Appr Aches

Nanotechnology: The Top-Down andBottom-Up Approaches

Parvez Iqbal,1 Jon A. Preece,2 and Paula M. Mendes11School of Chemical Engineering, The University of Birmingham, Birmingham, UK2School of Chemistry, The University of Birmingham, Birmingham, UK

1 Introduction 12 Nanofabrication 53 Conclusion and Outlook 11Acknowledgments 12References 12

1 INTRODUCTION

Nanotechnology involves the study, imaging, measuring,modeling, or manipulation of matter at scales falling inthe range of 1100 nanometers (nm). It is a highly mul-tidisciplinary field, drawing from fields such as chem-istry, materials science, colloidal science, applied physics,engineering, and biology. In a relatively short span ofabout 30 years of research, nanotechnology is already hav-ing an impact on society and several industrial sectors,and such impact will increasingly be felt as its prod-ucts increase in number and become more commercial-ized.1 Nanotechnology-based substances are now foundin a wide range of household products and productsintended for professional use, including sports gear, cosmet-ics, sunscreen lotions, food and food packaging material,clothing, household appliances, electronic devices, disin-fectants, paints, furniture varnishes, building materials, andmedicines. Nanotechnology is predicted to touch nearly

every industry and every part of our lives and become thebasis for remarkably powerful and inexpensive computers,fundamentally new diagnostic and therapeutic technologies(see Supramolecular Nanoparticles for Molecular Diag-nostics and Therapeutics, Nanotechnology) that couldenhance human health and longevity,2 advanced sensors3for military applications and environmental protection, andnew zero-pollution transportation technology.4, 5

This article provides an overview of nanotechnology,describing the origins of the field, present technology,ongoing research, and future aspirations. In addition, thetwo possible methodologies of fabricationthe top-downand bottom-up approachesare discussed, covering themerits and drawbacks of each approach.

1.1 Brief history of nanotechnology

Historically, the concept of nanotechnology was first pro-posed by the Nobel laureate Richard Feynman, when hegave a now-famous talk called Theres Plenty of Room atthe Bottom at an American Physical Society meeting atCaltech in 1959.6 With this visionary talk, Feynman dis-cussed both top-down and bottom-up possibilities of work-ing at the molecular level, most of which are still relevanttoday. Extrapolating from known physical laws, he arguedthe possibility of molecular writing, seeing and rearrangingindividual atoms, the prospect of designing molecules oneatom at a time, and the challenges involved in developingnanometer-scale devices. In his talk, Feynman made severalreferences to examples in nature such as cells, which arevery tiny, but they are very active; they manufacture vari-ous substances; they walk around; they wiggle; and they do

Supramolecular Chemistry: From Molecules to Nanomaterials, Online 2012 John Wiley & Sons, Ltd.This article is 2012 John Wiley & Sons, Ltd.This article was published in the Supramolecular Chemistry: From Molecules to Nanomaterials in 2012 by John Wiley & Sons, Ltd.DOI: 10.1002/9780470661345.smc195

2 Nanotechnology

all kinds of marvelous thingsall on a very small scale.Nonetheless, Feynman never used the term Nanotechnol-ogy to describe this new scientific field.

Several years later, in 1974, Professor Norio Taniguchicoined the term Nanotechnology in order to define top-down ultraprecision machining, describing the term asmainly consisting of the processing of separation, consol-idation, and deformation of materials by one atom or amolecule.7 Later, in the 1980s, Eric K. Drexler popu-larized the word nanotechnology through his biologicallyinspired bottom-up visions of building molecular machines,the so-called molecular assemblers, which could guidechemical reactions by positioning reactive molecules withatomic precision.8, 9 However, Drexlers sweeping visionsand theories of grey goo have proved highly contro-versial,10 and nanotechnology as a scientific field wasestablished in a way that diverged from Drexlers origi-nal vision of molecular manufacturing. Today, the goal fornanotechnology research is not to immediately create syn-thetic molecular assemblers but rather to understand theunique properties of the nanoscale and use that knowl-edge to create new, high-performance materials, devices,and processes.

Although nanotechnology was first theorized by Feyn-man,6 then coined by Taniguchi,7 and then later popularizedby Drexlers controversial vision of molecular manufactur-ing,810 a flurry of activity in the field was spurred by theinvention of the scanning tunneling microscope (STM), theatomic force microscope (AFM), and the first manipula-tion of atoms. In 1981, Gerd Binnig and Heinrich Rohrerdeveloped the STM at IBMs laboratories in Switzerland.11This microscope enabled atomic scale characterization ofconducting surfaces and opened the possibility of imaging

and mapping nanoscale materials. The next leg of the stan-dard story jumps us to 1986 with the development ofthe AFM, which enabled mapping on nonconducting sur-faces.12 In 1990, Don Eigler and Erhard Schweizer inventeda technique for picking up individual atoms using the tipof an STM and depositing them in patterns onto a sur-face.13 They used the technique to position 35 individualxenon atoms on a nickel metal surface to spell out theircorporate logo IBM, demonstrating how atoms could bemoved and positioned. The technique has since been usedto create a variety of structures out of many differentatoms.

In addition, the advances in supramolecular chem-istry, the constant pressure in device miniaturization inthe electronic industry, and the development of materialswith nanoscale dimensions, such as fullerene,14 metallicnanoparticles,15 graphene,16 carbon nanotubes (CNTs),17have contributed to the rapid growth of the field in thepast 30 years.

1.2 Current researchnanotools,nanostructured materials, and nanodevices

Current research into nanotechnology may be divided intothree broad categories: nanotools, nanostructured materials,and nanodevices. The various components of these cate-gories are schematically illustrated in Figure 1.

Nanotools are a collection of methods and techniquesemployed to produce and evaluate nanostructured materialsand nanodevices. The fabrication of nanostructured mate-rials has been led by the development of new syntheticmethods and major advances in supramolecular chemistry.

Medicine andhealth

Food andnutrition

Biotechnologyand agriculture

Textiles andclothing

National securityand defense

Energy andenvironment

Transportationand aerospace

Informationtechnology

NanotoolsSupramolecular chemistrySynthetic methodsSurface scienceNanolithographyAnalytic toolsComputer simulations

NanoparticlesNanowiresFullereneGraphene

NanomaterialsNanodevicesNanoelectronicsSpintronicsNano-optoelectronicsNanosensorsDrug delivery systems

Carbon nanotubesNanocompositesThin solid filmsNanopatterned surfaceSupramolecular systems

Medicine andhealth

Food andnutrition

Biotechnologyand agriculture

Textiles andclothing

National securityand defense

Energy andenvironment

Transportationand aerospace

Informationtechnology

Figure 1 Schematic illustration showing how nanotechnology and its nanotools, nanomaterials and nanodevices are impacting ourworld.


Nanotechnology: the top-down and bottom-up approaches 3

With advancements in synthetics methods, nanostructuredmaterials with exquisite control over size, morphology, andfunctionality have been achieved.18 Remarkable progress inthe field of supramolecular chemistry has enabled the designof molecular components to interact favorably with eachother in such a way that they can self-assemble, throughnoncovalent interactions, into larger, well-defined entitieson the nanoscale with tailored properties.19 Supramolecularapproaches have been successfully employed to construct,among other supramolecular systems, molecular devicesand machines (see Photochemically Driven MolecularDevices and Machines, Nanotechnology), functional, bio-logically derived supramolecular systems (see BiologicallyDerived Supramolecular Materials, Nanotechnology),polymeric nanomaterials (see Self-Assembled Nanopar-ticles, Nanotechnology), supramolecular hybrid nanoma-terials (see Supramolecular Hybrid Nanomaterials asProspective Sensing Platforms, Nanotechnology), andsupramolecular nanoparticles (see Supramolecular Nanopar-ticles for Molecular Diagnostics and Therapeutics, Nan-otechnology).

Furthermore, advances in surface science have led to thecreation of techniques to fabricate nanoscale molecular-assembly structures onto a variety of substrate surfaces,including self-assembled monolayers (SAMs),20 Lang-muirBlodgett technique,21 and two-dimensional supra-molecular assembly (see Two-Dimensional Supramolecu-lar Chemistry, Nanotechnology). Many lithographic tech-niques have emerged for patterning surfaces with nanome-ter resolution (see Nanolithography, Nanotechnology).Photolithography, electron-beam lithography, soft lithog-raphy, nanoimprinting, colloidal lithography, and dip-pennanolithography are but a few examples of such tech-niques. Analytic tools, such as the AFM (see Atomic ForceMicroscopy Measurements of Supramolecular Interac-tions, Nanotechnology), STM,11 and the near-field scan-ning optical microscope,22 have provided revolutionaryimprovements in our ability to investigate the structuresand functions of nanostructured materials and nanodevices.High-performance computer simulations based on advancedmathematical and physical modeling are at present a neces-sary tool in the development, design, and understanding ofnanoscopic systems.23 With these powerful contemporarytoolsadvanced synthetic methods, supramolecular chem-istry, surface science, nanolithography, new or improvedanalytical techniques, and high-performance computer sim-ulationsmany novel nanostructured materials and nan-odevices have been constructed and characterized. Theimprovement of existing and the development of new nan-otools is an unrenounceable condition for further progressin nanotechnology.

Nanostructured materials can be defined as any materialthat has structured components with at least one dimension

4 Nanotechnology

potential to add great value to a wide range of everydayproducts.

Nanodevices are systems with nanostructured materialsthat carry out specific functions with either improved per-formance or new attributes. Recent years have witnessedthe emergence of new device paradigms based on nanos-tructured materials, including nanoelectronic devices, nano-optoelectronic devices, spintronic devices, nanosensors, anddrug and gene delivery systems.

Nanoelectronics (see Nanoelectronics, Nanotechnology)have already revolutionized the semiconductor deviceindustry, in the form of integrated circuits with nanoscaletransistors that pack more and more functionality into com-pact devices. Since the development of the first integratedcircuits over 60 years ago, the semiconductor industry hasseen the size of transistor devices decrease by a factor of 2in every 18 months,30 a trend that was first pointed out byGordon Moore in the 1960s and is referred to as Mooreslaw.31 The miniaturization has primarily been achieved byoptimizing the photographic technique such as going fromusing visible light sources to UV and currently stands at22 nm; however, limits of the process are fast approaching,where further miniaturization will no longer be possiblewith the current setup. Therefore, to continue the trendset by Moores law, fairly dramatic changes in the waytransistors are designed and operate are required. Emerg-ing nanoelectronic devices, such as Si nanowire field-effecttransistors (FETs),32 carbon-nanotube FETs,33 and graphenenanoribbon FETs,34 are providing new opportunities forvery-large-scale integration circuits in order to achieve con-tinuing cost minimization and performance improvement,while simultaneously enabling the extension of Mooreslaw well into the next decade and beyond.35, 36 Anotherextremely important area of research in nanoelectronics ismolecular electronics, which is the utilization of a singlemolecule or group of molecules as key active componentsin electronic devices.37 Molecular electronic devices areexpected to not only address the ultimate limits of possi-ble miniaturization but also offer unlimited possibilities fortechnological development due to the potentially diverseelectronic functions of the component molecules, which canbe tailored by chemical design and synthesis.38

Advances in spintronics have already made their way intomagnetic hard discs, allowing for a huge increase in theirstorage capacity.39 In this rapidly growing field, researchersare paving the way for spin computers, which will usethe electrons spin state to store and process vast amountsof information more quickly while requiring less energyand generating less heat. Nanostructured materials, such asgraphene40 and lithographically nanopatterned surfaces,41are two promising candidates for use in spin computers.

Progresses in the synthesis of semiconducting nanostruc-tures, such as nanowires and nanoparticles, are leading to

the development of ultracompact and power-efficient opto-electronic devices such as photodetectors42 and lasers.43Nanowires have also been harnessed as building blocksfor the construction of nano-light emitting diodes (LEDs)with the ability of emitting in many different wavelengthsdepending on nanowire composition.44, 45 The conventional,low efficient photovoltaic cells, which have restricted large-scale production of electrical energy, can be now replacedby nanosolar cells with much higher efficiencies and lowercosts.46

Nanosensor devices incorporating nanostructured materi-als as sensing probes (see Supramolecular Hybrid Nano-materials as Prospective Sensing Platforms, Nanotech-nology) continue to advance toward commercialization for anumber of different applications, including, but not limitedto, medical diagnostics, food safety, environmental protec-tion, national security, and aerospace.3 While in the realmof medicine nanosensors can detect the onset of disease, inthe area of national security they could be used to detectradioactive materials or biological warfare agents (such asanthrax and smallpox). The detection principle in nanosen-sors is based on measuring the physical and chemical prop-erty changes, such as electrical, optical, magnetic, mass, andpH value, derived from the interaction of the target analytewith the nanodetection device. Today, numerous gas, chem-ical, and biosensors are being developed with substantiallysmaller size, lower weight, more modest power require-ments, greater sensitivity, better specificity, and, in somecases, with the ability to detect multiple analytes at the sametime through high-density nanoarrays.47 In order to achievesuch powerful nanosensor capabilities, researchers havebeen exploiting numerous nanostructured materials, includ-ing nanoparticles,24 CNTs,47 nanowires,48, nanoscaled thinfilms,49 and nanocantilevers.50 For instance, over the pasttwo decades, the evolution of fluorescent semiconductornanoparticles known as quantum dots (QDs) has helped tousher in a new era in biomedical research and applications(i.e., as labels for the detection of DNA and immunosensingof disease biomarkers, and to improve biomedical imaging).The defining features of QDs are their exceptional pho-tostability and size-dependent tunable photoluminescence.QDs emit different colors depending on their size, allowingthem to be used to color-code and track different cell pro-cesses, thereby providing high-resolution cellular imaging,long-term observation of individual molecules, and theirmovement within cells.24

Liposomal drug delivery systems and delivery systemsbased on drug conjugates are two classes of nanotechnologytherapeutic products that are used in clinical practices.51Present research on nanotechnology-based drug and genedelivery systems is focused on achieving targeted deliveryof drugs to specific cells or tissues, improved delivery ofpoorly water-soluble drugs, multiple drug administration,



monitoring drug delivery by combining therapeutic agentswith imaging biomarkers, and real-time read on the in vivoefficacy of a therapeutic agent.51, 52 Nanodevices are alsoexpected to be developed for both diagnosis and therapy,and such theragnostic devices hold great promises in per-sonalized medicine (see Supramolecular Nanoparticlesfor Molecular Diagnostics and Therapeutics, Nanotech-nology).

Nanodevices are in some ways the most complicatednanotechnological systems. They require the understandingof fundamental phenomena, the synthesis of appropriatenanostructured materials, the use of those materials tofabricate functioning devices, and the integration of thesedevices into working systems. Nevertheless, significantprogress is currently being reported, and development andavailability of increasingly sophisticated nanodevices aregreatly anticipated.

1.3 Possible applications of nanotechnology

The huge scope of nanotechnology is summed up in aquote from a report by the National Science Foundationin 1999,53 which quoted, nanoscience and technology willchange the nature of almost every human-made object in the21st century. The fascinating and often unrivaled propertiesof nanostructured materials and devices have been, and willcontinue, to open new and sometimes unexpected fields ofapplication. Today, the widespread applications range frominformation and communication technology, transportationand aerospace, energy and environment, national securityand defence to healthcare and medicine, food and nutrition,biotechnology and agriculture, textiles and clothing, andmany more (Figure 1).

Over the years, advances in the field of nanoelectronics,which deals with the miniaturization of electronic devices,have enabled the appearance of new products in a rangeof areas, including consumer electronics (e.g., computers,mobile phones, televisions, etc.), the automotive industry,healthcare and environmental management.53 Nanoelec-tronics are expected to have an impact in many areas of ourlives, as more and more functions are integrated into every-day products. For instance, nanoelectronic devices couldbe used to regulate energy use in buildings, while in carsadditional built-in electronics could allow for more assisteddriving.

Renewable energy technologies are often regarded asclean or green energy since they are far less harmful tothe environment than conventional fossil fuel technologies.Nanotechnology is offering a range of new opportunities,such as nanosolar cells that would be energy-intensiveand far less expensive to make,46 solar panels capable oftapping not only the visible light from the sun but also

from infrared light as well, thus significantly increasingenergy output.54 Wind, wave, and geothermal energy isalso expected to be harnessed more effectively using newnanostructured materials and stored or delivered moreefficiently through nanotechnological advances in batteriesand hydrogen fuel cells.55 Nanotechnology is playing amajor role in the development of hydrogen fuel cells, whichis considered to be one of the most promising clean energyconversion devices for a wide variety of power applicationsranging from portable and stationary power supplies totransportation.4, 5

More recently, subfields of nanotechnology, such asnanobiotechnology and nanomedicine, are contributingtoward the development of highly accurate and sensitiveearly-stage diagnostic devices. For instance, biosensors andmolecular probes are capable of directly interacting withthe biological molecule and converting the interactionsinto directly transduced or significantly amplified electricalor electromagnetic signals.2, 56 Dramatic breakthroughs arealso expected in life sciences research that could contributeto the treatment of a number of human diseases, includingcancer and neurodegenerative diseases, such as Alzheimersand Parkinsons. Therapeutic fields, such as gene and drugdelivery, tissue engineering, and drug discovery, will alsobenefit greatly from advances in nanobiotechnology andnanomedicine.57, 58 For instance, one of the current chal-lenges is developing multifunctional nanodevices that arecapable of targeting specific malignant cells, visualizingtheir location in the body, killing primarily the cancer cells,with minimal side effects on the bodys normal cells andtissues, and monitoring treatment effects in real time.59

2 NANOFABRICATION

Nanofabrication methods can be categorized into twogroups: the top-down and bottom-up approaches. Thetop-down approach revolves around fabrication via etch-ing away bulk material to achieve the required smallerstructural architectures and this is generally achieved bylithographic processes (see Nanolithography, Nanotech-nology).60, 61 This process could be likened to sculptinga block of stone to the required image. In contrast, thebottom-up approach involves the structures being craftedatom by atom or molecule by molecule through covalentor supramolecular interactions6063 in a similar manner tohow a house is built brick by brick. Both approaches havetheir merits and drawbacks, which will be discussed later.

2.1 Top-down approach

Presently, the top-down approach is dominantly used inindustry for the fabrication of many man-made materials,


6 Nanotechnology

a prime example being the semiconductor industry,64, 65where features of metal oxide semiconductor field effecttransistor (MOSFET) are imprinted onto a silica wafervia a lithography-based procedure termed photolithogra-phy.64, 65 The technique is based on a projection printingsystem, which is done in a device called a stepper, wherethe features of the transistors are projected through a pho-tomask onto a silicon wafer that has been prespinned witha photoresist (light-sensitive) material using UV light. Ifthe photoresist is positive, the regions of the photoresistthat are exposed to UV light become soluble to a partic-ular developing solvent and are washed away during thedeveloping step, leaving a pattern of raised features on thewafer identical to the dark regions on the mask. Conversely,for a negative resist, the regions of the photoresist that areexposed to UV light become insoluble to a particular devel-oping solvent and only the unexposed regions are washedaway during the developing step, leaving a pattern of raisedfeatures on the wafer identical to the clear regions on themask. For more details on the fabrication of MOSFET, referto article Nanolithography, Nanotechnology.

2.1.1 Limitations of the top-down approachSemiconductor technology is beginning to reach the lim-its of miniaturization, with Intel announcing in May 2011another milestone as they demonstrated a production-ready3D transistor technology for 22 nm called Tri-Gate. Cur-rently, the photolithography process uses 193 nm wave-length of light to pattern the wafers, but both technicaland material limitations are envisaged to be approachedas smaller features are to be obtained.60 For instance,quantum effects and defects formed during the patterningprocess (see Nanoelectronics, Nanotechnology) will playmore dominant roles as smaller features are fabricated.60 Ifsmaller features are to be generated, new fluids, lens mate-rials, and resist material with high index will be requiredor a new generation of lithography techniques such asextreme UV lithography (EUVL), which uses light withwavelengths in the range 1050 nm, needs to be introduced(see Nanoelectronics, Nanotechnology). If smaller wave-lengths of light and hence higher energy photons are used,it becomes deleterious to the materials used, such as thefocussing lenses, and resists layers.60

2.2 The bottom-up approach

Scientists curiosity to understand and mimic how bio-logical architectures are preprogrammed to self-assembleand self-organize into ordered, yet dynamic and functional,structures through supramolecular interactions (i.e., hydro-gen bonding, van der Waals, electrostatic, interactions,

hydrophilichydrophilic, and hydrophobichydrophobicinteractions), in nature has been a major inspiration towardthe development of the bottom-up approach. In fact,supramolecular chemistry provides an exciting tool thatcombines the concepts of self-assembly and molecularrecognition for the fabrication of three-dimensional intel-ligent nanodevices. Similar to nature, artificial systems areeither responsive to external stimuli, such as electrical,chemical/biochemical, temperature, and photons, or canpartake through intermolecular interactions with other iso-lated components to form functional materials. There area number of two-dimensional and three-dimensional self-assemblies as shown in Figure 2, which can be utilizedas the fundamentals for building novel nanotechnologi-cal devices. The nanofabrication and applications of thethree-dimensional self-assemblies in fields such as molec-ular diagnostics, therapy, and electronics are discussedin the articles to follow (see Self-Assembled Nanopar-ticles, Magnetically Responsive Self-Assembled Com-posite Materials, Supramolecular Nanoparticles forMolecular Diagnostics and Therapeutics, Advances inSupramolecular Chemistry of Carbon Nanotubes, Nan-otechnology) and hence will not be discussed further in thisarticle.

2.2.1 Two-dimensional thin solid filmsSelf-assembly of well-ordered two-dimensional ultrathinfilms on conducting, semiconducting, and insulating sur-faces provides a simple, cheap, and reproducible method ofobtaining nanoscale films with a wide variety of functionalgroups, which can be chemically manipulated.23, 6669 Thesesurfaces are increasingly being utilized as foundations forbuilding nanodevices such as sensors and for electronicapplications.23, 66, 67 Three types of ultrathin films used forsuch potential applications are LBFs,21, 70, 71 SAMs,20, 72 andtwo-dimensional supramolecular assemblies. The follow-ing discussion excludes two-dimensional supramolecularassemblies since they are covered in great detail in arti-cle Two-Dimensional Supramolecular Chemistry, Nan-otechnology.

LangmuirBlodgett films (LBFs)LBFs are the first practical example of ordered molecularassemblies.21, 70, 71 The approach involves the transfer ofmonolayers from a liquidair interface, which is denotedas a Langmuir film onto a solid substrate. The Langmuirfilms are produced by amphiphiles, which are moleculesthat have a hydrophobic end and a hydrophilic end.70, 71 Theamphiphile molecule is deposited onto a water subphaseavoiding the formation of multilayers in the Langmuir film.Initially, the distances between the molecules in the phaseare large relative to the molecular dimensions and the film



Self-assembly entities

Two-dimensionalultrathin films

Three-dimensionalassemblies

Self-assembledmonolayers

LangmuirBlodgettfilms

Micelles VesiclesMetallic,semiconducting,

magnetic, polymeric,supramolecular andhybrid nanoparticles

Two-dimensional supramolecular assemblies Carbonnanotubes

Nanorods Fullerenes

Figure 2 Schematic representation of examples of two-dimensional and three-dimensional self-assemblies.

is disordered and known as two-dimensional gas phase.The film is compressed to bring the molecules closer toeach other. The area per molecule decreases and the surfacepressure increases when the distance between the moleculesapproaches molecular dimensions. The profile betweenthe surface pressure against molecular area is known asthe pressure-area isotherm.70, 71 The pressure-area isothermprovides information on the stability of the monolayerformed, at the liquidair interface, the orientation of themolecules in the two-dimensional system, phase transitions,and conformational transformation.

There are two possible methods of transferring the mono-layers from the liquidair interface onto a solid sub-strate. The most conventional method that is used is thevertical deposition of the substrate (LangmuirBlodgettmethod).70, 71 The second method is the horizontal depo-sition of the substrate onto the Langmuir film, which isknown as the LangmuirSchaefer method.70, 71 In the Lang-muirBlodgett method, the monolayer is transferred ontothe substrate, as the substrate is either emersed (retractionor upstroke) or immersed (dipping or downstroke) into theLangmuir film, as shown in Figure 3(a).70, 71 When the sub-strate surface is hydrophilic, the monolayer is transferredas the substrate is retracted. However, if the substrate ishydrophobic the monolayer is transferred as the substrateis immersed. The speed at which the substrate is dippedand retracted is important for a quality film to be pro-duced. One of the unique benefits of LBFs is that mul-tilayers can be formed in a controlled manner. The number

of layers formed depends on the number of immersionsand emersions. There are three possible multilayer struc-tures that can be formed, which are X-, Y-, and Z-typemodes (Figure 3b). X and Z modes are formed only in thedownstroke and upstroke, respectively, whereas Y mode isformed both in the upstroke and downstroke and is the moststable and commonly formed mode.

In the LangmuirSchaefer method, a flat substrate isplaced horizontally onto a compressed monolayer on theliquidair interface. When the substrate is lifted horizon-tally and separated from the water subphase, the monolayeris transferred onto the substrate (Figure 4). The method isuseful to transfer viscous films as well as monolayers oflipids and proteins.73

Self-assembled monolayersSAMs are two-dimensional quasi-ordered molecular assem-blies, which are formed via adsorption of molecules fromsolution. SAMs have been increasingly used over LBFsas they offer a number of advantages.20, 72 First and fore-most, SAMs are more stable than the LBFs stemmingfrom the molecules being chemisorbed onto the substrate,whereas in LBFs the molecules are physisorbed. Secondly,SAMs provide more flexibility in the molecular designof the molecules because the molecules that form LBFsneed to be amphiphiles, whereas in SAMs such a need isnot required. Hence, the surfaces of the monolayers areeasily tunable, and a wide range of terminal functionali-ties have been studied (e.g., carboxylic acid, amine, nitro,


8 Nanotechnology

Substrate holder

(a)

(b)

X-type film

Hydrophobichydrophilicinteraction

Hydrophilichydrophilicinteraction

Y-type film

Hydrophilichydrophobicinteraction

Z-type film

Hydrophilic substrate

Figure 3 Schematic representation of (a) vertical deposition ofthe Langmuir film onto the solid substrate and (b) possiblemultifilms that can be formed by the vertical deposition process:X-type, Y-type, and Z-type.

hydroxyl, and methyl groups) and exploited to providecontrol over surface properties such as wettability23, 67 andadhesion.68, 69

The molecular structure of the adsorbate or surfactantmolecules can be divided into three components, headgroup, backbone, and terminal group (Figure 5). The headgroup is the anchor, which binds the adsorbate to thesubstrate. The choice of head group depends on thesubstrate used, as different groups have varied affinity forparticular substrates. The most common head groups arethiols (SH) on gold and silanes on silica substrates.20, 72 Thebackbone takes a major role in the molecular ordering andthe thermal stability of the SAM formed.70 The backboneconnects the head group with the terminal group and isgenerally made out of an aliphatic chain and/or aromaticcomponents. Each molecule in the SAM interacts withneighboring molecules through the backbone. Dependingon the groups in the backbone, the molecules can interactby van der Waals or interactions, leading to relativewell-ordered molecular layers. The terminal group is thesurface group, which plays a crucial role in the properties of

(a)

(b)

Substrate Direction of thebarrier movement

Barrier

Trough

Raising of thesolid substrate

(c)

Figure 4 Schematic representation of the LangmuirSchaefermethod. (a) Langmuir film, (b) the solid substrate is placedhorizontally on the Langmuir film, and (c) the solid substrate islifted with a LangmuirSchaefer film.

Head group

Surface group orterminal group

Backbone

Figure 5 Cartoon representation of the molecular structure ofan adsorbate and how they are typically tilted at an angle of inthe monolayer.

the surface such as wettability and corrosion. The terminalgroup also has an influence on the packing density in theSAM.71

Organosulfur-based SAMs Organosulfur-based SAMshave been extensively studied and are well understood.These types of molecules have shown to self-assemble ona number of metal surfaces including Au, Ag, Cu, Pt, andFe,74 the preferred surface being clean and hydrophilic. Auis the most commonly used for such type of SAMs, assulfur has a relative strong bond to Au. In addition, Audoes not have a stable oxide, hence is easy to handle in



SS

S

S

S

S

SS

SiSi

SiO

O

O

Si

SiO

Si

SiO

OOO

O OO OSiO

OOOO

(a) (b)

Figure 6 Representation of the structures of (a) alkanethiolateSAM on Au and (b) alkanesilane SAM on silica substrate, alsoshowing the cross-linking network between the molecules throughSiOSi bonds.

ambient conditions. Several different types of organosulfurmolecules have been shown to form well-ordered mono-layers on Au: alkanethiol,20, 72 dialkyl disulfide,75 dialkylsulfide,76 thioctic acid,77 and thiophene.78, 79 Alkanethiol-Au SAMs (Figure 6a) are the most studied and under-stood.

The organosulfur-Au SAMs are generally deposited from1 mM solution of the adsorbate molecule in an organicsolvent such as EtOH75, 76, 78 and CHCl3.79 The depositionprocess of alkanethiol-Au SAMs are well understood and isthought to go through four stages. The first stage involvesthe adsorbates physisorbing onto the surface (Figure 7a),followed by chemical bonds forming between the adsorbateand the substrate (Figure 7b). As the number of adsorbatesbound to the substrate increases, the adsorbates reorganizethrough van der Waals and other interactions to form islands(Figure 7c), eventually leading to the formation of a well-ordered SAM (Figure 7d). The molecules in the SAM arealways slightly tilted from the normal of the substrate(Figure 5) due to the optimization of the intermolecularinteractions between the molecules (i.e., van der Waalsinteractions) and the trans conformation in alkyl chains inthe backbone.

Organosilicon-based SAMs There are two types ofsilicon-based adsorbates studied, alkyltrioxysilanes(RSi(OR)3)80 and alkyltrichlorosilanes (RSiCl3).20 Theseare especially attractive for electronic applications becausethe SAMs can be formed on silica or silicon wafers(Si/SiO2). SAMs are formed on hydroxylated surfaces(usually the native oxide). The SAM is afforded via acondensation reaction between the organosilane, hydrox-ylated surface and the neighboring silane,80 due to theextensive cross-linking between the molecules (Figure 6b).The organosilane-Si/SiO2 SAMs are thermally more sta-ble than organosulfur-Au SAMs. The major drawback for

Adsorbatesphysisorb

Adsorbates chemisorbonto the substrate

Formationof islands

(b)

(a)

(c)

(d)

Self-assembled monolayer

van der Waalsinteraction

Figure 7 Schematic representation of the deposition pro-cess of alkanethiol on Au. (a) Physisorption, (b) chemisorption,(c) formation of islands, and (d) formation of SAM.

organosilane-based SAMs is their susceptibility to hydrol-ysis even in mild conditions.80, 81

Mixed SAMs Molecular level control over the densityand spatial distribution of functional groups on surfacesis important in a wide variety of applications, and, inparticular, for biomedical applications. In many areas ofmedical and biological research, the functional groupsmediate the immobilization of biomolecules to surfacesand their efficient immobilization not only requires thatbiomolecules preserve their activity after immobilizationbut also that biomolecules are presented on the surface at anoptimal density and spatial distribution, such that efficientbinding can occur between the immobilized biomoleculesand target species in solution. At present, mixed SAMsoffer the best option for controlling the density and spatialdistribution of the biomolecules on surfaces.

There have been a number of different approaches usedfor obtaining the mixed SAMs. The simplest approach isthe formation of a monolayer via deposition of a substratein a mixed solution that contains an active (containing thebinding functional group) and inert surfactant (acts as thespacer to distribute the functional group in the first sur-factant).82 This method is simple, but has some limitationssuch as the fact that the ratio of active surfactants in themonolayer is rarely identical to the ratio of active sur-factants in the solution due to the preferential adsorptionof one of the components, thus affecting control over the


10 Nanotechnology

density of the functional group on the mixed monolay-ers.83 Further, the formation of two-component monolayershas been reported by some authors8486 to lead to phase-segregated mixed SAMs, that is, formation of two distinctlocal domains, each of which was mostly composed of oneconstituent surfactant. In order to address some of theseissues, asymmetric dialkyl sulfide87 and dialkyl disulfide88SAMs have been prepared, which consist of the activefunctional moiety on only one of the chains bound tothe sulfur. Although this approach gives a more homo-geneous monolayer compared to the mixed SAMs men-tioned above, the spacing between the active moleculesis restricted. Recently, Tokuhisa et al.89 proposed usingdendrons as spacers. The dendrons were bound to lipoicacid through an ester linkage.89 After the formation of themonolayer, the dendrons were removed by hydrolysis underbasic conditions and subsequently, a second surfactant wasdeposited onto the vacant areas to form a mixed monolayer.The spacing between the active sites was controlled by thesize or generation of the dendron used. Novel approachesfor molecular level control over the functional groups onsurfaces will continue to aid in the development of moreadvanced surface materials.

Switchable SAMs Recently, there has been an increas-ing activity in fabricating stimuli-responsive SAM surfaces,where the surface properties are manipulated through exter-nal stimulus providing an on and off switch for regulat-ing the immobilization of biological and chemical partic-ulates (Figure 8).56 Such SAM surfaces potentially pro-vide a wide range of applications to many areas in sci-ence and technology, especially in the life sciences. TheseSAM surfaces enable modulation of biomolecule activ-ity, protein immobilization, and cell adhesion at the liq-uidsolid interface for applications including biofouling,cell culture, regenerative medicine, and tissue engineer-ing.56 This field is in its infancy and early examples ofswitchable biological SAM surfaces include surfaces that

switch between bioinert and bioactive states, under an exter-nal thermal-, photo-, chemical/biochemical-, or electrical-induced stimulus, to trigger capture or release of biologi-cal entities.56 However, existing switchable SAM surfacesrely mostly on nonspecific interactions (i.e., hydropho-bic/hydrophilic and electrostatic) of the biomolecules withthe active surface, thus lacking biospecificity and selec-tivity and substantially limiting the application potential-ities of such surface systems. There are relatively fewreported examples in which specific biomolecular inter-actions have been dynamically controlled in response toapplied stimuli.9092 Electro-switchable oligopeptide SAMsurfaces have been successfully used to reversibly controlbiomolecular interactions upon application of an electri-cal stimulus.92 In another example, a thermoresponsiveoligo(ethylene glycol) derivative has been exploited to con-trol the affinity binding between surface-tethered biotingroups and streptavidin.91 Albeit the progress and scien-tific advances in the field, exciting future developmentsare ahead of us. One of the major challenges in thefield of switchable biological surfaces today is the designof new and more versatile surfaces with tunable biospe-cific interactions. For a detailed discussion on the progressmade on stimuli-responsive surfaces, refer to the review byMendes.56

2.2.2 Hybrid approach: Combination of thetop-down and bottom-up approaches

Currently, a significant challenge in nanotechnology isto spatially self-organize self-assembled nanoscale compo-nents (such as nanoparticles, CNTs, proteins, cells, organicmolecules, polymers, etc.) onto surfaces to fabricate func-tional nanostructured systems for electronic, optoelectronic,or sensing applications.93, 94 Precise control over the rel-ative position and orientation of the nanocomponents isfrequently required in such systems to obtain useful prop-erties. Moreover, the integration and the stability of inter-faces to these nanostructures from the micron-length andmacroscopic scales are key to the success of future appli-cations.

Stimulus

Chemical /BiochemicalThermalElectricOptical

Inactive surface Active surface

Figure 8 Schematic representation depicting the range of stimuli and how they can be used to modulate the binding of particulateson stimuli responsive-based self-assembled monolayers.



Nanostructuring surfaces of macroscopic materials bytop-down nanolithography techniques, and subsequentlybuilding into the third dimension utilizing bottom-upself-organization of self-assembled nanoentities, is anattractive approach for creating such a bridge betweenmacroscopic systems and the nanoscale dimensions thatmany modern technologies demand. With this in mind,different hybrid top-down and bottom-up fabrication appr-oaches have been investigated to create three-dimensionalnanostructured surfaces.93, 95, 96 Several strategies rely onthe formation of an ultrathin film (e.g., SAMs) on the sur-face material of interest, followed by chemical transforma-tion/damage on the film caused by a lithographic technique(see Nanolithography, Nanotechnology), and finally theimmobilization of nanoscale components on the modifiedsurfaces through either covalent or supramolecular inter-actions to form functionalized three-dimensional nanos-tructures. SAMs are attractive ultrathin organic films forpatterning high-resolution features on a number of tech-nologically relevant surface substrates. The attraction ofthese systems as ultrathin resists is driven primarily becauseSAMs eliminate depth of focus, transparency issues andcan be prepared with a discrete number of well-definedchemical functional groups to permit further nanoscalematerials attachment. Currently, patterning SAMs can beachieved by several different types of lithographic tech-niques, which include (i) stamping or moulding methods,that is, soft lithography,64 (ii) scanning probe lithogra-phy (SPL)-based techniques such as dip-pen nanolithogra-phy,98, 99 nanografting,100, 101 and nanoshaving,102 and (iii)radiative techniques that include ultraviolet/visible (UV/vis)light, X-rays, and electron-beam. Generation of three-dimensional nanostructures by self-organization of vari-ous self-assembled nanoscale components onto nanopat-terned SAM surface templates has also been demon-strated.93, 9597 The integration of the top-down and bottom-up methodologies is representing a new paradigm for

creating nanostructured materials with high degrees ofcontrol, and is being actively pursued by a number ofgroups within the scientific and engineering communi-ties.9397

3 CONCLUSION AND OUTLOOK

Nanotechnology is going to play an increasingly importantrole in a number of sectors, addressing important issuessuch as health, energy, environment, transportation, water,food, and security. For instance, in medicine, nanotech-nology has a role to play in developing novel, highlyaccurate and sensitive early-stage diagnostic devices, aswell as providing novel methodologies for the treatmentof chronic (such as diabetes) or life threatening diseasessuch as cancer through gene therapy or drug delivery.Both the top-down, which relies on dimensional reductionthrough selective etching and various nanoimprinting tech-niques, and bottom-up methods, which assemble atoms ormolecules into nanostructured materials, in several casesthrough use of supramolecular chemistry, are at the heartof such developments. As a result of the numerous fun-damental breakthroughs made in the past three decades,neither the top-down nor the bottom-up approach is superiorat the moment; each has its advantages and disadvantages(Table 1).

The top-down nanofabrication has been used with obvi-ous success by the semiconductor industry for severaldecades now, with physicists and engineers manipulatingprogressively smaller pieces of matter by photolithogra-phy and related techniques, but the top-down approachis quickly reaching its physical and economic limits. Onthe other hand, the bottom-up nanofabrication offers ulti-mate limits of miniaturization, opens virtually unlimitedpossibilities concerning the design and construction of func-tional nanostructured materials, and has the potential to be

Table 1 Summary of the merits and drawbacks for the top-down and bottom-up approaches.

Top-down approach Bottom-up approach

Advantages Already well understood and have established techniques Self-assembly provides a simple, fast, and low-costmethod for producing nanostructured materials

Provides control and precision when patterning the surfacesthrough lithography

Offer ultimate limits of miniaturization

Procedures reproducible Opportunities open to the fabrication of a wider rangeof functional nanostructured materials by chemicalsynthesis

Disadvantages More sensitive to defects as features become smaller At present, the mastery of self-assembly is limitedto fairly simple nanostructured materials, not beingable, for example, to create integrated devices

Tighter tolerance as features become smallerMore expensive as compared to the self-assembly methods


12 Nanotechnology

more cost-effective than top-down nanofabrication. How-ever, at present our ability to build nanostructured materialsfrom the bottom-up approach is fairly limited in scope,even though chemical synthesis has been developed to abreathtaking level of sophistication. The integration of thesophisticated techniques of the top-down and bottom-upapproaches is an exciting development, which is offeringa unique opportunity to fabricate complex nanostructurednanomaterials with high degrees of control and signifi-cantly expanding the possibilities of nanofabrication andfunctions.

We can peer into the future with reasonable confi-dence. We can be confident that we will witness manybreakthroughs based on bottom-up approaches in the nextdecades, leading to nanostructured materials with noveland unique material properties and functionalities, and toincreasingly sophisticated nanodevices. While current indi-cations are that bottom-up nanofabrication methods will notcompletely replace top-down nanofabrication techniques, inthe decades to come we will see more applications originat-ing either from bottom-up techniques alone or from hybridapproaches combining the strengths of bottom-up and top-down methods.

ACKNOWLEDGMENTS

The authors thank the Leverhulme Trust (F/00094/AW) fortheir support.

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