Docsity
Docsity

Prepare for your exams
Prepare for your exams

Study with the several resources on Docsity


Earn points to download
Earn points to download

Earn points by helping other students or get them with a premium plan


Guidelines and tips
Guidelines and tips

Fabrication Methods in Nanoscience: Top-Down and Bottom-Up Approaches, Transcriptions of Bioethics

An overview of fabrication methods used in nanoscience, distinguishing between top-down and bottom-up approaches. Top-down methods employ physical processes to form nanomaterials from bulk materials, while bottom-up methods involve chemical reactions to build materials from the atomic or molecular level. various thermal and chemical fabrication methods, including annealing, electrohydrodynamic atomization, electrospinning, extrusion, and chemical etching.

What you will learn

  • Which chemical fabrication methods are discussed in the text?
  • How does electrohydrodynamic atomization (EHDA) contribute to the production of nanoparticles?
  • What role does annealing play in the formation of nanocrystallites and the transformation of nanomaterials?
  • What is the difference between top-down and bottom-up fabrication methods in nanoscience?
  • Which thermal fabrication methods are discussed in the text?

Typology: Transcriptions

2019/2020

Uploaded on 11/21/2021

zico3434
zico3434 🇺🇸

4 documents

1 / 17

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
FABRICATION METHODS
Manufacturing takes place in very large facilities. If you want to build a computer
chip, you need a giant semiconductor fabrication facility. But nature can grow
complex molecular machines using nothing more than a plant.
RALPH MERKLE
Chapter 4
48058_C004.indd 17748058_C004.indd 177 3/29/2008 5:32:53 PM3/29/2008 5:32:53 PM
pf3
pf4
pf5
pf8
pf9
pfa
pfd
pfe
pff

Partial preview of the text

Download Fabrication Methods in Nanoscience: Top-Down and Bottom-Up Approaches and more Transcriptions Bioethics in PDF only on Docsity!

FABRICATION M ETHODS

Manufacturing takes place in very large facilities. If you want to build a computer

chip, you need a giant semiconductor fabrication facility. But nature can grow

complex molecular machines using nothing more than a plant.

RALPH M ERKLE

Chapter 4

178 Introduction to Nanoscience

THREADS

Characterization methods have been presented, addressed, and discussed, albeit without providing significant detail. The catalog nature of chapter 3 is deliberately extended into this chapter, which is the last chapter in the “Nanotools” division of the text. Because reference is made continually to vari- ous kinds of fabrication techniques throughout the text, it is prudent to place introductory material concerning fabrication early in the book. In this way, the student should be able to establish a level of comfort with, perspective on, and understanding of fabrication methods when the subjects emerge

time and time again later in the text. The physics division of the text—chapter 5 through chapter 8— engages the study of nanomaterial properties and phenomena. Please take note that the fabrication methods listed in this chapter are but a few of the multi- tude that actually exist. We have tried to catego- rize in a generic sense the major forms and tried to illustrate the processes with commonly prac- ticed fabrication techniques. Much can be learned about nanomaterials by understanding how they are made.

4.0 FABRICATION OF NANOMATERIALS

There is nothing more gratifying, arguably, than holding in one’s hand the physical

manifestation of an idea, concept, or theory. The link between the idea, concept, or

theory and its physical form is the process of fabrication. The fabrication process

begins in a laboratory with atomistic simulations, experiments, mock-ups, and

prototypes. Eventually, after a battery of testing, the physical embodiment of the

idea, concept, theory, simulation, mock-up, and prototype makes it way into a

manufacturing facility. We have already listed several characterization methods. It

is now time to discuss the fabrication of nanomaterials.

Nanomaterials are made by two generalized processes: top down (e.g., sub-

traction from bulk starting materials) or bottom up (e.g., addition of atomic or

molecular starting materials). Each scheme has a unique set of advantages and

disadvantages. We recommend that you make a checklist of the advantages, dis-

advantages, limitations, and issues confronting each method as we discuss them

through the course of this chapter.

We also add a brief section on molecular modeling, which is a fabrication

tool. It is part of the design process. Molecular modeling has become one of the

most powerful tools in nanoscale research, development, and material design.

There exists a perfect fit between simulation and nanomaterials since atoms and

molecules in nanoscale materials are finite and countable, and computer capa-

bility in this day and age is still limited with regard to capacity. Depending on

the quality of input parameters, molecular simulation is able to generate an

accurate rendition of nanoscale material behavior. Low-energy states, structure,

dynamical behavior, chemical reactions, fluxes and flows, and more have been

modeled with some form of atomistic-molecular simulation.

4.0.1 Background

Like anything else that we present in this text, boundaries are drawn for the

sake of convenience and clarity, although sharp ones are not always possible.

180 Introduction to Nanoscience

chemical and physical techniques developed over millennia. Engineers tend to

manufacture components from the top down and then assemble them to make

a device. Chemists, on the other hand, have always made materials by reacting

atoms and molecules to form chemicals in bulk quantities—from the bottom-up.

Chemical synthesis is by definition a bottom-up process. With regard to the

biological processes, all structures are formed from the bottom up. Are you able

to think of any exceptions to this rule?

The convergent nature of nanotechnology is well represented by fabrication

methods. Engineers, physicists, chemists, and biologists respectively bring top-

down, top-down, bottom-up, and bottom-up methods to the same table. The

future of fabrication will require more cooperation between and among the

disciplines, and the design parameters of future nanofabs must include such for-

ward thinking in order to accommodate diversity and to enhance interaction

among all the participants.

It is not practical to build an automobile engine from the bottom up and,

conversely, it is not practical to synthesize aspirin from the top down. However,

in nanotechnology, similar structures can be built from either fabrication

perspective [2]. Features on a silicon wafer can be produced by a standard top-

bottom procedure called lithography (bulk wafer → application of a photoresist

layer → mask-UV exposure → etch) or by a bottom-up procedure (bulk wafer →

polymer or seed crystals → self-assembly) [2]. Once again, nanotechnology and

nanoscience are changing the way we do things and fabrication methods are no

exception.

4.0.2 Types of Top-Down Fabrication Methods

We begin our catalog of fabrication methods with top-down methods. Physical

fabrication techniques are considered to be mostly from the top down. Top-down

methods are extremely diverse. Nanomaterials are formed from the top down

by mechanical-energy, high-energy, thermal, chemical, lithographic, and natural

methods.

Top-Down Mechanical-Energy Fabrication Methods. Cutting, rolling, beating,

machining, compaction, milling, and atomization comprise a few examples of

mechanical methods used to produce nanomaterials from the bulk. A mechanical

method employs a physical process that does not involve chemical change—

according to the traditional definition of chemical change (a reaction). Beating

metals into a thin film is an ancient mechanical procedure used by the Egyptians

and other pre-Hellenistic cultures to make swords, spear tips, and ornamental

coatings. Mechanical energy methods such as ball milling operate on the principle

of mechanical attrition. Kinetic energy, translated by hard, high-speed pellets, is

imparted to samples by collision and friction. Samples are ground into fine

powders by this method. An overview of mechanical top-down methods is shown

in Table 4.1.

Top-Down Thermal Fabrication Methods. In the purest sense, a thermal fabri-

cation method employs a physical process (heating) that does not initiate

a chemical change in the sample—according to the traditional definition

of chemical change (a reaction). Once again, it has proven difficult to place

Fabrication Methods 181

specific thermal methods into this category. Some of the top-down mechanical

methods also involve thermal exchange. During the ball mill process, heat is

obviously generated and plays a role in the outcome of the nanomaterial

structure. Heat may be deliberately added during ball milling. Several compaction

methodologies involve heating of samples during processing. The methods

listed in Table 4.2, although extremely diverse, involve direct and deliberate

heating of the sample during the fabrication process; chemical change may

happen or not.

The process of combustion occurs in the presence of oxygen and causes a

chemical change. Thermolytic and pyrolytic methods imply a process called

“lysis” (from the Greek lysis, “a loosening, setting free, releasing, dissolution,”

from lysein, “to unfasten, untie”) and usually involve chemical changes to the

starting materials. Pyrolysis is the conversion of one material into another mate-

rial by the application of heat in the absence of oxygen. Thermolysis or thermal

reaction is often used synonymously with pyrolysis. Nanomaterials are routinely

formed during the combustion of bulk organic materials. In the absence of

oxygen, polyaromatic hydrocarbons (PAHs) with nanometer dimensions are

formed by chemical reaction in pyrolytic processes. Sublimation, on the other

hand, is the process of a solid phase of a material becoming a gaseous phase

without experiencing an intermediate liquid phase (top down).

Top-Down High-Energy and Particle Fabrication Methods. High-energy sources

such as electric arcs, lasers, solar flux, electron beams, and plasmas are commonly

used to produce nanomaterials from the top down. A by-product of high-energy

methods is superheating: a desirable or undesirable outcome depending on the

objective. Although heat is produced during operation, these are not labeled as

thermal methods because the origin of the heat is not a conventional thermal

source per se. Table 4.3 lists several commonly used high-energy methods that

TABLE 4.1 Top-Down Mechanical-Energy Fabrication Methods

Method Comments Ball milling Production of nanoparticles by mechanical attrition to produce grain size <5 nm [3] High-energy ball milling uses steel balls to transfer kinetic energy by impact to the sample. Highly polydisperse products and contamination are problems. Rolling/beating Traditional mechanical methods to minimize material thickness and refi ne structure. Gold can be beat into a 50-nm thick fi lm [4]. Extrusion; drawing

High-pressure processes of forcing materials in a plastic phase through a die to form high-aspect ratio parts like wires. Bi metal forced through nanopore alumina is an analogous process at the nanoscale and can be considered a thermal–mechanical process. Mechanical Machining, polishing, grinding, and ultramicrotome

Also known as conventional machining; resolution limit: 5 μm [5] Other techniques analogous to mechanical machining perform the same function with laser beams, focused ion beams, and plasmas. Mechanical grinders/cutters are used to thin TEM samples. These include dimple grinders, diamond saws, ultrasonic disc cutters, and ultramicrotomes (<100 nm sections). Compaction; consolidation

Metal powder ball milled and compacted. Powders are considered to be bulk materials; therefore, compaction of powders to form bulk material is not considered to be a bottom-up method. Atomization Conversion of a liquid into aerosol particles by forcing through a nozzle at high pressure

Fabrication Methods 183

result in nanomaterials. Evaporation, a thermal method based on resistive heating,

is considered to be a crossover technique in that a bulk material is converted

into small particles (molecules or clusters)—a top-down process—that are then

deposited to form a nanomaterial (thin film)—a bottom-up process.

Top-Down Chemical Fabrication Methods. If chemical transformations occur

during a fabrication process, we shall designate that process as a chemical fabri-

cation method. Although fabrication (a.k.a. synthesis) methods that employ

chemical procedures rightfully reside within the domain of the bottom up, there

are several that can be considered to be top down. Combustion is an ambiguous

TABLE 4.3 Top-Down High-Energy and Particle Fabrication Methods

Method Comments Arc discharge High-intensity electrical arc discharge directed on a graphite target (anode) + catalyst to produce single-walled carbon nanotubes that accumulate on the cathode Temperature ~4000 K [15,16] Laser ablation High-intensity laser beam directed on a graphite target + catalyst to produce single-walled carbon nanotubes; sample warmed to 1200–1500°C by furnace, laser Sample is collected on water-cooled copper collector [17]. This process can be considered to be a thermal and a high-energy method. Solar energy vaporization

Solar energy focused on graphite target + catalyst to produce single-walled carbon nanotubes Temperature ~3000+K [18] RF sputtering Ion bombardment of metal, oxide, or other material targets to form thin fi lm coatings Usually performed under moderate vacuum (10–3^ torr). Atoms, molecules, and clusters are formed by this process. Ion milling Argon ion plasma is used to subtract material from a surface. The purpose is to clean surface or remove (thin) materials for TEM. No change in the chemical nature of the sample happens during this process. Electron beam evaporation

This is similar to evaporation in Table 4.2 but uses an electron beam source to heat material. Evaporated material condenses on target substrate. High vacuum is required. Thin-layer antirefl ection, scratch-resistant coatings are formed by this technique. Reactive ion etching

Sensitive materials are etched by reactive chemical species in charged plasma. Chemical change of the etched material takes place during this process. The etching process is guided by maskant materials. Pyrolysis Pyrolysis can also be considered a high-energy method. Application of high-energy source like fi re to bulk hydrocarbon materials (like a steak) in the absence of oxygen creates polyaromatic hydrocarbons (PAHs)—a top-down process (or if considering intermediates—for example, carbon atoms—it can be considered to be a bottom-up process). Pyrolysis of solid refractory nanoscale materials like Si–C–N substrate to form nanotubes at 1500–2200°C is a crossover technique [19]. Large-scale synthesis of multiwalled carbon nanotubes occurs in fl ame environments by burning carbon sources such as methane, ethylene, or benzene. Combustion Combustion can be considered to be a high-energy, thermal, or chemical fabrication method. High-energy sonication

Ultrasonication uses high-energy sound waves to make nanomaterials from bulk materials. The technique is also used to disperse carbon nanotubes in a suitable solvent. The dispersion of bundles of nanotubes into individual tubes is top down. Probe tips are made of titanium, vanadium, and other metals and alloys. Micron- to nanosized residual tip metal is introduced into solutions during the sonication process.

184 Introduction to Nanoscience

chemical top-down method, depending on the starting material. The chemical

structure of solid constituents is completely altered following a combustion

process. Nanosized PAHs and fly ash are by-products of a top-down pyrolysis

process, e.g., the burning of coal.

Chemical etching of solid substrates like a silicon wafer (masked or other-

wise) is a top-down chemical method. Chemical etching processes, on the other

hand, adhere to a slightly different classification criterion—specifically, that

chemical alteration occurs only in the layers exposed to, and subsequently

removed from, the solid substrate. In other words, although nanofacets or

porous structures are formed on or within the solid substrate, the chemical

structure of the solid substrate remains intact. Only the surface is altered (passiv-

ated, oxidized). The process of chemical alteration is only applicable to substrate

material removed during the etching process, e.g., transformation of the solid

into a water-soluble oxide.

Anodizing is a chemical etching process that involves electricity (e.g., electro-

chemical etching). This process is a crossover technique and consists of four

parts.

1. Metal is electrochemically removed top down from the surface and

released into solution in ionic form, Al 3+, during the anodizing of

aluminum metal. The cationic products of anodizing are not nano-

materials; they are ions.

2. Hexagonally distributed, monodisperse scalloped structures (nano-

facets) are formed on the surface of the aluminum anode during

anodizing. The diameter and curvature of individual nanoscale scallops

are dependent on the applied anodic voltage. This is a true example of

top-down fabrication. The other two parts of the anodize equation

are bottom-up procedures.

3. The reaction of metal cations with anions originating from the cathode

reaction or with solution anions leads to the formation of nanoscale

colloidal oxides that eventually form the porous layer (from the bottom

up). Anionic species include oxides, hydroxides, and other negatively

charged species (phosphates, sulfates, oxalates, or chromates).

4. The hexagonal porous anodic oxide layer is formed from the bottom

up by the electrochemical reaction of Al 3+^ cations with various oxide

anions. The scalloped top-down metal surface structures direct the

size, orientation, and distribution of the bottom-up pore channels.

Overall, if we had to choose we should probably consider anodizing as a top-

down fabrication process. Top-down chemical fabrication methods are listed in

Table 4.4.

Top-Down Lithographic Fabrication Methods. Many powerful top-down tech-

niques involve some form of lithography. Lithographic techniques are what

made the integrated circuit industry what it is today, and it continues to be the most

viable method to form nanostructures that actually has widespread applications.

The history of lithography was presented briefly in chapter 1. Traditionally,

electromagnetic sources ranging from the visible wavelengths are still the most

popular—especially in MEMS and circuit fabrication. Ultraviolet and x-ray

sources are increasingly in demand as smaller features are required. Electron

186 Introduction to Nanoscience

TABLE 4.5 Top-Down Lithographic Fabrication Methods

Method Comments LIGA techniques LIGA is a German acronym for “Lithographie Galvanoformung Abformung,” a microlithographic method developed in the 1980s. It was one of the fi rst major techniques to demonstrate the fabrication of high-aspect ratio structures. Beam sources include x-ray, ultraviolet, and reactive ion etching. MEMS devices are fabricated using LIGA techniques. Photolithography Light is used to transfer patterns onto light-sensitive photoresist substrates. Photolithography is primarily used in the manufacture of integrated circuits and MEMS devices. The wavelength range of optical lithography techniques ranges from the visible to the near ultraviolet—ca. 300 nm. The resolution of photolithography techniques is ~100 nm [20]. Immersion lithography

Just like with immersion optical microscopy, resolution can be enhanced by 30–40% with application of a liquid medium between the aperture and the sample with higher refractive index. The medium needs to conform to the following criteria: (1) refractive index n > 1, (2) low optical absorption at 193 nm l , (3) immersion fl uid compatible with the photoresist and the lens, and (4) be noncontaminating. Deep ultraviolet lithography (DUV)

Resolution with deep ultraviolet with l = 248–193 nm, resulting in features on the order of 50 nm

Extreme ultraviolet lithography (EUVL)

Short wavelength ultraviolet, l = 13.5 nm. EUVL resolution: ~30 nm [20]. The major problem with EUVL is that all matter absorbs EUV and damage to substrates is very likely. High vacuum is required and mask must be made of Mo–Si. X-ray lithography (XRL)

X-rays are produced by synchrotron sources. XRL is capable of producing features down to 10 nm. Problems include damage to substrate materials. Electron beam lithography (EBL)

An electron beam source is used instead of light to generate patterns. Although e-beams can be generated below a few nanometers, the practical resolution is determined by the electron scattering of the photoresist material. Just like in SEM, electron interaction volumes are generated during exposure. Line width <20 nm and electron energy: 10–50 ke V Electron beam writing (EBW)

EBW is a direct-writing procedure and, therefore, no pattern masters are required. Direct-write e-beam resolution is ca. 20 nm with lateral dimensions <10 nm [20]. Operation of electron beam parameters and patterning are computer controlled. Electron beam projection lithography (EPL)

This technique is similar to TEM in that electrons are focused through a lens and projected onto a surface. In this case, however, a pattern is placed near the aperture. EPL is a high-throughput technique. A diamond membrane is used as stencil mask material. The process is not limited by diffraction as are the photolithographic techniques [21]. Focused ion beam lithography (FIBL)

Utilizes a liquid metal ion source (LMIS) with beam size of 10+ nm. FIBL resolution is 30 nm [20]. There is less backscattering than EBL and FIBL resists are more sensitive. FIBL, however, is restricted by limitations in reliable ion sources, diffi culty in focusing, shorter penetration depth, swelling of resist, and whimsical ion implantation episodes. FIBL is also more expensive and slower that optical methods. Microcontact printing methods

The George Whitesides group at Harvard University invented the lithographic method of microcontact printing. A topographical master is created by standard lithographic techniques that employ electron, ion, or electromagnetic beams. A negative replica of the master is made by pouring an elastomeric polymer, usually polydimethylsiloxane (PDMS), over the master. Upon curing, the elastomer is removed and coated with a self-assembled monolayer such as hexadecanethiol. Application of gold then reproduces the master pattern. Sub-100 nm features are possible by this technique [20]. Nano-imprint lithography (NIL)

Nano-imprint lithography is used to fabricate nanometer-scale patterns. It is a straightforward economical process with high throughput and high resolution. Patterns are created by stamping a resist material with a prefabricated stamp. The stamp can be used over and over. There are two types: thermoplastic (TNIL) [22,23] and photo (PNIL) [22,23]

Fabrication Methods 187

Method Comments

Nanosphere lithography (NSL)

NSL is used to fabricate nanometer-scale patterns. It is a straightforward economical process with high throughput and high resolution. It is diffi cult to categorize this technique as top down or bottom up. Micron-scale latex spheres are often used as the template material. The interstices are nanoscale in size. NSL utilizes nanospherical materials in close-packed confi guration as a mask to aid in the fabrication of periodic particle arrays (PPAs). Polymer nanospheres (diameter <300 nm) are in a single or double layer over insulator, semiconductor, metal, inorganic ion insulator, or organic π-electron semiconductor materials. Depending on the sphere diameter, nanoscale facets on the order of 22 nm are easily formed [24]. Scanning AFM nanostencil

An evaporated particle beam source is focused through a hole in an AFM cantilever. The procedure is good for metal deposition. This technique combines the ability to pattern a surface simultaneously with the ability to image the surface with the same cantilever. It is diffi cult to classify this technique as top down or bottom up (e.g., as it is for the thermal evaporation technique discussed before).

Scanning probe nanolithographies

There exist several forms of scanning probe nanolithographies. Some impart mechanical stress via the probe tip to a sensitized surface, followed by a chemical treatment; others apply an STM current to a substrate to create dangling bonds that react further to produce nanofeatures. These methods can be considered as top down in that nanofacets and features are produced from a solid bulk substrate. 2-Photon polymerization

Photopolymerization causes polymer to solidify to form three-dimensional image. Resolution of ~120 nm, although the laser l is 780 nm [25]. This is, in the clearest sense of the term, a top-down process.

TABLE 4. (CONTD.)

Top-Down Lithographic Fabrication Methods

TABLE 4.6 Top-Down Natural Fabrication Methods

Method Comments Erosion Conversion of macroscopic mineral-based materials into micro- and nanoparticles.

Etching Etching of silicate rocks by carbonic acid from the environment contributes to erosion. Hydrolysis The decomposition of organic (and inorganic) matter by hydrolysis is a common way to make nanomaterials in the natural world. Volcanic activity Formation of fl y ash and other materials by volcanic activity. The dispersion of volcanic byproducts is mostly airborne. Volcanic by-products contribute to the formation of clays like montmorillonite (a nanostructured material discussed in chapter 13).

Forest and brush fi res

Formation of combustion gases, nanometer scale PAHs, amorphous carbons, and particulates

Solar activity Radiation degradation of bulk synthetic, inorganic, and organic materials

Pressure and temperature

Formation of diamond crystallites from pressure and temperature processes applied to bulk materials; application to bulk carbon deposits (coal) Biological decomposition

Decomposition is a process that begins at the bulk, micro-, or nanoscale level and terminates at the nano, molecular, or atomic level. Biological decomposition is mitigated by bacterial and other life forms in addition to inorganic natural processes.

Digestion Reduction of bulk biological materials into nanometer and subnanometer scale components by the action of acids and hydrolysis; the formation of nitrogenous wastes is a bottom-up procedure, so to speak.

Fabrication Methods 189

Bottom-up lithography methods are limited to a few kinds, based on template

processes or direct writing ( Table 4.9 ).

Bottom-Up Biological and Inorganic Fabrication Methods. Biological processes

are overwhelmingly formed from the bottom up ( Table 4.10 ). More detail is

allotted to this topic in chapter 14.

TABLE 4.7 Bottom-Up Gas-Phase Fabrication Methods

Method Comments Chemical vapor deposition (CVD)

CVD involves the formation of nanomaterials from the gas phase, usually at elevated temperatures, onto a solid substrate or catalyst. Carbon nanotubes are formed by catalytic decomposition of carbon feedstock gas in inert carrier gas at elevated temperature. Single-walled carbon nanotube production by CVD requires nanoscale Fe, Co, or Ni catalyst plus Mo activator on high surface area support (alumina) at >650°C. Methane gas serves as the carbon source [26]. Atomic layer deposition (ALD)

ALD is an incredibly precise sequential surface chemistry layer deposition method to form thin fi lms on conductors, insulators, and ceramics. The layer formed by ALD conforms to surface topography. Precursor materials are kept separate until required. Atomic scale control pinhole-free layers are formed. Al 203 layers are generated from hydroxylated Si substrate + Al(CH 3 ) (^) 3(g), then H 2 O vapor is applied to remove methyl groups. The process is repeated until a target thickness is attained. Layer thickness: 1–500 nm Combustion The formation of Si nanoparticles from the combustion of SiH 4 (silane gas) and other silicon-containing gases like hexamethyldisiloxane under low-oxygen conditions produces Si nanoparticles as small as 2 nm. Al 2 O 3 and TiO 2 can also be formed by combustion. Thermolysis; pyrolysis

Solid Si nanoparticles can also be formed by the thermal decomposition of silane gas in the absence of oxygen. The bottom-up decomposition of ferrocene to form Fe nanoparticles is one of the best examples of a bottom-up gas-phase fabrication method. Metal oxide (MOCVD) Organometallic vapor phase epitaxy (OMVPE)

Chemical characteristics of precursor materials utilize reactive gas-phase-organometallic compounds that decompose to form nanometer-scale thin fi lms or nanoparticles. H 2 carrier gas, group III metal–organic compounds + group V hydrides 500–1500°C at 15- to 750-torr pressure are representative conditions under which MOCVD is performed. Molecular beam epitaxy (MBE)

MBE is a thin fi lm growth process conducted under high vacuum. A heated Knudsen cell or effusion cell is used to introduce reactants by molecular beams. MBE is able to deposit one atomic layer per application. Examples include alternate layers of GaAs and AlGaAs with each layer of 1.13 nm in thickness and InGaAs quantum dots [27]. The temperature used in MBE is commonly 750–1050°C in H 2 carrier gas. Ion implantation This is a tough method to categorize. Nanovoids, for example, can be created by ion implantation of Cu ions into silica and subsequent annealing [28]. It is bottom-up action performed on a bulk material. If the ions come from a bulk source, it has a bottom up component. Once the ions are formed, ion implantation is bottom up. Gas phase condensation; thermolysis

Formation of Fe nanoparticles by decomposition of ferrocene at 200°C is an example of gas-phase process to form nanoscale Fe. Formation of lithium nanoclusters by decomposition of LiN 3 is another example [7]. Temperature at decomposition depends on the material. Solid template synthesis

Provides a solid template substrate for gas-phase deposition of materials on the solid substrate. This is considered to be a mixed bottom-up system. Final nanomaterial size, shape, and orientation are predetermined by template parameters.

190 Introduction to Nanoscience

TABLE 4.8 Nonbiological Bottom-Up Liquid-Phase Fabrication Methods

Method Comments Molecular self-assembly

This generic process is supported in liquid media. From some perspectives, supramolecular chemistry is a subset of molecular self-assembly. Almost all molecular self-assembly takes place in liquids. The liquid plays a major role in supporting intermolecular interactions and intermediate metastable species. Supramolecular chemistry

Supramolecular chemistry, for reasons to be explained in chapter 11, is conducted in liquid media. Weak intermolecular forces are supported in liquids that allow many kinds of intermolecular interactions to take place. All signifi cant biological metabolic processes occur in a liquid medium. Nucleation and sol–gel processes

Precursor chemicals in a supersaturated state combine by self-assembly or chemical reaction to form seed particles. Thermodynamics drives a nucleation process that forms nanoparticles. The nucleation process depends on prevailing conditions of pH, temperature, ionic strength, and time [5]. Due to van der Waals attractions, colloids are formed. Sol–gel methods are irreversible chemical reactions of homogeneous solutions that result in a three-dimensional polymer. Sol–gel methods yield nanostructured materials of high purity and uniform nanostructures formed at low temperatures [5]. Negative replicas of colloidal hierarchical structures, upon drying, yield aerogels or xerogels. Such gels can be back-filled to produce nanocomposites or hybrid materials [5]. These are all pure bottom-up processes. Reduction of metal salts

Noble metal clusters and colloids are formed by the reduction of metal salts like HAuCl 4 and H 2 PtCl 6. Common reducing agents come in the form of organic salts like sodium citrate— Na3C 6 H 5 O7. By means of phase transfer reactions (consisting of an interface between two immiscible liquids), metal clusters and colloids are stabilized by the addition of organic ligands. For example, phosphine or thiols are adsorbed onto gold-55 to produce a stable cluster [29]. Single-crystal growth Nucleation process to form single crystals in liquid media Electrodeposition Electroplating

Electrodeposition is direct deposition of metals from metal salt solutions to form thin layers or fi lms on a solid conducting substrate. Electrodeposition is an electrolytic process that forms thin metal fi lms on the cathode of the cell. The process conforms to Faraday’s law. Electroless deposition Electroless deposition is the autocatalytic deposition of metals without electrical assistance. It requires metal cation + catalytic (activated) surface + reducing agents like formaldehyde, alkali diboranes, alkali borohydrides, or hypophosphorous acid. Pt, Ni, Co, Au, and numerous other metals can be deposited on many kinds of substrates, including plastics. Electroless deposition has been used to create negative or positive replicas of porous nanostructures [30]. Anodizing We have already characterized anodizing as a top-down process. We mentioned earlier that anodizing method contains a top-down component (formation of scalloped structure). Here, we focus on the bottom-up formation of the porous alumina. Aluminum metal is made the anode in an electrolytic cell consisting of a polyprotic acid (usually sulfuric, phosphoric, or oxalic). Pore diameter of <5 nm → >200 nm; with pore density: 20–80+% and fi lm thickness: <1 μm → > 100 μm. Anodized titanium several nanometers thick generates bright interference colors. Electrolysis in molten salt solutions

Utilization of molten alkali halide salts with graphite electrodes with 3- to 5-A current [31] Erosion at the cathode to form tubes The product is transferred to toluene. Solid template synthesis

Provides a solid template substrate for electrochemical, chemical, polymerization, and other liquid-phase reactions. Most methods are accomplished in a liquid medium. Final nanomaterial size, shape, and orientation predetermined by template parameters. Liquid template synthesis

Liquid templates (micelles and reverse micelles) are commonly used to make quantum dots from the bottom up. Supercritical fl uid expansion

Solvent removal under hypercritical conditions forms aerogels and xerogels that contain nanometer-sized voids. Supercritical conditions imply that the medium is in neither liquid nor solid phase.

192 Introduction to Nanoscience

TABLE 4.10 Bottom-Up Biological and Inorganic Fabrication Methods

Method Comments Protein synthesis Formation of proteins from precursor amino acids by elaborate process of protein synthesis Transfer RNA transports amino acids to ribosomal RNA and link with peptide bonds. Nucleic acid synthesis Synthesis of nucleic material (RNA, DNA) from sugars, phosphate, and nuclides (adenosine, guanine, cytosine, and thymine) from the bottom up The processes of mitosis and meiosis are template (replication) methods. Membrane synthesis Bottom-up agglomeration of lipids, phospholipids to form organized membrane structures that make life possible Inorganic biological structures

Mother of pearl (nacre) 95% Inorganic aragonite (platelets 200–500 nm thick) + organic biopolymer Deformable nanograins [38] Crystal formation methods

Nucleation depends on P, T, concentration, and composition. Flaws reduce surface energy by nucleation. Direction of growth depends on nanostructure.

defects that have nanoscale dimensions. Are these considered to be “nanomate-

rials” or nanofacets? Or are they merely nanodomains of the bulk type material?

Voids formed by ion implantation do agglomerate to form nanovoids from the

bottom up. We address this nebulosity in more detail later.

4.0.5 The Nano Perspective

There are many kinds of nanomaterials. When discussing fabrication methods,

it is essential that the nature of the end product be understood. For example,

some types of nanomaterials retain their nanoscale dimensions (e.g., quantum

dots). Others form into components of more complex structures (e.g., one-

dimensional, two-dimensional, or three-dimensional arrays of quantum dots).

In these instances, the quantum dot retains its identity as a unique nanomaterial.

In other cases, nanomaterials form the structure of an integrated bulk material.

An example of a bulk material that is composed of nanostructured components

is a Cu–Fe alloy in which nanodomains of one or the other metal exist within a

bulk material. Steel made of nanosized grains has better mechanical properties

than steel made of micron-sized grains.

Silk, collagen, elastin, and keratin tissue found in animals are composed of a

hierarchy of increasingly larger structures [39]. The hierarchy begins with sub-

nanometer materials and ends with a functional macroscopic material [39,40].

The relationship of nanostructure, muscle fibers, and connective tissue is shown

in Table 4.11. A similar table can be created for bone tissue and other organ

systems in animal bodies. From the purely structural point of view, it is clear

that nature begins from the bottom up to build any kind of macroscopic

functional material.

Fabrication of inorganic nanomaterials is bottom up, but some well-known

methods such as erosion certainly operate from the top down. The construction

of a snowflake is a nucleation process that emphasizes eccentricities in the unit

cell of each snowflake, a bottom-up process. With regard to nanoscience

and technology, materials are constructed from the top down, bottom up, or a

Fabrication Methods 193

combination of both. Although some fabrication categories are placed in one of

the two major types, note that several cross borders. Lithographic techniques,

when taking in the whole, are a combination of several techniques.

There are numerous challenges facing any kind of nanofabrication technique.

According to George Whitesides of Harvard [41], “In almost all applications of

nanostructures, fabrication represents the first and one of the most significant

challenges to their realization.” As with any process, there are advantages and

disadvantages. Problems with top-down approaches include alteration of the

surface structure during the process [42,43]. Lithography is a method that is

capable of causing undesirable changes to the crystal structure and more damage

from subsequent chemical etching steps. Reduction in conductivity and genera-

tion of excess heat due to surface imperfections in nanowires, for example, could

be problematic [43]. Top-down methods can be extremely energy intensive

because energy is required to create new surfaces (chapter 6). Nanoscale materi-

als made from bottom-up methods may lack long-range order and structural

integrity. There are more disadvantages.

We started our civilization with stone, bone, and wood and then metal and

ceramics. These materials are classified as hard matter. Biological materials like

leather and gut, materials based on biological soft matter sources, were also put

to good use. With semiconductors, advanced alloys, and other hard materials,

the tradition continued. The advent of plastics, polymers, pharmaceuticals, and

other organically based materials ushered in a new era of chemistry: the chemis-

try of the covalent bond. We have entered the age of soft matter.

4.1 TOP-D OWN FABRICATION

Top-down approaches remove, reduce, subtract, or subdivide a bulk material

to make nanomaterials. Top-down methods, therefore, are considered to be

subtractive. Top-down fabrication methods logically reside within the realm of

TABLE 4.11 The Nanostructure of Tendons

Structural component Dimensions Description/function Amino acids (^) <1-nm The building blocks of proteins Collagen 1.5-nm Diameter Primary structure polypeptide (the protein of connective tissue) Triple-helix coil (tropocollagen) 1.5-nm Diameter; 300 nm length

Three polypeptide strands form a cooperative quaternary structure. Microfi brils Subfi bril Fibril

<4-nm Diameter 10–20-nm Diameter 50–500-nm Diameter Connective tissue called^ endomysium^ based on collagen subunits that surround muscle fi brils Fascicle 50–300 μm Bundle of muscle fi brils (~10) surrounded by perimysium (connective tissue) [40]. The endo-, peri-, and epimysia converge to form the tendon. Tendon 10–50 cm Attachment of muscle to bone support structure Provides fl exibility and strength