systems with Professor P. Yager. She spent one year with Professor G. in 1964. A Mallinckrodt Professor of Chemistry from 1982 to 2004, he is Ertl at the Fritz Haber Institute in Berlin, Germany, where she studied the now a Woodford L. and Ann A. Flowers University Professor. Prior to adsorption of gases onto carbon nanotubes. She is currently in her fifth joining the Harvard faculty in 1992, he was a member of the chemistry year as a graduate student in Chemical Physics with Professor G. faculty of the Massachusetts Institute of Technology. His research interests Whitesides at Harvard University. Her thesis work explores molecular include physical and organic chemistry, materials science, biophysics, electronics by studying electron transport through self-assembled mono- complexity, surface science, microfluidics, self-assembly, micro- and layers. She is especially interested in correlating the metal−molecule nanotechnology, and cell−surface biochemistry. interfaces with the current response through a two-terminal junction. in materials science as catalysts, in medicine as components of systems for drug delivery, in magnetic storage media, and in electronic and optical devices. Biology is a source of inspiration for nanoscience. The cell (the fundamental unit of life) is, in one view, essentially a collection of sophisticated nano- machines. Some of the components of the cell with nanometer-scale dimensions include catalysts and other functional systems (enzymes, ribozymes, pro- teins, and protein-RNA aggregates), lipid bilayers, ion channels, cytoskeletal elements (actin filaments and microtubules), DNA and RNA, motor proteins, vacuoles, and mitochondria.14 These biological sys- tems interact with one another through complex chemical pathways that regulate their activities; they Ralph Nuzzo received his B.S. degree in Chemistry from Rutgers University self-assemble in a hierarchical manner to generate in 1976 and his Ph.D. degree in Organic Chemistry from the Massachusetts complicated, “soft” structures; they act cooperatively Institute of Technology in 1980. After completing his graduate studies, to sense their local environment and modify it; they he accepted a position at Bell Laboratories, then a part of AT&T, where enable collective functions such as motility, replica- he held the title of Distinguished Member of the Technical Staff in Materials Research. He joined the faculty of the University of Illinois at Urbana− tion, metabolism, and apoptosis. Biological systems Champaign in 1991. He is the Senior Editor of Langmuir and, among offer many examples of nanostructures interacting various honors, was awarded the ACS Arthur Adamson Award for in complex networks and suggest new strategies with Distinguished Contributions in the Advancement of Surface Chemistry in which to build artificial nanosystems, from the “bot- 2003. tom up”. New tools for observing and manipulating atomic-, ment,3 electron tunneling,4-6 and ballistic transport7) molecular-, and colloidal-scale objects, such as scan- or subdomain phenomena (superparamagnetism,8,9 ning probe and electron microscopies, have also been overlapping double layers in fluids10). a significant factor in the emergence of nanoscience Chemistry has played a key role in the develop- and nanotechnology. The remarkable ability to visu- ment of nanoscience. Making and breaking bonds alize, manipulate, and shape nanometer-scale struc- between atoms or groups of atoms is a fundamental tures with atomic resolution has, in turn, led to some component of chemistry; the products of those reac- fantastic ideas for new technologies, such as “as- tions are structures-molecules-that range in size semblers”, nanorobots, and “grey goo”, that have from 0.1 to 10 nm. The development of new synthetic attracted popular and regulatory attention.15 Al- methods has made it possible to produce uniform though these ideas are more science fiction than nanostructures with sizes ranging from 1 to 100 nm science/technology, they have contributed (for better and with new shapes (spheres, rods, wires, half- and for worse) to a public interest in research in shells, cubes) and compositions (organics, metals, nanoscience that is now producing the beginnings of oxides, and semiconductors); examples include nano- potentially important technologies; examples include crystals,9 nanowires,11 block copolymers,12 and nano- composite materials with tailored toughness, electri- tubes.13 Some of these new structures will be applied cal conductivity, or other physical properties, ul- 1106 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al. tradense memories, organic electronics, new classes high affinity for the surface and displaces adsorbed of biosensors, and electronic devices based on quan- adventitious organic materials from the surface. tum effects. There are a number of headgroups that bind to specific metals, metal oxides, and semiconductors 1.2. Surfaces and Interfaces in Nanoscience (Table 1). The most extensively studied class of SAMs is derived from the adsorption of alkanethiols on One distinguishing characteristic of nanometer- gold,19-27 silver,26,28,29 copper,26 palladium,30,31 plati- scale structures is that, unlike macroscopic materials, num,32 and mercury.33 The high affinity of thiols for they typically have a high percentage of their con- the surfaces of noble and coinage metals makes it stituent atoms at a surface. The volume of an object possible to generate well-defined organic surfaces (V ∝ l3, where l is the characteristic length) decreases with useful and highly alterable chemical function- more quickly than its surface area (S ∝ l2) as the size alities displayed at the exposed interface.23,34 diminishes: S/V ∝ l-1, where l has atomic or molec- ular dimensions. This scaling behavior leads, in the 1.4. SAMs as Components of Nanoscience and most extreme case, to structures where nearly every Nanotechnology atom in the structure is interfacial. In some sense, nanostructures are “all surface”.16 SAMs are themselves nanostructures with a num- We believe that surfaces represent a fourth state ber of useful properties (Figure 1). For example, the of matter-they are where the gradients in properties thickness of a SAM is typically 1-3 nm; they are the are greatest. (In bulk phases of matter-gas, liquid, most elementary form of a nanometer-scale organic solid-the gradients are usually zero.) Atoms or thin-film material. The composition of the molecular molecules at the surface of a material experience a components of the SAM determines the atomic com- different environment from those in the bulk and position of the SAM perpendicular to the surface; this thus have different free energies, electronic states, characteristic makes it possible to use organic syn- reactivities, mobilities, and structures.17,18 The struc- thesis to tailor organic and organometallic structures ture and chemical composition within macroscopic at the surface with positional control approaching objects determines many physical properties, e.g., ∼0.1 nm. SAMs can be fabricated into patterns thermal and electrical conductivity, hardness, and having 10-100-nm-scale dimensions in the plane of plasticity. In contrast, the physical properties of a surface by patterning using microcontact printing nanostructures depend to a much greater extent on (µCP),130,131 scanning probes,132-134 and beams of their surface and interfacial environment than do photons,135-138 electrons,139 or atoms.140,141 Phase- bulk materials. separated regions in SAMs comprising two or more constituent molecules can have ∼100-nm2-scale di- 1.3. SAMs and Organic Surfaces mensions.142 SAMs are well-suited for studies in nanoscience Bare surfaces of metals and metal oxides tend to and technology because (1) they are easy to prepare, adsorb adventitious organic materials readily be- that is, they do not require ultrahigh vacuum (UHV) cause these adsorbates lower the free energy of the or other specialized equipment (e.g., Langmuir- interface between the metal or metal oxide and the Blodgett (LB) troughs) in their preparation, (2) they ambient environment.18 These adsorbates also alter form on objects of all sizes and are critical compo- interfacial properties and can have a significant nents for stabilizing and adding function to pre- influence on the stability of nanostructures of metals formed, nanometer-scale objectssfor example, thin and metal oxides; the organic material can act as a films, nanowires, colloids, and other nanostructures, physical or electrostatic barrier against aggregation, (3) they can couple the external environment to the decrease the reactivity of the surface atoms, or act electronic (current-voltage responses, electrochem- as an electrically insulating film. Surfaces coated istry) and optical (local refractive index, surface with adventitious materials are, however, not well- plasmon frequency) properties of metallic structures, defined: they do not present specific chemical func- and (4) they link molecular-level structures to mac- tionalities and do not have reproducible physical roscopic interfacial phenomena, such as wetting, properties.(e.g., conductivity, wettability, or corrosion adhesion, and friction. resistance). Self-assembled monolayers (SAMs) provide a con- 1.5. Scope and Organization of the Review venient, flexible, and simple system with which to tailor the interfacial properties of metals, metal This review focuses on the preparation, formation, oxides, and semiconductors. SAMs are organic as- structure, and applications of SAMs formed from semblies formed by the adsorption of molecular alkanethiols (and derivatives of alkanethiols) on gold, constituents from solution or the gas phase onto the silver, copper, palladium, platinum, mercury, and surface of solids or in regular arrays on the surface alloys of these metals. It emphasizes advances made of liquids (in the case of mercury and probably other in this area over the past 5 years (1999-2004). It liquid metals and alloys); the adsorbates organize does not cover organic assemblies formed by Lang- spontaneously (and sometimes epitaxially) into crys- muir-Blodgett techniques,143 from alkylsiloxanes talline (or semicrystalline) structures. The molecules and alkylsilanes,144 or from surfactants adsorbed on or ligands that form SAMs have a chemical function- polar surfaces.145 The objectives of this review are as ality, or “headgroup”, with a specific affinity for a follows: (1) to review the structure and mechanism substrate; in many cases, the headgroup also has a of formation of SAMs formed by adsorption of n- Self-Assembled Monolayers of Thiolates Chemical Reviews, 2005, Vol. 105, No. 4 1107 Table 1. Combinations of Headgroups and Substrates Used in Forming SAMs on Metals, Oxides, and Semiconductors Figure 1. Schematic diagram of an ideal, single-crystalline SAM of alkanethiolates supported on a gold surface with a (111) texture. The anatomy and characteristics of the SAM are highlighted. alkanethiols on metals, including an analysis of the to illustrate applications of SAMs where (i) they act thermodynamics and kinetics of these systems; (2) as nanostructures themselves, e.g., ultrathin films, 1108 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al. (ii) they enable other nanosystems, e.g., nanopar- terizing the structure-property relationships of SAMs ticles, (iii) they interact with biological nano- because they are convenient (easy to prepare) and structuressproteins, etc., (iv) and they form patterns compatible with a number of techniques for surface on surfaces with critical dimensions below 100 nm; analysis and spectroscopic/physical characterization (3) to outline what is not understood about these such as reflectance absorption infrared spectroscopy SAMs and which of their properties are not yet (RAIRS),155,156 Raman spectroscopy,151 X-ray photo- controlled; and (4) to sketch some of the important electron spectroscopy (XPS),157,158 high-resolution opportunities that still remain for future progress in electron energy loss spectroscopy (HREELS),158 near- research involving SAMs. edge X-ray absorption fine structure spectroscopy (NEXAFS),159 helium atom scattering,160,161 X-ray 2. Preparation of SAMs diffraction,161,162 contact angle goniometry,154 optical ellipsometry,21,156 surface plasmon resonance (SPR) The early literature on SAMs (1983-1993) focused spectroscopy,156 mass spectrometry,163 and scanning largely on the assemblies formed by the adsorption probe microscopy (SPM).5,153,164,165 Other metallic of organosulfur compounds from solution or the vapor nanostructures, such as nanoparticles or those formed phase onto planar metal substrates of gold and by templating, also can support SAMs, and these silver.20,21,29,88,146-153 These studies used three types systems have been characterized by many techniques of organosulfur compounds: alkanethiols (HS(CH2)nX), including electron microscopy,166 SPM,167,168 edge dialkyl disulfides (X(CH2)mS-S(CH2)nX), and dialkyl X-ray absorption fine structure spectroscopy (EXAFS) sulfides (X(CH2)mS(CH2)nX), where n and m are the and X-ray absorption near-edge spectroscopy number of methylene units and X represents the end (XANES),169 infrared spectroscopy,170,171 UV-vis spec- group of the alkyl chain (-CH3, -OH, -COOH). The troscopy,172 differential scanning calorimetry experiments established many of the basic structural (DSC),170,173 mass spectroscopy,174 high-pressure characteristics of these systems (surface structure, liquid chromatography,175 electrochemistry (see sec- chain organization, orientation), practical protocols tion 9.5),176 and NMR spectroscopy.170 for preparing SAMs (concentrations, length of time The criteria important for selecting the type of for immersion, solvents, temperature), and some substrate and method of preparation are dependent details of the thermodynamics and kinetics governing on the application for which the SAM is used. For the process of assembly. Comprehensive reviews of example, polycrystalline films are sufficient for many the early work are available.22,144,154 applications on planar substrates such as etch resists A major portion of the research on SAMs since the (section 8.1), templates for crystallization (section early 1990s has continued to expand the types of 8.3), and model surfaces for biological studies (section substrates used to support SAMs, and, to some 8.4) because a wide range of materials can be degree, the types of molecules used to form them. deposited easily and these substrates are inexpensive Table 1 indicates, however, that the variety of ligands relative to single crystals. Other applications, such studied is still limited to functionalities formed from as measurements of electron transport through or- a small set of elements in a narrow range of oxidation ganic molecules (section 8.2), benefit from substrates states and that much of the work has continued to that are single crystals or polycrystalline films with focus on SAMs formed from thiols. Nevertheless, the minimal grain boundaries. past decade has seen a significant expansion in studies that exploit the assembly of SAMs on nano- 2.1.1. Preparation of Thin Metal Films as Substrates for structures. The availability of new types of nano- SAMs structures with well-defined shapes and sizes on planar supports (metal structures on silicon wafers The most common planar substrates for SAMs or glass slides) and in solution (nanocrystals, tem- formed from alkanethiols are thin films of metal plated structures) has stimulated wide application supported on silicon wafers, glass, mica, or plastic of SAMs for stabilizing these new structures of substrates. These substrates are easy to prepare by metallic (and other) nanoscale materials and ma- physical vapor deposition (PVD) methods (thermal nipulating the interfacial/surface properties of these or electron beam (e-beam) evaporation),177 electrodepo- materials. This section of the review describes some sition,178 or electroless deposition.179-183 PVD and of the types of substrates most widely used for electrodeposition can generate thin films of a wide supporting SAMs and reviews what is known about range of metals (including gold, silver, copper, pal- the methods for preparing SAMs from different ladium, platinum, and nickel) and alloys. organosulfur compounds in solution and from the Thin Films on Glass or Silicon by PVD. A vapor phase. typical thin film deposited onto a silicon wafer or glass support consists of a thin primer or adhesion 2.1. Types of Substrates layer of titanium, chromium, or nickel (1-5 nm) and a layer of coinage or noble metal (10-200 nm). The The surface on which a SAM forms and the physi- primer improves the adhesion of metals that do not cal object supporting that surface often are referred form oxides readily (especially gold) to substrates to as the “substrate”. Types of substrates range from with an oxidized surface, e.g., silicon wafers with the planar surfaces (glass or silicon slabs supporting thin native oxide, and glass slides. Metal films on glass films of metal, metal foils, single crystals) to highly or silicon are polycrystalline and composed of a curved nanostructures (colloids, nanocrystals, nano- continuous layer of contiguous islands or grains of rods). Planar substrates are used widely for charac- metal that can range in size from ∼10 to 1000 nm Self-Assembled Monolayers of Thiolates Chemical Reviews, 2005, Vol. 105, No. 4 1109 on glass from ∼200 to 106 nm2.185 The size and shape of the grains change from small and round to large and terraced. Abbott et al. demonstrated that deposi- tion of metal films at oblique angles changes the grain size and roughness of the resulting metal films.186 For example, at a particular rate of deposi- tion the average grain size of gold can decrease from ∼36 to ∼14 nm as the angle of incidence onto glass substrates increases from 15° to 60°. The composition of thin films also influences their topography. Metals with high melting points such as palladium (1552 °C) and platinum (1772 °C) tend to produce films with smaller grains than metals with lower melting points such as gold (1064 °C) when deposited at comparable deposition rates. For ex- ample, the grains in a thin film of palladium pre- pared on a silicon wafer by e-beam deposition are ∼15-30 nm in diameter (Figure 2b); thin films of gold prepared in the same manner had grains of ∼45-60 nm.30 Differences in the sizes of grains can impact the utility of the materials in different ap- plications of SAMs. Polycrystalline films with the smallest possible grains are desirable as substrates for microcontact printing and etching (section 8.1) structures with dimensions less than 100 nm because the small grain sizes minimize the roughness of the edges of the etched structures. Large grains are important in applications where the SAM provides an insulating barrier against electrochemical pro- cesses or biased electron transport (section 8.2). Figure 2. Scanning probe micrographs of metal thin films Glassy metal substrates, that is, ones with no grains prepared by different techniques. AFM images of (a) a gold and no long-range ordering, likely would be useful film (200 nm thick) deposited by electron-beam evaporation for many applications of SAMs, but there is no (note that the range of the topographical heights in the z-direction is expressed by the grayscale shading of the significant data available for SAMs on these types image, where white denotes the highest feature and black of materials, which typically are complex alloys of denotes the lowest one; the full range of the z-scale between metals. these two extremes in (a) is 25 nm), (b) a palladium film The primary method used to change the grain sizes (200 nm thick) deposited by electron-beam evaporation of metal films after their preparation by PVD is (range of z-scale ) 10 nm), (c) a thermally evaporated gold film treated with dilute piranha and aqua regia solutions thermal annealing.180,187 Twardowski and Nuzzo (range of z-scale ) 15 nm), (d) a thermally annealed gold demonstrated a chemical method for recrystallizing film (15 nm) deposited on a glass microscope slide func- gold and gold/copper films.188 Treatment of thick tionalized with 3-aminopropyltrimethoxysilane (range of (180-200 nm) gold films with hot piranha solution z-scale ) 3.5 nm), and (e) a gold film prepared by the (3:1 concentrated H2SO4:30% H2O2) followed by im- template-stripping method while immersed in a solution mersion in a dilute aqua regia solution (3:1:16 of octadecanethiol (range of z-scale ) 3 nm). STM image of (f) a gold film prepared by electroless deposition on a HCl:HNO3:H2O) led to coalescence of the grains and glass microscope slide (range of z-scale ) 80 nm). (c, d, and recrystallization of the surface that enhanced the f) (Reprinted with permission from refs 188, 187, and 180. (111) texture of the surface (Figure 2c).189 Chemo- Copyright 2002, 2004, and 1998 American Chemical Soci- mechanical and electrochemical polishing can also ety.) (e) (Reprinted with permission from ref 202. Copyright generate flat surfaces on thick films of metal.190 2003 Wiley-VCH.) Metal films that are optically transparent are (Figure 2a). As typically deposited, these films tend important for applications of SAMs in biology because to have a dominant (111) texture-for fcc metals, a experiments in this field (and especially in cell hexagonal presentation of the atoms at the surface- biology) often require observation by transmission at the exposed interface.22,26,30 The use of single- optical microscopy. The opacity of a thin film of metal crystal substrates has allowed the study of SAMs depends on the electrical resistivity of the metal; the forming on other low-index planes, particularly the thickness of the film at the point where the trans- (100) surface of gold.27,184 mission of light is nearly zero is referred to as its ‘skin The morphology of the grains of thin films on glass depth’.191 Partial transparency tends to be seen in or silicon can vary substantially depending on the films that are thick compared to their formal skin experimental methods and conditions used in their depth. For example, gold films less than ∼15 nm formation. Semaltianos and Wilson have shown that thick are semitransparent and commonly used as changing the temperature of the substrate from room substrates for SAMs in biology.192 The morphology temperature to 400 °C during thermal deposition of the thin film also influences its optical properties: increases the average area of gold grains deposited evaporation of gold or other noble metals onto bare 1110 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al. glass tends to produce island-textured films when separation.202 They removed the mica film in an their thickness is less than ∼100 nm. Deposition of ethanolic solution containing thiol (200 µM), and a a primer (e.g., Ti, Cr, Ni) promotes the formation of SAM formed on the gold surface as it was exposed a mostly continuous metal film structure on a sub- (Figure 2e). The roughness of these surfaces was strate such as glass for thicknesses >∼5-10 nm, but ∼0.3-0.7 nm (rms), and the advancing and receding the primers tend to diffuse through the overlying contact angles of water on the SAMs were essentially metal film to the surface over time.193 “Blooming” of indistinguishable, that is, there was almost no hys- the primer is a problem because chromium and nickel teresis (∼1-5°). (The hysteresis measured for SAMs are toxic to cells adherent to the SAM and bonding of alkanethiolates prepared on polycrystalline sub- of sulfur to alloy surfaces is not understood. The strates with no additional treatments is ∼10- presence of admetal impurities such as tin can also 20°.)26,30 lead to cell death, and therefore, stringent cleaning Electroless Deposition of Thin Films. Processes of the glass substrates should be carried out for for depositing thin films by chemical reduction of studies of this sort. Titanium does not seem to affect metal salts onto surfaces are known as “electroless” the viability of the cells and is the adhesion layer of processes.179 One advantage of these methods over choice for those systems. Chemical primers such as PVD is that they do not require vacuum processing 3-aminopropyl-trimethoxysilane provide an alterna- equipment; the chemical solutions are commercially tive method for promoting the formation of continu- available and only require mixing. Unlike conven- ous films of gold or other noble metals on substrates tional electrodeposition, electroless deposition does such as glass or silicon;194 thermal annealing of films not require a conductive electrode and, therefore, can deposited on chemically modified substrates can deposit films onto nonconductive materials. increase the grain sizes (from ∼50-100 to ∼200- Because electroless methods are solution-based, 500 nm diameter) and the degree of crystallinity they are attractive for depositing thin films on (Figure 2d).187 Whatever the method of preparation, nanostructures, such as colloids and nanopores, the optical properties of so-called “transparent” gold which are easily suspended or immersed in solutions, thin films are complex and depend sensitively on the or on structures that have internal surfaces, e.g., nature and evolution of their granular structure pores.181,183,203,204 Stroeve et al. investigated the mor- during the course of an experiment.195 phology of electroless gold deposits and its implica- An excellent quartz-substrate-supported gold thin tions for SAMs.180-182 The roughness of electrolessly film for studies of SAMs by SPM is provided by a deposited gold on glass was greater than that for flame annealing protocol. The method uses brief films prepared by thermal evaporation (by a factor exposure of a supported film to the flame of an of ∼4) (Figure 2f). X-ray diffraction studies showed oxygen-hydrogen torch.196 This method is capable of that the primary crystalline texture of the electroless producing exceptional quality-nearly single-crystal- deposits was (111) but that the surface orientation line, low step density-gold surfaces for the assembly was more heterogeneous than that of films prepared of SAMs over areas as large as a few square mi- by evaporation. Other highly expressed orientations crometers. included Au(200), (220), and (311).180 Thiols form densely packed SAMs on these surfaces, but RAIR Thin Films on Mica. Freshly cleaved mica sup- spectra for SAMs of n-alkanethiolates on these porting a thin film of metal is a common substrate surfaces suggest that there is a mixture of structures used as a pseudo-“single crystal” for microscopic present that result from the heterogeneity in surface studies of SAMs by scanning tunneling microscopy orientations.180,182 (STM) or atomic force microscopy (AFM).197,198 Gold Underpotential Deposition. One technique used films grow epitaxially with a strongly oriented (111) to modify the composition at the surface of thin films texture on the (100) surface of mica. The films usually is underpotential deposition (UPD). UPD is an elec- are prepared by thermal evaporation of gold at a rate trochemical method for generating submonolayer of ∼0.1-0.2 nm/s onto a heated (400-650 °C) sample coverage of one metal on another metal; the atomic of mica. The grain sizes of these films are ∼1000 nm adlayer forms epitaxially, that is, it adopts the with flat (111) terraces of ∼100 nm in width. ordering of the underlying surface.205 The deposited A method called template stripping can generate metal can alter the nature of the surface and, surfaces with roughness 5 nm) that cannot be formed directly with a protective layer of thiols.466 For example, dodecanethiol has been used to extract gold 6.2. Strategies for Functionalizing Nanoparticles nanoparticles from water (where they were formed with Ligands via ascorbic acid reduction in the presence of CTAB) There are three common strategies for tailoring the into organic solvents. 438,446 Caruso and co-workers composition of the SAM on nanoparticles and the also demonstrated the extraction of gold nanopar- functional groups exposed at the SAM-solvent in- ticles from toluene into aqueous solutions; this method terface (Scheme 4). They are (1) forming the nano- relies on the displacement of hydrophobic n-al- particles directly in the presence of ω-functionalized kanethiols with water-soluble thiols.435,446 Self-Assembled Monolayers of Thiolates Chemical Reviews, 2005, Vol. 105, No. 4 1131 6.2.3. Covalent Modification Murray and co-workers demonstrated the use of chemical reactions to modify the terminal functional groups of alkanethiolates on nanoparticles (Scheme 4).467 Many of the same reactions discussed in section 5.2 have been used to functionalize nanoparticles, including cross-metathesis,468 peptide coupling reac- tions,463,465,467,469 and nucleophilic substitution reac- tions.470 The chemical reactivities of thiols are, how- ever, different on nanoparticles than on thin films (section 6.3.2) because the structure of SAMs on highly curved surfaces (nanoparticles) are different than that for SAMs on planar surfaces (section 6.3). These differences make it possible to use other classes of organic reactions that typically are pre- cluded by steric effects on planar surfaces, for ex- ample, SN2 reactions.470 6.3. Structure of SAMs on Highly Curved Surfaces Much of the research involving gold nanoparticles has been carried out without a detailed understand- ing of the structure of the organic films formed by SAMs on the surfaces of nanoparticles.235 Two ex- perimental approaches are used to investigate the structure of SAMs on nanoparticles: physical ana- lytical techniques (IR, NMR, differential scanning calorimetry (DSC), HRTEM, AFM167) and chemical methods (reactivities). Before summarizing experi- mental results, we will discuss two physical charac- teristics of nanocrystals (geometric shape and radius of curvature) that are important in determining the structure of SAMs supported on them. Gold nanocrystals larger than 0.8 nm are believed Figure 9. (a) Model of Au140 nanocluster with a truncated to have a truncated octahedral or cubooctahedral octahedral geometry. (b) Equilibrium configurations of shape, depending on the number of gold atoms in the dodecanethiol-passivated Au140 clusters obtained through core, with eight {111} faces truncated by six smaller a molecular dynamics simulation: (left) 350 and (right) {100} faces (Figure 9a).57,471-473 There is a higher 200 K. (Reprinted with permission from ref 473. Copyright 1998 American Chemical Society.) (c) Schematic diagram percentage of regions where the surface construction of a gold cluster (radius ) Rcore) protected with a branched changes from one type to another (gold atoms on the and unbranched alkanethiolate. R is the radial distance, corners and edges of the truncated octahedron) on and b is the half-angle of the conical packing constraint. nanocrystals than on the planar substrates commonly (Adapted from ref 168.) used for SAM formation. For example, on a 1-2 nm Au cluster ∼45% of all surface atoms are located on chain density moving away from the surface of the edges or corners (Figure 9a).171 Thermal gravimetric core.166 For n-alkanethiols the decreasing density analysis (TGA) has suggested that small nano- translates into enhanced mobility of the terminal particles (60%) than an ideal, passes the area available to each chain on a nano- perfectly flat, single-crystal 2-D Au(111) surface particle with a given diameter; the alkyl chain (33%).166 This high coverage has been attributed to completely fills the volume of the cone at the surface the occupancy of alternative binding sites (edges and of the nanoparticle but is unable to fill the larger end corners) and can be modeled with both a simple of the cone (Figure 9b and c).474,475 geometric model166,168,170,171 and a computational Measurements of the hydrodynamic radii of mono- model.473 Such models need, however, to be consid- layer-protected gold nanoparticles support the hy- ered with some caution as the mixture of reaction pothesis that the outer part of the thiol layer is products from nanocluster preparations, especially loosely packed.175 A nanoparticle coated with a well- the distributions of mass, has not been characterized packed SAM (similar to those formed on thin films) as fully as is required to make direct structural is expected to have a hydrodynamic radius equal to assignments of this sort. the sum of the radius of the gold core and the fully Another distinguishing characteristic of SAMs extended alkanethiolate. This expectation does not, formed on the surfaces of nanoparticles is the high however, match experimental results: all measured radius of curvature of the substrate. An important hydrodynamic radii of monolayer-protected gold nano- consequence of this curvature is a decrease in the particles are smaller than the prediction, suggesting 1132 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al. that the monolayer is not well packed along the outer dicates an ordered inner core with increased fluidity edge. of the carbon chains in the outer shell.170 Monolayers on nanoparticles composed of branched 6.3.1. Spectroscopic Evidence for SAM Structure on alkane chains offer a higher degree of protection to Nanoparticles chemical etching (sodium cyanide) than do straight Spectroscopy (IR and NMR) provides information chain alkanes.470,476 Murray and co-workers found about the conformation and packing of the alkyl that SAMs formed from 2-butanethiol decreased the chains on the nanoparticles.166,168,170,171,467,476,477 A rate of etching by NaCN to the same degree that solid-state IR study of the structure of SAMs of hexanethiol did.470 In a related study, Rotello and co- n-alkanethiolates on 1-2 nm gold clusters showed workers formed SAMs from alkanethiols functional- that the major difference between planar SAMs and ized with a variety of amides and esters with branched SAMs on nanoparticles is that the SAMs on nano- end groups and evaluated the stability of the mono- particles exhibit a higher number (10-25%) of chain layer-protected clusters using cyanide etching and IR end-gauche defects (for all chain lengths) than SAMs spectroscopy.476 They hypothesized that “cone-shaped”, on planar substrates.171 The same study found that branched molecules would more effectively occupy the SAMs on nanoparticles have a number of near- volume available at the outer edge of the monolayer surface and internal kink defects similar to that of than simple n-alkanethiolates, which have a linear planar SAMs formed from alkanethiols of similar geometry when extended in an all-trans conformation lengths. IR spectra of the same nanoparticles in (Figure 9). carbon tetrachloride show a degree of disorder com- The chemical reactivities of terminal functional parable to that of liquid n-alkanes.470 One interpreta- groups displayed on SAMs formed on the surfaces of tion of the difference in IR spectra between solution nanoparticles are different than those on SAMs on and the solid phase is that the packing of the planar surfaces. For example, Murray and co-workers nanoparticles in the solid state induces some degree demonstrated that SN2 reactions occur more readily of order on the alkanethiolates. Alternatively, the on the surfaces of nanoparticles than on planar solvation of the alkyl chains by carbon tetrachloride surfaces.470 The headgroups of ω-bromoalkanethi- could account for the observed disorder. olates are less densely packed on curved surfaces As the size of the particle increases, the properties than they are on planar surfaces; this lower density of the SAM become more similar to a SAM on a allows backside attack of the incoming nucleophile planar surface: particles with a core diameter greater (amine) to occur. The rate is a function of the steric than 4.4 nm, coated with a SAM of dodecanethiolates, bulk of the incoming amine as well as of the relative have spectroscopic and physical properties approxi- chain lengths of the bromoalkanethiolates and the mating that of a planar SAM.166 For 4.4 nm particles surrounding alkanethiolates. The measured rates are the majority of the surface comprises flat {111} similar to solution-phase rates for SN2 substitutions, terraces rather than edges and corners; this geometry in agreement with the spectroscopic data (section leads to “bundles” of ordered alkanethiolates with 6.3.1) regarding the fluidity of the SAMs on nano- gaps (areas with a disordered organic layer) at the particles. corners and vertexes (Figure 9b).170,472,473 These “bundles” have been hypothesized to play an impor- 6.4. SAMs and the Packing of Nanocrystals into tant role in the solid-state packing of nanoparticles Superlattices into lattices (see section 6.4). The same surfactants that are used to control the size and shape of nanocrystals also influence the 6.3.2. Evidence for the Structure of SAMs on organization of the particles into superlattices and Nanoparticles based on Chemical Reactivity colloidal crystals.480-483 In colloidal crystals the nano- The chemical reactivities, both of the metal core particles are sometimes referred to as “molecules” and the alkanethiolate ligands, have been used to and the van der Waals contact of surfactant layers evaluate the structure of SAMs on nanoparticles. For on neighboring particles as intermolecular “bonds”.54 example, Murray and co-workers studied the kinetics When spherical nanocrystals, protected by a layer of and thermodynamics of the displacement of one alkanethiolates, are allowed to self-assemble on a alkanethiolate for another on the surfaces of gold TEM grid via slow evaporation of solvent, they form nanoparticles (2 nm diameter) as a function of chain hexagonal close-packed 2-D arrays (Figure 10).483-486 length.446,459,478 They find that the alkanethiolates Preparations with shorter alkyl chains (hexanethiol) bound to the vertexes and edges have a higher rate assemble in solution to form ordered 3-D, colloidal of exchange than those in the dense, well-packed crystals.486 The separation of the close-packed spheres planar faces. The rate of exchange decreases as the is linearly dependent on the length of the alkyl chains chain length and/or steric bulk of the initial SAM (Figure 10b).486,487 The increase in particle spacing increases. per additional carbon (∼1.2 Å) is about one-half of The susceptibility of differently protected gold cores the expected value; this observation suggests that the to a cyanide etchant gives an indication of the density alkyl chains might interdigitate with the chains on of packing of the alkanethiolates in the SAM (section neighboring particles.81,482,486,488 There are also ex- 8.1).470,476,479 For nanoparticles, the rate of dissolution amples of hydrogen-bonding control over interparticle (etching) decreases with increasing chain length; the spacing for gold nanoparticles with carboxylic-acid- rate remains constant when the chain length is terminated SAMs.447,489 greater than 10 carbons, however.470 This result Nanocrystals with other morphologies assemble, complements the spectroscopic evidence, which in- promoted by the hydrophobic tails of the capping Self-Assembled Monolayers of Thiolates Chemical Reviews, 2005, Vol. 105, No. 4 1133 Figure 10. (a) TEM image of a long-range-ordered array of dodecanethiolate-protected gold nanoparticles (5.5 nm Figure 11. (a) (Left) TEM image of a face-centered, cubic- diameter) deposited from toluene onto a silicon nitride packed, array of silver nanoparticles, passivated with a membrane. (Upper right inset) Enlarged view of the dodecanethiolate monolayer, with a truncated octahedral individual particles. (Left inset) Diffraction pattern of the morphology (see inset). (Right) Representation of the array obtained by Fourier transformation of a portion of proposed packing of the particles via interdigitation of the the image. (Reprinted with permission from ref 484. bundled alkyl chains on each face. (Reprinted with permis- Copyright 2001 American Chemical Society.) (b) Depen- sion from refs 450 and 490. Copyright 2000 and 1998 dence of the particle spacing in lattices of gold nano- American Chemical Society.) (b) (Left) TEM image of a particles on thiol chain length. The slope of the line gives monolayer of self-assembled silver tetrahedra passivated an increase of 1.2 Å per additional carbon atom. (Reprinted with dodecanethiolates. The bracketed area most closely with permission from ref 486. Copyright 2000 American matches the proposed model. (Reprinted with permission Chemical Society.) from ref 450. Copyright 2000 American Chemical Society.) (Right) Possible model of the short-range orientational order of the assemblies of tetrahedra. (Reprinted with agents, with close-packed geometries when allowed permission from ref 488. Copyright 1998 Wiley-VCH.) (c) to assemble on a TEM grid. Several examples of (Left) TEM image of stacks of ruthenium hexagonal superlattices formed by nanocrystals with different platelets protected with a monolayer of dodecanethiolates. morphologies are shown in Figure 11. As discussed (Right) Proposed model for the anisotropic packing of the in section 6.3, there is an uneven distribution of thiols platelets with low concentrations of thiols. (Reprinted with on the surfaces of angular polyhedra.472,473 For ex- permission from ref 81. Copyright 2003 American Chemical Society.) ample, the superlattices formed by silver tetrahedra and truncated octahedra have been analyzed using suggests that the thiols preferentially bind to the energy-filtered TEM to locate areas of high organic edges and leave the flat faces bare and prone to density.450,488,490 The authors assume that the alkane aggregation (Figure 11c). Similar behavior has been chains on each face of a tetrahedron “bundle” and observed for silver rods that fuse into wires when leave areas of lower organic density at the corners.472 assembled on TEM grids.491 For both particle morphologies the packing of the nanoparticles that they observe is consistent with 7. Patterning SAMs In Plane this model (Figure 11a and b).480 Physical tools capable of selectively positioning or The uneven distribution of thiolates on the surface damaging organic molecules enable the fabrication of nanoparticles can also lead to 1-D assembly (Figure of surfaces with well-defined patterns of SAMs in the 11c).81,491 For example, the assembly of hexagonal plane of the surface with lateral features ranging platelets of ruthenium can be altered by changing the from 10 nm to 10 cm. The techniques developed to ratio of thiol to particle.81 At high concentrations of generate patterns of SAMs on surfaces belong thiol (where the nanoparticles are assumed to be to a general class of techniquesstermed “soft completely coated with a monolayer), the particles lithography”130,492sthat can replicate patterns of or- arrange themselves in hexagonal lattices. At lower ganic (or organometallic) molecules and other mate- thiol concentrations, however, the platelets pack rials on substrates with planar or nonplanar topog- anisotropically into 1-D columns; this observation raphies. One strategy employed for patterning SAMs 1134 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al. on surfaces is physical transfer of the molecular components of a SAM to the substrate in an imposed pattern. Microcontact printing (µCP, section 7.1) and scanning probe lithography are examples of methods that use this principle. (There are many variations of scanning probe lithographies for patterning SAMs that deposit molecules from the tip to the substrate or that scratch patterns into preformed SAMs.493 A number of recent reviews address these topics ex- plicitly, and the topic is not covered in this re- view.132,133,362) A key difference between µCP and scanning probe methods is that µCP can generate many features simultaneously on the surface in a single step, whereas scanning probes are serial techniques that only write one feature at a time. A disadvantage of the probe methods, therefore, is that they require long times (minutes to hours per cm2) to write patterns; new technologies for arrays of independent scanning probes may improve the prac- ticality of these methods for prototyping structures.494 Another strategy for generating in-plane patterns of SAMs relies on damage to a preformed SAM; an energetic beam of photons, electrons, or atoms, or mechanical scratching213 can cause either chemical Figure 12. (a) Schematic illustration depicting the ap- plication of a PDMS stamp containing thiols to a polycrys- or physical damage to the SAM. Yet another strategy talline metal film. The primary mechanisms of mass uses the composition or topography of the substrate transport from the stamp to the surface are shown. The itself to determine the defect sites in the SAM. Both grayscale gradient approximates the concentration of thiols of these strategies may be combined with methods adsorbed in the stamp itself. (b) Magnified schematic view for exchanging SAMs to replace the damaged regions that illustrates the variety of structural arrangements with a SAM presenting different functional groups. found in SAMs prepared by µCP when the stamp is wetted with a 1-10 mM solution and applied to the substrate for 1-10 s. 7.1. Microcontact Printing Microcontact printing is a method for patterning surface energy makes it easy to remove the stamp SAMs on surfaces that is operationally analogous to from most surfaces and makes the surface relatively printing ink with a rubber stamp on paper: SAMs resistant to contamination by adsorption of organic form in the regions of contact between a topographi- vapors and dust particles. The flexibility of the stamp cally patterned elastomeric stamp, wetted with (or also allows conformal, that is, molecular level or van containing dissolved) reactive chemical ‘ink’ consist- der Waals, contact between the stamp and substrate; ing of n-alkanethiols (or other molecules that form it is thus critical to molecular-scale printing. This SAMs), and the bare surface of a metal, metal oxide, flexibility also makes it possible to print on curved or semiconductor (Figure 12).130 When forming pat- (nonplanar) substrates. Another advantage of PDMS terned SAMs of n-alkanethiolates on gold, the stamp is that it is compatible with a wide variety of organic usually is left in contact with the surface for a few and organometallic molecules because it is unreactive seconds (5-10 s) before it is removed. The lateral toward most chemicals; it is, however, swollen by a dimensions of the SAMs formed depend on the number of nonpolar organic solvents.497 dimensions of relief features on the stamp used for The stamps are formed by casting a PDMS pre- printing; the size of the stamp determines the total polymer (a viscous liquid) against a rigid substrate area over which the pattern is formed. Typical patterned in reliefsthe ‘master’. Fabrication methods patterns generated by µCP cover areas of 0.1-100 typically used to produce masters include photoli- cm2 with critical in-plane dimensions of ∼50 nm to thography, micromachining, or anisotropic chemical 1000 µm. If required, another SAM can be generated etching.498 Commercially available micro- and nano- in the bare regions of the surface that remain after structured elements, such as diffraction gratings, also removing the stamp by immersion of the substrate are practical structures to use as masters. Because in a solution containing another thiol for a few the pattern transfer element is formed by molding, minutes (∼1-10 min) or application of a second the size of the molecular precursors for the cross- stamp wetted with another thiol. linked polymer determines, in principle, the limita- tions on the minimum dimensions of the features 7.1.1. Composition of Topographically Patterned Stamps replicated in the stamp. The smallest features that The most common material used for the stamp in have been replicated to date by molding are ap- µCP is poly(dimethylsiloxane) (PDMS). PDMS is a proximately 1.5 nm; these features are defined in the nontoxic, commercially available silicone rubber. It direction normal to the surface of the stamp.499 is well-suited for forming stamps because it is elas- Sylgard 184 is the most common formulation of tomeric (Young’s modulus ≈ 1.8 MPa) and has a low PDMS used for forming stamps because it is com- surface energy (γ ) 21.6 dyn/cm2).495,496 The low mercially available, inexpensive, and easy to use.500 Self-Assembled Monolayers of Thiolates Chemical Reviews, 2005, Vol. 105, No. 4 1135 It can replicate features on the order of 0.3 µm raised regions of the stamp. Another technique without significant distortions of the features and (called ‘contact inking’) uses a flat slab of PDMS without mechanical instabilities.131,501 Sylgard 184 is soaked in a solution of thiols or a glass slide coated a convenient material for replicating masters with with a thin layer of thiols as an ‘ink pad’. A stamp features >1 µm that are separated by distances ∼1- placed against the surface of the pad adsorbs thiols 10 times the feature sizes. Large separations of only in the regions of contact.509,511,512 features (∼10 times the size of the features) lead to Nonpolar thiols, especially n-alkanethiols, diffuse collapse of the stamp in regions between features, into the bulk of the hydrophobic stamp upon applica- and small distances between features (ratios of tion. The favorable partition coefficient raises the feature sizes to separation distances less than ∼0.5) effective concentration of the thiols in the stamp lead to lateral collapse of the features.131,502,503 The relative to that in the applied solution (∼1-10 mM). surface tension of the PDMS elastomer also distorts Polar molecules, however, do not partition into the small (∼20-100 nm), replicated features.504 These stamp and remain entirely on the surface of the mechanical instabilities make it difficult to reproduce stamp.497 Plasma oxidation of the PDMS stamp reliably and accurately features that have lateral improves the wettability of the surface of the stamp dimensions-and distances separating them from for polar molecules and, therefore, the uniformity of other features-smaller than ∼500 nm and aspect the patterns generated by printing with these types ratios >1 (vertical dimensions greater than the of molecules.513 lateral dimensions). An alternative formulation of PDMS developed by 7.1.3. Mechanism for Forming SAMs by Printing Schmid and Michel505 is more rigid than Sylgard. The basic process for forming SAMs of alkane- This ‘hard PDMS’ (h-PDMS) has a Young’s modulus thiolates on gold is conceptually simple: the stamp of 9.7 MPa and can replicate features as small as ∼20 impregnated with thiols is placed in contact with a nm with high fidelity.503 The material is, however, bare gold surface, and the thiols diffuse from the too brittle to use as a stamp: it cracks or breaks in stamp onto the surface where they assemble into handling. Composite stamps comprising a thin (∼30- ordered structures. Studies of the details of the 40 µm) layer of h-PDMS and a thick (∼1 mm) layer process suggest, however, the process is complex and of 184 PDMS combine the advantages of both mate- depends on a number of parameters, including choice rials and yield a stamp that can accurately mold of the SAM-forming molecules, concentration of small features and easily peel away from surfaces.503 molecules in the solution applied to the stamp, Attaching the composite stamp to a thin, rigid glass duration of contact, and pressure applied to the support allows large-area (>10 cm2) printing of stamp.501,506,514 features that are less distorted than those produced when the stamp is applied manually to a surface.505 The mechanisms for mass transport of thiols dur- A photocurable formulation of PDMS with physical ing µCP include, at least, the following: (1) diffusion properties between those of h-PDMS and 184 PDMS from the bulk of the stamp to the interface between also has been reported.495 the stamp and the surface of the gold contacted by the stamp; (2) diffusion away from the edges of the Another material used for stamps in µCP is block stamp and across the surface of the gold; or (3) vapor copolymer thermoplastic elastomers.506 These stamps transport through the gas phase (Figure 12). The first are less susceptible than 184 PDMS to sagging or mechanism is important for the formation of SAMs collapse during printing, even under applied loads. in the regions where the stamp is intended to be in The stamps are formed by compression molding at contact with the surface but little information is temperatures above 100 °C and with loads of ∼200 available regarding relevant parameters such as the g. These conditions may be appropriate for mechani- rates of diffusion of thiols (or other nonpolar mol- cally strong masters, e.g., micromachined silicon, but ecules) in PDMS. The second and third mechanisms are not directly compatible with masters generated are important for understanding (and controlling) the by photolithography, which consist of a patterned lateral diffusion of SAMs into regions that are not layer of organic photoresist on silicon wafers. contacted by the stamp; these processes lead to 7.1.2. Methods for Wetting Stamps with Thiols distortions of the lateral dimensions of the printed features and gradients of mass coverage at the edges Common methods for applying thiols to the surface of structures (determined by wet chemical etching). of a stamp include rubbing a cotton swab or foam The relative contributions of each of these mecha- applicator wet with a solution of thiols (0.1-10 mM), nisms in the formation of the SAMs in the regions placing a drop of thiol-containing solution onto the contacted by the stamp and in nonprinted regions, surface of the stamp, or immersing the stamp in a however, are not completely understood.514 solution of thiol 220,507-510 The excess solvent (usually The degree to which thiols spread across the ethanol) evaporates from the surface under a stream surface in a liquid phase during µCP is not clear. of nitrogen; the surface appears visibly dry afterward. SAMs of alkanethiolates are autophobic, that is, the Ethanol is only slightly soluble in PDMS,497 but the low-energy surface generated by the formation of the effect of residual ethanol dissolved in the stamp on SAM is not wetted by liquid thiols. This characteristic the process of forming SAMs by µCP is not known. limits the spreading of thiols past the edge of the The common methods for applying inks do not SAM once formed. This effect can be observed mac- distinguish between flat and raised regions of the roscopically: the surface of a SAM on gold is dry stamp, that is, thiols are applied in both recessed and when it removed from a solution of thiols.515 Spread- 1136 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al. ing is, however, a process that does occur in micro- requires printing times from 1 min to 1 h; this time contact printing and is one factor that limits the is less than that required in solution (12-18 h).30,507,511 performance of this method of replication. RAIR spectra also suggest that concentrated solu- Vapor transport is a primary mechanism for the tions of thiols (∼100 mM) generate SAMs with a spreading of SAMs in regions not contacted by the higher degree of chain organization than low con- stamp, but it is not clear what role, if any, it plays centrations (1-10 mM) when the stamps are applied in forming SAMs where the stamp contacts the to the surface for the same amount of time (60 s).30 surface. On a polycrystalline film with variations in Studies using SFG microscopy have shown that the roughness of ∼3-10 nm it is possible that the stamp edges of 10 µm features printed on metal surfaces does not make van der Waals contacts with the entire are not sharp and lead to regions of mixed SAMs exposed surface, especially in the crevices between when the bare regions of the substrate are filled with the grains of the thin film. Whether surface-mediated a second SAM.521 diffusion or vapor transport through the air leads to Overall, the data indicate that the SAMs formed the formation of SAMs in the crevices is not under- by µCP on polycrystalline films of metal and used in stood. Experiments using wet-chemical etchants to most applications are not equivalent to those formed transfer patterns of SAMs into underlying metal in solution when formed by printing for 1-10 s with films suggest that the boundaries between the grains stamps inked with 1-10 mM solutions of thiol. The of the thin film are susceptible to corrosion220,516,517 thiols present near the surface of the stamp are and may indicate that SAMs have a higher degree responsible for the nonequilibrium state generated of disorder when formed in the crevices than on the when printing for only a few seconds; it requires tops of the grains. additional time for thiols to diffuse from the bulk of the stamp to the surface to increase the mass 7.1.4. Structure of SAMs Formed by µCP coverage. The composition, mass coverage, and organization The transition in the structure of a SAM from the of SAMs formed by µCP have been studied by contact printed to nonprinted regions has, to the best of our angle goniometry,511,518 STM,508,518 AFM,507,509,519 knowledge, not been observed directly, but the de- XPS,511,520 RAIRS,30,511 ellipsometry,514 electrochem- pendence of the surface structure on the concentra- istry,508 time-of-flight secondary-ion mass spectrom- tion of thiols loaded into the stamp also implies that etry (TOF-SIMS),520 GIXD,519 NEXAFS,507 and sum- the structure of SAMs near the edges of printed frequency generation (SFG) spectroscopy.511,521 Direct features is different than that in the centers. On the comparisons between studies of the organization of basis of STM data for different concentrations of SAMs formed by µCP are complicated by a lack of thiols applied by printing,518 one possible structural standards for conducting the printing experiments transition could include an increase in the size of the (methods for applying thiols to the stamps, duration (x3 × x3)R30° domains close to the edges of printed of printing times, etc.). Taken together, however, the features and a high percentage of low-density striped data from these studies indicate that the SAMs phases (>60%). As the distance away from the edges formed by µCP are usually a complex mixture of of the printed regions increases, the mass coverage phases but can reach a state of organization that is must decrease; the low mass coverage would imply spectroscopically indistinguishable from SAMs formed a more disordered or liquidlike state. Such variations by adsorption from solution. in structure have been observed for SAMs patterned by dip-pen lithography.522 STM studies show that the SAMs formed by µCP for 3-5 s with 1-10 mM solutions of dodecanethiol 7.1.5. Transfer of PDMS to the Surface during Printing on Au(111) exhibit a mixture of structures.518 The structures observed include disordered, liquidlike Some studies report that trace contaminants of regions, striped phases with p × x3 packing ar- PDMS are left on the surface after printing.511,520,523 rangements (p ) 3.5, 4, 8, 5), and dense (x3 × x3)- The effect of these contaminants on the structure and R30° structures with a c4 × 2 superlattice. The SAMs properties of the SAMs is not clear. The composition formed in these experiments consisted of islands of of the prepolymer, the time over which the cross- dense (x3 × x3)R30° structures (∼50-200 nm linked PDMS is cured, the exact ratio of components diameter) surrounded with striped phases and dis- in the prepolymer, and the procedures used to extract ordered regions; the crystalline islands were sepa- low molecular weight siloxanes probably determine rated by distances of ∼100 nm and occupied only 20- the degree of contamination.497 40% of the surface. SAMs formed by µCP with 100 mM solutions of thiol were nearly identical to those 7.1.6. Fabrication of Nanostructures by µCP formed from solution (1 mM for 18 h): they contained It is possible to form nanostructures by µCP with only (x3 × x3)R30° structures and c4 × 2 superlat- lateral dimensions as small as 50 nm, but the tices of the (x3 × x3)R30° structures. The experi- fabrication of such structures by µCP remains a more ments suggested that the percentage of each type of significant challenge than producing micrometer- structure and the domain sizes of the structures scale patterns by µCP.31,131,220,524-527 Two key factors depend on the concentration of thiol used for printing that determine the limits of resolution are lateral and not small variations in contact time (0.3-30 s). diffusion of the molecules and distortions of the RAIR spectra and contact angle measurements stamp. Lateral broadening of the printed features suggest that the elimination of conformational defects results from diffusion of the molecular ink through (and probably low mass coverage phases as well) the gas phase or through a surface-mediated process. Self-Assembled Monolayers of Thiolates Chemical Reviews, 2005, Vol. 105, No. 4 1137 Delamarche and co-workers have shown the extent that occupy areas smaller than ∼0.25 nm2 and (2) of broadening of features depends on the vapor they are very thin ( 16) exhibit less over µCP for generating patterns in SAMs in-plane spreading than short ones: eicosanethiol (CH3(CH2)19- is that the resolution is determined by the size of the SH) is a good choice among inks for printing submi- beam applied to the SAM and not by other factors crometer-scale features that subsequently are trans- related to the molecules forming the SAM such as ferred into the substrate by etching. Alkanethiols diffusion by vapor transport or by surface-mediated with more than 20 carbons are less soluble in ethanol processes. A significant disadvantage of these meth- and in the PDMS stamp than shorter ones with 16- ods, however, is the cost of the equipment and 20 carbons and therefore are less suitable choices for infrastructure required, especially for high-resolution printing. Macromolecules with molecular weights ( 1000 Da) exhibit much less diffusion than alkanethiols and can 7.2.1. Photolithography produce patterns of organic materials on surfaces Irradiation of a SAM of alkanethiolates with UV with critical dimensions less than 50 nm.525 The light through a pattern of apertures in a chromium concentration of ink adsorbed in the stamp and the film on glass leads to photooxidation of the SAM in time of contact for printing provide two parameters the exposed regions.138,527,533 The oxidized species can useful in controlling the extent of broadening in be removed from the substrate by rinsing the sub- printed features, but in practice, there is still a large strate in a polar solvent, e.g., water or ethanol. The degree (10-50%) of variability in the size of small optical elements of the system determine the mini- (1 m2) may supersede requirements for high-quality SAMs. Neutral atoms of rare gases excited into metastable states (∼8-20 eV above the ground state) also can 7.3.3. Topographically Directed Assembly damage SAMs of alkanethiolates.140,541 This system, SAMs formed on metal substrates patterned with in principle, provides the basis for a form of 1:1 topographical features-steps, edges-have different projection lithography that effectively is unlimited by degrees of order depending on the topography. SAMs the effects of diffraction (which is unimportant for of alkanethiolates formed in the planar regions of the atomic systems).542 The energy released when the substrate adopt the organization and structure de- metastable atom collides with the SAM and returns scribed in section 3, but the regions where the to its ground state seems to ionize the organic topography changes drastically-edges of topographi- material in the SAM543 and induce conformational cal features-induce a higher degree of disorder in disorder in the alkane chains;141 it is also possible the SAMs formed there than on the planar sur- that the collision generates secondary electrons, faces.212,214 The width of the disordered regions are which contribute to the damage.544 somewhat dependent on the cross-sectional profile of Dosages of >10 metastable atoms per thiolate are the topographic features; sharp changes in topogra- necessary to generate useful contrast between dam- phy (∼90°) produce regions of disorder as small as aged and undamaged regions of SAMs (as determined 50 nm. The thiolates in the disordered regions are by wet chemical etching); these levels of flux are susceptible to exchange with other thiols, and thus, difficult to achieve in periods of time less than 1 h.544 a SAM containing a second functional group can be Low dosages of metastable atoms (1 µm) electrodeposited films (Figure 15).565 chemistry and cell biology.562,563 These studies depend The density of critical defects (pinholes) that primarily on the ability to synthesize interfacial films penetrate the entire thickness of a thin film and the with specific compositions both in the plane of the roughness of the edges of etched features have surface and out of plane, but some, for example, limited the use of µCP and selective wet etching for electron-transfer processes, are extremely sensitive fabricating structures with lateral dimensions 16) can protect metal films from is that they have small grain sizes (∼15-30 nm); this corrosion by aqueous wet-chemical etchants.564 Com- morphology is better suited than that of gold (grain bining this ability with techniques for generating in- sizes ∼35-75 nm) for fabricating metal lines with plane patterns of thiols (e.g., µCP) makes it possible widths as small as 50 nm.31,131,220 Unlike gold, pal- to fabricate micro- and nanostructures composed of ladium is compatible with complementary metal- gold, silver, copper, palladium, platinum, and gold- oxide semiconductor (CMOS) manufacturing pro- palladium alloys. Some of the parameters that de- cesses.221 termine the minimum critical dimensions and quality (as measured by the density of pinhole defects on etching and on the edge