Synthesis and Fabrication
Top-down
Mechanical Approaches
Mechanical cleavage (aka the Scotch tape method) uses adhesive tape to peel thin layers off a bulk crystal. It
works by weakening van der Waals interactions without breaking in-plane covalent bonds. This method was
famously used by Andre Geim and Konstantin Novoselov to isolate graphene and win the 2010 Nobel Prize in
Physics. It has since been used on a number of other 2D nanomaterials (e.g. BN nanosheets, TMDs).
Liquid exfoliation disperses layer materials into solvents in order to separate the constituent layers. This
process may occur by:
Mechanical force: Sonication or shear mixing;
Ion intercalation: Insertion of ions (typically Li+) between layers, weakening van der Waals forces;
Ion exchange: Replaces intercalated ions, resulting in further destabilization;
Redox: Introduces defects (e.g. converting graphite into graphene oxide), enabling dispersion in solution;
Selective etching: Chemical removal of regions to release sheets.
Lithography and Direct Writing
Photolithography: coating a substrate w/ light-sensitive resist (called a photoresist), then exposing it to UV
light through a photomask. exposed or unexposed region (depending on whether a positive or negative
photoresist is used) is dissolved to create a pattern. pattern can be transferred to underlying layers via
etching or deposition.
Electron beam lithography (or EBL) writes patterns directly onto a resist-coated surface with an electron
beam. It is less precise than photolithography and much slower, since the beam has to scan through the
pattern.
Focused ion beam (or FIB) uses a narrow beam (typically Ga+) to sputter atoms directly from a surface. This
directly sculpts the material, eschewing the need for resist layers.
Nanoimprint lithography (or NIL), also known as stamp lithography, uses a patterned stamp (typically
PDMS), to mechanically press nanoscale features into a resist. Soft lithography is a class of related
techniques, where these stamps are used for a range of techniques:
Microcontact printing (or uCP): raised stamp is inked with molecules, then transferred onto a substrate,
forming a self-assembled monolayer;
Replica molding (or REM): A liquid polymer is cast using a mold with a relief pattern, cured, then peeled off
the mold;
Micromolding in capillaries (or MIMIC): A liquid polymer is run through microfluidic channels by exploiting
capillary action.
Etching and Pattern Transfer
Wet etching uses solvents to dissolve material away from exposed areas of substrate. Most wet etching is
isotropic, removing from all directions equally, but some can be anisotropic (in particular, KOH etching of Si).
Dry etching uses ions to remove substrate, allowing for far greater control and precision over wet etching.
The most common form of dry etching is reactive ion etching (or RIE), which uses a chemically reactive
plasma.
Bottom-up
Chemical vapor deposition (CVD) grows films and nanostructures by reacting gas-phase precursors (volatile
compound of a material to be deposited) with other gases and produce a nonvolatile solid that deposits
atomistically on a heated substrate. is like baking with gases.primary method of synthesizing carbon
nanotubes (CNTs), graphene, and Transition Metal Dichalcogenide (TMD) monolayers.
Atomic layer deposition (ALD) is a subclass of CVD, which deposits material one monolayer at a time by
alternating surface reactions. These alternating dose and purge steps provides sub-nanometer thickness
control when synthesizing thin films.
Sol-gel processing converts molecular precursors (typically metal alkoxides) into networks via hydrolysis and
condensation. Solvent is then removed from the resulting gel through calcination. The sol-gel method is the
primary means of synthesizing ceramic nanoparticles (e.g. silica, alumina, titania, zirconia).
Hydrothermal and solvothermal methods perform reactions in sealed autoclaves. Hydrothermal methods
use water, solvothermal methods use organic solvents. high temperature+pressure of the autoclave
promotes supersaturation of starting materials, enabling crystallization of nanostructures that would be
unstable under ambient conditions. Solvothermal methods: primary method of synthesizing MOFs.
Molecular self-assembly sees molecules spontaneously organize into ordered structures through non-
covalent interactions (e.g. H-bonding, pi stacking, van der Waals forces). Common systems include: self-
assembled monolayers (SAMs), DNA origami and block copolymer (BCP) self-assembly.
Colloidal synthesis of nanoparticles begins by dissolving precursors within an organic solvent with ligands,
which are converted to monomers that eventually rise in concentration, before exceeding a critical
supersaturation. followed by nucleation, growth and termination steps. can be tuned to produce particles
with a narrow size distribution. Colloidal synthesis is commonly used to synthesis CdSe and PbS quantum
dots, as well as various nanocrystals.
Molecular beam epitaxy (MBE) directs atomic /molecular beams (often from Knudsen cells) in ultra-high
vacuum onto a substrate. offers atomically precise growth of crystal thin films, with extremely high control
over stoichiometry and doping. is central to the synthesis of semiconductor layers in transistors. Growth
during monitored using reflection high-energy electron diffraction (RHEED).
Vapor-liquid-solid (VLS) nanowire growth employs a catalytic droplet (often Au), which absorbs vapor
precursors until supersaturated, before crystallizing into a nanowire at the liquid-solid interface. Related
vapor-solid-solid (VSS) techniques use solid catalysts. VLS is the primary mode of synthesizing Si and Ge
nanowires.
Characterization
Zero-dimensional
Nanoparticles (NPs) are particles w/ all three dimensions <100 nm.
extremely high surface area to volume ratio makes them highly
effective in applications such as catalysis and drug delivery. Their
properties often differ significantly from that of their bulk-scale
counterparts.
Examples: Gold nanoparticles (AuNPs), silver nanoparticles
(AgNPs), iron oxide nanoparticles
Applications: Catalysis, drug delivery, medical imaging
Quantum dots (QDs) are semiconductor nanocrystals. They have
discrete energy levels due to quantum confinement.
Examples: CdSe, CdTe, PbS, carbon dots, perovskite QDs
Applications: QD-LED TVs, fluorescent imaging probes
One-dimensional
Nanowires (NWs): crystalline wires with nanometer-scale diameters
but lengths that can range up to the micrometer scale.
Examples: Si, Ge, GaN, ZnO, InP, SnO2
Applications: Optoelectronics, sensors
Nanotubes are hollow cylindrical nanostructures with nanometer-
scale diameters. They are classified as either single-walled or multi-
walled. Nanotubes are of interest for their tensile strength, thermal
conductivity and electrical properties.
Examples: Carbon nanotubes (CNTs), boron nitride nanotubes
(BNNTs)
Applications: Electrodes, drug delivery
Nanorods are rod-shaped nanocrystals that are shorter than
nanowires. They exhibit anisotropic properties. Controlling their
aspect ratio allows for tuning of their optical and plasmonic behavior.
Examples: Gold nanorods, ZnO nanorods
Applications: Catalysis, solar cells
Two-dimensional
Graphene is a single, atomic layer of carbon atoms in an sp
hybridized lattice. extremely high strength, conductivity,
transparency and electron mobility. Common derivatives include
graphene oxide (GO) and reduced graphene oxide (rGO).
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Applications: Electrodes, flexible electronics, energy storage,
biosensors, membranes
Hexagonal boron nitride (h-BN) has graphene-like structure, but with
alternating B and N atoms. It has a wide bandgap and its stability and
smoothness makes it a common substrate for graphene/TMD devices.
Applications: Substrate for 2D heterostructures, lubricants,
coatings
Transition metal dichalcogenides (TMDs) are 2D semiconductors with
formula MX (M = Mo, W; X = S, Se, Te). Each layer of M atoms is
sandwiched between two layers of X atoms. Unlike the electrically
conductive graphene, TMDs have an adjustable bandgap.
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Applications: Spintronics, optoelectronics
MXenes are transition metal carbides and nitrides with formula
M X T (M = Ti, V, Nb; X = C, N; T = -OH, -O, -F, etc.). They are formed
from selective etching of MAX phases.
n+1 n x
Applications: Energy storage
Three-dimensional
Nanoporous materials are solids with nanometer-scale pores. Their
high surface area makes them highly effective adsorbents and
catalysts.
Examples: Zeolites, metal-organic frameworks (MOFs), porous
carbons
Applications: Catalysis, sensing, drug delivery
Nanocomposites are hybrid materials, in which a nanoscale filler is
dispersed in a bulk matrix. This improves mechanical strength and
thermal stability.
Examples: Polymer (polymer + clay platelets, CNTs, graphene,
etc.), metal metrix (nanoparticles or CNTs reinforcing metals),
ceramic (reinforced ceramics)
Applications: Automotive/aerospace materials, conductive
plastics, flame retardants, medical implants
Hierarchical nanostructures are materials that combine multiple
length scales.
Examples: Core-shell aggregates (nanoparticles forming porous
spheres), branched nanostructures (nanotubes/nanowires growing
from a backbone)
Applications: electrodes, photocatalysis, superhydrophobic
coatings, tissue scaffolds
Van der Waals Forces
Weak electric forces attract neutral molecules.
Present in gasses, liquefied/solidified gasses, and most organic
liquids/solids.
Named after Dutch physicist Johannes Diderik van der Waals
(1873).
Postulated while developing a theory on real gas properties.
Solids held by van der Waals forces have lower melting points and
are softer compared to those held by ionic, covalent, or metallic
bonds.There are four main types of structures that are covered in
Material Science: FCC, BCC, HCP, and SC.
Face Centered Cubic
Face centered cubic. Callister, W. D., & Rethwisch, D. G. (2018).
Materials Science and Engineering: An Introduction.
Also known as cubic packed
Atoms are located at each of the 8 corners as well as in the
centers of each of the 6 faces
Follows an ABCABC close packing pattern - there are 3 repeating
layers, where the atoms of the third layer are located above holes
in the first and second layers
Densest of the cubic packing arrangements, with an atomic
packing factor of 0.74
Each unit cell contains 4 atoms and has a side length of A = 4R√2
Each atom in the matrix has a coordination number of 12
Body Centered Cubic
Materials Science and Engineering: An Introduction.
Atoms are located at each of the 8 corners as well as in the center
of the cubic cell
Less dense than FCC, with an atomic packing factor of 0.68
Each unit cell contains 2 atoms and has a side length of A = 4R√3
Each atom in the matrix has a coordination number of 8
Microscopy
Light microscopy or optical microscopy uses visible light to
magnify samples through glass lenses. The resolution limit of a
standard light microscope is given by the Rayleigh criterion. For
visible light, the diffraction limit (~200 nm) makes it impossible to
directly image nanoscale features using light microscopy. However,
advanced forms of light microscopy allow for higher resolution.
Optical Contrast Methods
Bright-field microscopy: Standard transmission microscopy
Good for stained, opaque materials
Very poor for transparent materials (like many nanomaterials)
Resolution diffraction limited
Dark-field microscopy: Blocks direct light with a special condenser,
only collecting scattered light
Good for unstained, transparent materials
Very sensitive to small features that scatter light
Insensitive to phase shifts (i.e. transparent regions)
Can visualize nanoscale structures/features, well below the
diffraction limit
Phase-contrast microscopy (PCM): Phase plate in the objective to
convert phase shifts from transparent samples into intensity
differences
Excellent for unstained, transparent materials
Can visualize edges without staining
Often produces halos and other imaging artifacts
Can visualize colloids, thin films, hydrogels
Differential Interference Contrast (DIC) microscopy:
Interferometry converts differences in sample index of refraction
into intensity differences
High-contrast, sharp images, with fewer halos than PCM
Better than phase-contrast for thick or complex samples
Images appear like a 3D relief
Can visualize nanostructured surfaces/membranes
Fluorescence Methods
Widefield microscopy: Entire field illuminated, all fluorophores
emit simultaneously
Pros: Fast, simple, high-throughput
Cons: Blurring out of focal plane, poor depth penetration, out-of-
focus light (from out-of-plane fluorophores)
Confocal microscopy: Out-of-focus light is rejected using a laser /
pinhole setup
Pros: Sharp images at specific depths (can be reconstructed with
Z-stacking), pinhole reduces out-of-focus light
Cons: Photobleaching, slower
Multiphoton microscopy: Two (or more) photons are absorbed
simultaneously by the fluorophore
Pros: Deeper penetration, less photobleaching
Cons: Expensive, complicated, slower
Super-Resolution Microscopy
Stimulated Emission Depletion Microscopy (STED): Fluorophores
around the focal area are deactivated using a donut-shaped beam,
shrinking the excitation area
Pros: Real-time imaging, no special fluorophores
Cons: Needs strong laser, complicated and expensive optics
Resolution: ~20-50 nm
Photoactivation Localization Microscopy (PALM) & Stochastic
Optical Reconstruction Microscopy (STORM): Only activate a few
fluorophores at a time, but repeat many times (thousands), then
reconstruct into a single image
Pros: Single-molecule sensitivity
Cons: Slow, needs special dyes
Resolution: ~10-20 nm
Structured Illumination Microscopy (SIM): Multiple images are
taken with different illumination patterns, generating Moire fringes
that encode high-frequency information, which are reconstructed
into a single image
Pros: Fast, relatively gentle, compatible with most fluorophores
Cons: Lower resolution improvement compared to STED or
PALM/STORM, reconstruction is computationally expensive
Resolution: ~100 nm
Simple Cubic
Simple cubic is a very basic arrangement, only containing atoms at each
corner of the unit cell
SC is the least dense, with an atomic packing factor of 0.52
Each unit cell contains 1 atom and has a side length of A = 2R
Each atom in the SC matrix has a coordination number of 6
Atomic Packing and Coordination Number
The atomic packing factor (APF) is a measure of how efficiently atoms are
packed within a unit cell of a crystal structure. It is defined as the ratio of the
total volume of atoms within a unit cell to the volume of the unit cell itself. To
find the APF, one assumes that atoms are hard spheres and calculates the
volume occupied by these spheres within the unit cell. The formula for APF
is: APF = (volume of atoms in a unit cell) / (total unit cell volume). For
example, in the face-centered cubic (FCC) structure, with four atoms per unit
cell, the APF is 0.74, meaning that 74% of the unit cell's volume is occupied by
atoms, while the rest is empty space.
The coordination number refers to the number of nearest-neighbor atoms
surrounding a given atom in a crystal structure. It indicates how many other
atoms are in direct contact with an atom. To find the coordination number,
look at the arrangement of atoms in the unit cell and count the nearest
neighbors. In the FCC structure, for example, each atom has 12 nearest-
neighbor atoms, which can be visualized as four atoms surrounding it on the
same plane, four atoms in the plane above, and four in the plane below. In
contrast, the body-centered cubic (BCC) structure has a coordination
number of 8, as the central atom is in contact with the eight corner atoms of
the unit cell.
Hexagonal Close Packing
Science and Engineering: An Introduction.
Another close-packed arrangement
Composed of two hexagons of 6 atoms each, an additional atom in the
center of each hexagon, and a triangle of atoms in between the two
hexagons
Differs from FCC in that HCP follows an ABAB packing pattern - there are
only 2 repeating layers, where the atoms of the third layer are located
above the atoms of the first layer, not above gaps
Has an atomic packing factor of 0.74, the maximum possible
Each unit cell contains 6 atoms and has two parameters, A (side length) and
B (height)
Each atom in the matrix has a coordination number of 12
Crystalline:
Crystalline solids are composed of atoms, molecules, or ions arranged in an ordered pattern
extending in all three spatial dimensions
Large crystals are identifiable by their macroscopic geometrical shape, with flat faces and
specific, characteristic orientations
Crystal structures are formed by repeating units called unit cells
Poly-Crystalline:
Semi-crystalline structures have both crystalline and amorphous properties, also known as
semi-crystalline structures
These structures contain true crystal portions with mixed sizes and orientations
Semi-crystalline solids are heavily bonded but lack the rigidity and constant structure of
fully crystalline solids
Almost all metals and many ceramics are polycrystalline
Amorphous:
Amorphous structures have little to no crystal properties
They possess short-range order but have significantly less chain linkage compared to
crystalline structures
Common types of amorphous solids include gels, thin films, and glass
