Intermediate Nanotechnology Engineering interview questions

150 questions with answers (NAN-151 to NAN-300). Try each one aloud before reading the answer.

Nanoscience Fundamentals

NAN-151 · How do you calculate the fraction of atoms at the surface of a nanoparticle?

Approximately, surface fraction ≈ 4d_atom ÷ D for a sphere of diameter D, from the shell volume divided by the total volume. For a 3 nm particle with 0.3 nm atoms this is about 40%; for 30 nm it is about 4%.

NAN-152 · Calculate the specific surface area of 10 nm silica spheres (density 2.2 g/cm³).

SSA = 6 ÷ (ρD) = 6 ÷ (2200 kg/m³ × 10 × 10⁻⁹ m) ≈ 2.7 × 10⁵ m²/kg, or about 270 m²/g.

NAN-153 · Why do nanoparticles tend to agglomerate?

Their high surface energy makes the dispersed state thermodynamically unstable, and van der Waals attraction pulls particles together. Reducing total surface area by clustering lowers free energy unless electrostatic or steric stabilization opposes it.

NAN-154 · What is the Hall-Petch relationship and how does it change for nanocrystalline metals?

Yield strength rises as grain size falls: σ_y = σ₀ + k ÷ √d. Below roughly 10–20 nm the trend weakens or reverses (inverse Hall-Petch) because grain-boundary sliding and diffusion replace dislocation pile-up.

NAN-155 · What is the Gibbs-Thomson effect?

Curved surfaces raise the chemical potential by about 2γV_m ÷ r. Small particles therefore have higher solubility and vapour pressure and lower melting point than large ones.

NAN-156 · What is Ostwald ripening?

Growth of large particles at the expense of small ones, because small particles have higher solubility (Gibbs-Thomson) and dissolve while material redeposits on larger particles. It broadens and coarsens size distributions over time.

NAN-157 · What forces dominate at the nanoscale compared with the macroscale?

Surface forces such as van der Waals, electrostatic, capillary and hydrogen bonding dominate, while gravity and inertia become negligible because they scale with volume (L³) and surface forces scale with area or length.

NAN-158 · What is the Reynolds number regime for nanoparticles moving in fluids?

Re = ρvL ÷ µ is far below 1, so flow is viscous-dominated (Stokes flow). Inertia is irrelevant and motion stops almost instantly when driving force is removed.

NAN-159 · What is Brownian motion and why does it matter for nanoparticles?

Random motion caused by collisions with solvent molecules. For nanoparticles it dominates over sedimentation, keeping colloids suspended and governing diffusion, aggregation rates and DLS measurements.

NAN-160 · What is the Stokes-Einstein equation?

D = k_BT ÷ (6πηr), where D is the diffusion coefficient, η the viscosity and r the hydrodynamic radius. It links diffusion to particle size.

NAN-161 · What is DLVO theory?

A model of colloidal stability that sums van der Waals attraction and electrical double-layer repulsion between particles. A high energy barrier prevents aggregation; adding salt compresses the double layer and lowers the barrier.

NAN-162 · What is the Debye length?

The characteristic thickness of the electrical double layer around a charged surface. In water at 25 °C it is about 0.304 ÷ √I nm (I in mol/L), so about 1 nm at 0.1 M and 9.6 nm at 1 mM for a 1:1 salt.

NAN-163 · What is steric stabilization?

Stabilizing colloids by attaching polymers or long ligands to particle surfaces. When particles approach, the overlapping chains create osmotic and entropic repulsion, preventing aggregation even at high ionic strength.

Quantum & Size Effects

NAN-164 · What are the energy levels of a particle in a 1D infinite square well?

Eₙ = n²h² ÷ (8mL²), with n = 1, 2, 3… Energy scales as 1/L², so shrinking the well raises the levels and widens their spacing.

NAN-165 · Calculate the ground-state energy of an electron in a 1 nm infinite well.

E₁ = h² ÷ (8mL²) = (6.626 × 10⁻³⁴)² ÷ (8 × 9.11 × 10⁻³¹ × 10⁻¹⁸) ≈ 6.0 × 10⁻²⁰ J ≈ 0.38 eV.

NAN-166 · What is the Brus equation?

An effective-mass estimate of quantum dot bandgap: E(R) ≈ E_g,bulk + (ħ²π² ÷ 2R²)(1/mₑ* + 1/mₕ*) − 1.8e² ÷ (4πε₀εᵣR). The confinement term grows as 1/R² and dominates the Coulomb term at small R.

NAN-167 · What is the difference between strong and weak confinement?

Strong confinement occurs when the particle radius is much smaller than the exciton Bohr radius, so electron and hole are confined separately. Weak confinement occurs when the radius is somewhat larger, and the exciton moves as a whole with quantized centre-of-mass motion.

NAN-168 · How does tunnelling probability depend on barrier width?

For a rectangular barrier, T ≈ exp(−2κd), where κ = √(2m(V − E)) ÷ ħ. Every ångström of extra width cuts transmission by a large factor, which is why STM and gate-oxide leakage are so thickness-sensitive.

NAN-169 · What determines the colour of a gold nanoparticle colloid?

The localized surface plasmon resonance. Spherical Au particles of about 10–50 nm absorb near 520 nm, appearing red; larger particles, aggregates or rods shift the resonance to longer wavelengths, giving purple or blue.

NAN-170 · How does nanoparticle shape affect plasmon resonance?

Anisotropic shapes split the resonance into modes. Gold nanorods have a transverse mode near 520 nm and a longitudinal mode that red-shifts into the near-infrared as aspect ratio increases.

NAN-171 · What is the superparamagnetic blocking temperature?

The temperature below which a nanoparticle's magnetization appears stable over the measurement time. It relates to the anisotropy barrier KV: Néel relaxation τ = τ₀ exp(KV ÷ k_BT). Smaller volume gives a lower blocking temperature.

NAN-172 · What is the charging energy of a quantum dot, and when is Coulomb blockade observable?

E_C = e² ÷ 2C (or e²/C for adding an electron, depending on convention). Blockade is observable when E_C ≫ k_BT (about 26 meV at 300 K) and tunnel resistance exceeds h/e² ≈ 25.8 kΩ.

NAN-173 · What is the Landauer formula?

Conductance of a mesoscopic conductor G = (2e²/h) Σ Tₙ, summing transmission probabilities of each conduction channel. For perfect transmission it gives integer multiples of the conductance quantum.

NAN-174 · Why does thermal conductivity drop in nanostructures?

Phonons scatter at boundaries and interfaces when dimensions approach the phonon mean free path, reducing their effective mean free path. Silicon nanowires can have thermal conductivity tens of times lower than bulk silicon.

NAN-175 · How does nanoparticle size affect catalytic activity?

Smaller particles have more surface atoms and more low-coordinated edge and corner sites, which often bind reactants more strongly. Gold, inert in bulk, becomes catalytically active for CO oxidation below about 5 nm.

NAN-176 · What is the mean free path of electrons, and why does it matter for nanoscale wires?

The average distance an electron travels between scattering events, about 40 nm in copper at room temperature. When wire width approaches it, surface and grain-boundary scattering raise resistivity above bulk.

Top-Down Fabrication & Lithography

NAN-177 · What is the Rayleigh resolution criterion for lithography?

Minimum feature (critical dimension) CD = k₁λ ÷ NA, where λ is wavelength, NA numerical aperture and k₁ a process factor (practical minimum about 0.25 for single exposure).

NAN-178 · Calculate the resolution for 193 nm immersion lithography with NA = 1.35 and k₁ = 0.3.

CD = 0.3 × 193 ÷ 1.35 ≈ 43 nm.

NAN-179 · What is depth of focus in lithography?

DOF = k₂λ ÷ NA². Increasing NA improves resolution but shrinks depth of focus quadratically, demanding flatter wafers and thinner resists.

NAN-180 · How does immersion lithography improve resolution?

Filling the gap between the lens and wafer with water (n ≈ 1.44 at 193 nm) allows numerical apertures above 1, up to about 1.35, reducing the minimum printable feature.

NAN-181 · What is multiple patterning?

Splitting a dense pattern into two or more exposures or using spacer techniques to print features finer than a single exposure allows. Examples are LELE (litho-etch-litho-etch) and self-aligned double or quadruple patterning.

NAN-182 · What is self-aligned double patterning (SADP)?

A spacer film is deposited conformally on a patterned mandrel, etched back to leave sidewall spacers, and the mandrel is removed. The spacers define lines at half the original pitch.

NAN-183 · What is anisotropic etching of silicon with KOH?

KOH etches the (100) and (110) planes much faster than (111) planes, so on (100) wafers it forms V-grooves and pits bounded by (111) sidewalls at 54.7° to the surface.

NAN-184 · What is etch selectivity?

The ratio of etch rates of the target material to the mask or underlying layer. High selectivity lets thin masks survive and stops etching cleanly on an etch-stop layer.

NAN-185 · What is the proximity effect in electron-beam lithography?

Backscattered and secondary electrons expose resist outside the intended area, widening or merging nearby features. It is corrected by adjusting dose per shape (proximity correction).

NAN-186 · What limits electron-beam lithography throughput?

It writes pattern elements serially with a single beam, so writing time scales with area and pattern density. A full wafer can take many hours, so it is used for masks, prototypes and research.

NAN-187 · What is optical proximity correction (OPC)?

Deliberate modification of mask shapes, such as serifs, hammerheads and sub-resolution assist features, to compensate for diffraction and process effects so printed features match the design.

NAN-188 · What is a phase-shift mask?

A mask that changes the phase of transmitted light by 180° in selected regions, creating destructive interference at feature edges that sharpens image contrast and improves resolution.

NAN-189 · What is sputtering and how does it differ from evaporation?

Sputtering ejects target atoms by ion bombardment in a plasma, giving good adhesion, step coverage and alloy composition control. Thermal or e-beam evaporation heats source material in high vacuum, giving directional, purer films suited to lift-off.

Bottom-Up Synthesis

NAN-190 · How does chemical vapour deposition (CVD) work?

Precursor gases are delivered to a heated substrate, adsorb, react or decompose on the surface, and deposit a solid film or nanostructure while by-products desorb and are pumped away. Variants include LPCVD, PECVD and MOCVD.

NAN-191 · What drives self-assembly?

Minimization of free energy through non-covalent interactions: hydrogen bonding, van der Waals forces, electrostatics, π–π stacking and hydrophobic effects. Structures form spontaneously when the ordered state is thermodynamically favoured.

NAN-192 · What is the LaMer model of nanoparticle formation?

Monomer concentration rises above a critical supersaturation, triggering a short burst of nucleation that lowers concentration below the nucleation threshold. Existing nuclei then grow uniformly, giving narrow size distributions.

NAN-193 · What is hot-injection synthesis?

Rapidly injecting precursors into a hot coordinating solvent to create instantaneous supersaturation and a single nucleation burst, followed by controlled growth. It is used for monodisperse quantum dots such as CdSe.

NAN-194 · What is the critical nucleus radius in classical nucleation theory?

r* = 2γV_m ÷ (k_BT ln S), where γ is surface energy, V_m molecular volume and S supersaturation. Nuclei smaller than r* dissolve; larger ones grow. Higher supersaturation gives smaller critical nuclei.

NAN-195 · How does PECVD differ from thermal CVD?

Plasma-enhanced CVD uses a plasma to dissociate precursors, so deposition occurs at much lower substrate temperatures, often 200–400 °C. This suits temperature-sensitive substrates but can incorporate hydrogen and plasma damage.

NAN-196 · What is the vapour-liquid-solid (VLS) growth mechanism?

A metal catalyst droplet, such as gold, absorbs vapour-phase precursor until supersaturated, then precipitates solid crystal at the droplet-substrate interface. The droplet rides on top, so the nanowire diameter follows the droplet size.

NAN-197 · What is the difference between ALD and CVD?

ALD separates precursor exposures with purges, so surface reactions are self-limiting and growth is about one monolayer or less per cycle, giving atomic-level thickness control and conformality. CVD supplies precursors together and grows continuously, faster but less conformal.

NAN-198 · What is block copolymer self-assembly?

Two chemically distinct polymer blocks joined covalently microphase-separate into periodic domains (spheres, cylinders, lamellae) typically 10–100 nm, set by molecular weight and the χN product. It is used as a lithographic template.

NAN-199 · What is DNA origami?

Folding a long single-stranded DNA scaffold into a designed 2D or 3D shape using hundreds of short staple strands that bind by base pairing. It positions nanoparticles and molecules with nanometre precision.

NAN-200 · What parameters control nanoparticle size in colloidal synthesis?

Precursor concentration, reducing agent strength, ligand type and concentration, temperature, reaction time, pH and mixing rate. These set the nucleation-to-growth balance.

NAN-201 · What is the Stöber process?

A sol-gel synthesis of monodisperse silica spheres by hydrolysis and condensation of tetraethyl orthosilicate in ethanol and water with ammonia as catalyst. Sizes from tens of nanometres to about 2 µm are tuned by reagent ratios.

NAN-202 · What is epitaxy, and what is lattice mismatch?

Epitaxy is growth of a crystalline layer that follows the substrate's crystal orientation. Lattice mismatch f = (a_film − a_sub) ÷ a_sub creates strain; beyond a critical thickness it relaxes by misfit dislocations or island formation.

Carbon Nanomaterials (CNT, Graphene, Fullerenes)

NAN-203 · What are graphene's key properties?

Very high carrier mobility (above 10⁵ cm²/V·s when suspended or encapsulated), Young's modulus about 1 TPa, intrinsic strength about 130 GPa, thermal conductivity of several thousand W/m·K, about 2.3% optical absorption per layer and impermeability to gases.

NAN-204 · What is the difference between single-walled and multi-walled CNTs?

SWCNTs are one rolled graphene sheet, 1–2 nm in diameter, and are metallic or semiconducting depending on chirality. MWCNTs are concentric tubes spaced about 0.34 nm, usually metallic in behaviour, larger, cheaper and easier to make.

NAN-205 · What is the chiral vector (n, m) of a CNT?

The vector Cₕ = na₁ + ma₂ on the graphene lattice that wraps around the tube circumference. It defines diameter and helicity: (n, 0) is zigzag, (n, n) is armchair and others are chiral.

NAN-206 · How do you predict whether a CNT is metallic or semiconducting?

If (n − m) is divisible by 3, the tube is metallic (or a very small-gap semiconductor, except armchair which is truly metallic); otherwise it is semiconducting. Statistically about one-third of tubes are metallic.

NAN-207 · Calculate the diameter of a (10, 10) CNT.

d = (a ÷ π)√(n² + nm + m²) with a = 0.246 nm. √(100 + 100 + 100) = 17.32, so d ≈ 0.246 × 17.32 ÷ π ≈ 1.36 nm.

NAN-208 · How does the bandgap of a semiconducting SWCNT depend on diameter?

It scales inversely with diameter, approximately E_g ≈ 0.8 eV·nm ÷ d (some sources use about 0.7–0.9). A 1 nm tube has a gap of roughly 0.8 eV.

NAN-209 · How is graphene grown by CVD on copper?

Methane and hydrogen flow over copper foil near 1000 °C. Carbon decomposes on the surface, and because carbon solubility in Cu is very low, growth is self-limiting to mostly a monolayer. The film is then transferred using a polymer support.

NAN-210 · How does Raman spectroscopy identify graphene layers?

Graphene shows a G band near 1580 cm⁻¹ and a 2D band near 2700 cm⁻¹. Monolayers have a single sharp, symmetric 2D peak about twice as intense as G; more layers broaden the 2D peak and lower its ratio. A D band near 1350 cm⁻¹ indicates defects.

NAN-211 · Why does graphene have a linear band structure near the Dirac point?

Its two equivalent sublattices produce conduction and valence bands that touch at the K and K′ points with E = ±ħv_F|k|, where v_F ≈ 10⁶ m/s. Carriers behave as massless Dirac fermions.

NAN-212 · What is the radial breathing mode in CNT Raman spectra?

A low-frequency mode (about 100–350 cm⁻¹) from uniform radial vibration of the tube. Its frequency is inversely proportional to diameter, roughly ω ≈ 248 ÷ d (cm⁻¹, d in nm), so it is used to measure SWCNT diameter.

NAN-213 · How are CNTs dispersed in liquids or polymers?

By ultrasonication with surfactants (such as SDS), polymer wrapping, or covalent functionalization such as acid oxidation to add carboxyl groups. Covalent routes improve dispersion but can damage electrical properties.

NAN-214 · What is liquid-phase exfoliation of graphene?

Sonicating or shear-mixing graphite in solvents with matching surface energy, such as NMP, or in surfactant solutions, then centrifuging to separate few-layer flakes. It is scalable but gives small, multilayer flakes.

NAN-215 · What is the isolated pentagon rule for fullerenes?

Stable fullerenes have every pentagon surrounded by hexagons, because adjacent pentagons create high strain. C₆₀ is the smallest fullerene satisfying it, and C₇₀ is the next.

Nanoparticles & Quantum Dots

NAN-216 · How does quantum dot size relate to emission wavelength?

Smaller dots have larger bandgaps and emit at shorter wavelengths. For CdSe, dots of about 2 nm emit blue-green and about 6 nm emit red.

NAN-217 · Why are quantum dots coated with a ZnS shell?

ZnS has a wider bandgap than CdSe, confining carriers in the core and passivating surface trap states. This raises quantum yield and photostability and reduces Cd²⁺ leaching.

NAN-218 · What are type-I and type-II core-shell quantum dots?

In type-I, the shell bandgap straddles the core's, confining both electron and hole in the core (e.g. CdSe/ZnS). In type-II, band edges are staggered, separating electron and hole into different regions, red-shifting emission and lengthening lifetime.

NAN-219 · What is photoluminescence blinking in quantum dots?

Random switching of a single dot between bright and dark states, attributed to charging by Auger ionization or surface trapping. Thick shells and good passivation suppress it.

NAN-220 · How do you estimate the number of atoms in a gold nanoparticle?

N = (D ÷ d_atom)³ × packing, or more simply N = ρ × (πD³/6) × N_A ÷ M. For a 5 nm Au particle (ρ = 19.3 g/cm³) this gives about 3,800 atoms.

NAN-221 · What is ligand exchange, and why is it done?

Replacing the original synthesis ligands with new ones, such as swapping oleic acid for thiolated PEG. It transfers particles from organic to aqueous media, adds functional groups or improves charge transport in films.

NAN-222 · How can silver nanoparticle shape be controlled?

By capping agents that bind preferentially to specific facets, such as PVP on {100}, seeds with defined twinning, and reaction kinetics. This yields cubes, prisms, rods or wires with different plasmon spectra.

NAN-223 · What is the seed-mediated growth of gold nanorods?

Small Au seeds are added to a growth solution of HAuCl₄, a weak reducing agent (ascorbic acid), CTAB surfactant and a little AgNO₃. CTAB and silver direct anisotropic growth along one axis.

NAN-224 · Why are cadmium-free quantum dots being developed?

Cadmium is toxic and restricted by regulations such as EU RoHS. InP/ZnSe/ZnS and perovskite quantum dots are commercial alternatives for displays.

NAN-225 · What are perovskite nanocrystals?

Colloidal nanocrystals of lead halide perovskites such as CsPbBr₃. They are defect-tolerant with high quantum yields and narrow emission tunable by halide composition, but are sensitive to moisture and light.

NAN-226 · What is upconversion in nanoparticles?

Absorption of two or more low-energy photons, usually near-infrared, followed by emission of one higher-energy photon. Lanthanide-doped NaYF₄ nanoparticles are common upconverters for bioimaging.

NAN-227 · How is magnetic hyperthermia achieved with nanoparticles?

An alternating magnetic field of about 100 kHz–1 MHz makes superparamagnetic iron oxide particles release heat through Néel and Brownian relaxation, raising local tissue temperature to about 42–46 °C.

NAN-228 · What is the specific absorption rate (SAR) in magnetic hyperthermia?

The heating power per unit mass of magnetic material, SAR = c × (dT/dt) × (m_sample ÷ m_magnetic), in W/g. It depends on particle size, anisotropy, field amplitude and frequency.

Characterization Techniques

NAN-229 · Which characterization tools are commonly used for nanomaterials, and what does each measure?

SEM and TEM image morphology and structure, AFM and STM map surfaces at nanometre to atomic scale, XRD gives crystal phase and crystallite size, DLS gives hydrodynamic size in suspension, and Raman identifies bonding and carbon structure.

NAN-230 · What is the Scherrer equation?

τ = Kλ ÷ (β cosθ), where τ is the crystallite size, K ≈ 0.9 the shape factor, λ the X-ray wavelength, β the peak broadening (FWHM in radians, after removing instrumental broadening) and θ the Bragg angle.

NAN-231 · Calculate crystallite size from an XRD peak at 2θ = 38.2° with FWHM 0.5° using Cu Kα (0.154 nm).

β = 0.5° = 0.00873 rad, θ = 19.1°, cosθ = 0.945. τ = 0.9 × 0.154 ÷ (0.00873 × 0.945) ≈ 16.8 nm.

NAN-232 · Why can DLS size differ from TEM size?

DLS measures the hydrodynamic diameter, including ligand shell and solvation layer, and is intensity-weighted (scales roughly as d⁶), so a few aggregates skew it upward. TEM measures the dry, number-weighted core size.

NAN-233 · What is the difference between contact, tapping and non-contact AFM modes?

Contact mode drags the tip on the surface and measures deflection. Tapping mode oscillates the cantilever near resonance and intermittently touches, reducing lateral forces. Non-contact mode oscillates above the surface and senses attractive forces, minimizing sample damage.

NAN-234 · What limits the resolution of a TEM?

Not the electron wavelength (about 2.5 pm at 200 kV) but lens aberrations, mainly spherical and chromatic aberration, plus sample thickness, stability and beam damage. Aberration correctors push resolution below 0.1 nm.

NAN-235 · What is selected-area electron diffraction (SAED)?

A TEM technique that inserts an aperture to select a region and records its diffraction pattern. Spots indicate single crystals, rings indicate polycrystalline material and diffuse halos indicate amorphous material.

NAN-236 · What is HAADF-STEM?

High-angle annular dark-field scanning TEM collects electrons scattered to high angles. Intensity scales roughly with Z^1.7–2, giving atomic-number (Z) contrast that distinguishes heavy atoms in nanostructures.

NAN-237 · What is the difference between secondary and backscattered electron imaging in SEM?

Secondary electrons are low energy (below 50 eV) from near the surface and give topographic contrast. Backscattered electrons are high-energy beam electrons reflected by nuclei, giving compositional (atomic number) contrast.

NAN-238 · Why must non-conductive samples be coated for SEM?

Insulating samples accumulate charge from the beam, causing image drift and distortion. A thin conductive coating of gold, Pt or carbon, or low-vacuum and low-voltage imaging, prevents charging.

NAN-239 · What is nanoparticle tracking analysis (NTA)?

A technique that images light scattered by individual particles in suspension and tracks their Brownian motion, giving number-based size distributions and concentration, typically for about 30–1000 nm particles.

NAN-240 · How does surface-enhanced Raman spectroscopy (SERS) work?

Molecules on or near plasmonic nanostructures (Au, Ag) experience strongly enhanced local fields at "hot spots", amplifying Raman signals by factors up to about 10⁶–10¹⁰, enabling trace and single-molecule detection.

Nanoelectronics & Spintronics

NAN-241 · Why did Dennard scaling end?

Dennard scaling assumed voltage scales with dimensions to keep power density constant. Threshold voltage could not keep falling without exponential subthreshold leakage, so supply voltage stalled near 1 V around the mid-2000s and power density rose.

NAN-242 · What is subthreshold swing, and what is its thermal limit?

The gate voltage needed to change subthreshold drain current by a decade. For conventional MOSFETs it is limited to (k_BT/q) ln10 ≈ 60 mV/decade at 300 K.

NAN-243 · Why are high-κ dielectrics used in modern transistors?

As SiO₂ thinned below about 1.2 nm, tunnelling leakage became excessive. HfO₂ (κ ≈ 20–25) gives the same capacitance with a physically thicker layer, cutting leakage. It is paired with metal gates.

NAN-244 · What is equivalent oxide thickness (EOT)?

EOT = t_high-κ × (κ_SiO₂ ÷ κ_high-κ), with κ_SiO₂ = 3.9. A 3 nm HfO₂ film with κ = 20 has EOT ≈ 0.59 nm.

NAN-245 · What is drain-induced barrier lowering (DIBL)?

In short channels, the drain field lowers the source-channel potential barrier, reducing threshold voltage as drain voltage rises. It is measured in mV/V and increases leakage.

NAN-246 · What is spin-transfer torque (STT)?

A spin-polarized current transfers angular momentum to a ferromagnetic layer and can switch its magnetization. STT-MRAM uses it to write bits without external magnetic fields.

NAN-247 · How is TMR ratio defined?

TMR = (R_AP − R_P) ÷ R_P, where R_AP and R_P are antiparallel and parallel resistances. Julliere's model gives TMR = 2P₁P₂ ÷ (1 − P₁P₂) from the spin polarizations of the electrodes.

NAN-248 · What is spin-orbit torque (SOT)?

Switching of a magnetic layer by spin current generated from charge current in an adjacent heavy metal (Pt, Ta, W) via the spin Hall effect. Write and read paths are separated, improving endurance and speed.

NAN-249 · What are 2D semiconductors such as MoS₂ used for in nanoelectronics?

Monolayer transition-metal dichalcogenides have atomic thickness and a bandgap (about 1.8 eV direct for monolayer MoS₂), enabling excellent gate control in ultra-short channels where silicon thins poorly.

NAN-250 · What is a tunnel FET (TFET)?

A transistor that switches by band-to-band tunnelling at a gated p-i-n junction instead of thermionic emission, so its subthreshold swing can fall below 60 mV/decade. Low on-current is a main drawback.

NAN-251 · How does a phase-change memory (PCM) work?

Chalcogenide material such as Ge₂Sb₂Te₅ switches between a high-resistance amorphous state (melt-quench by a short high pulse) and a low-resistance crystalline state (longer moderate pulse).

NAN-252 · What is a spin valve?

A GMR structure with a pinned ferromagnetic layer (fixed by an antiferromagnet) and a free layer separated by a non-magnetic metal. External fields rotate the free layer, changing resistance; it was used in hard-disk read heads.

MEMS & NEMS

NAN-253 · What is the resonant frequency of a cantilever?

f = (1 ÷ 2π)√(k ÷ m_eff). For a rectangular beam, f₁ ≈ 0.162 (t ÷ L²)√(E ÷ ρ), so frequency rises with thickness and falls with the square of length.

NAN-254 · How does a resonant mass sensor work?

Added mass Δm lowers the resonant frequency: Δf ≈ −(f ÷ 2m_eff)Δm. Smaller, stiffer NEMS resonators with low mass and high frequency detect attogram to zeptogram changes.

NAN-255 · What is the quality factor (Q) of a resonator?

Q = f₀ ÷ Δf (FWHM), or 2π × energy stored ÷ energy lost per cycle. High Q gives sharper resonance and better frequency resolution; air damping, clamping and thermoelastic losses limit it.

NAN-256 · What is the spring constant of a rectangular cantilever?

k = Ewt³ ÷ (4L³), where E is Young's modulus, w width, t thickness and L length.

NAN-257 · Calculate k for a silicon cantilever with E = 169 GPa, w = 30 µm, t = 2 µm, L = 200 µm.

k = 169 × 10⁹ × 30 × 10⁻⁶ × (2 × 10⁻⁶)³ ÷ (4 × (200 × 10⁻⁶)³) = 4.06 × 10⁻⁸ ÷ 3.2 × 10⁻¹¹ ≈ 1.27 N/m.

NAN-258 · What is electrostatic pull-in?

In a parallel-plate electrostatic actuator, beyond a critical voltage the electrostatic force grows faster than the spring force and the plate snaps to the electrode. Pull-in occurs at one-third of the initial gap: V_PI = √(8kg₀³ ÷ 27ε₀A).

NAN-259 · How does capacitive sensing work in MEMS?

Proof-mass motion changes the gap or overlap area of capacitor plates, C = ε₀A ÷ g. Differential comb-drive or parallel-plate arrangements convert displacement to a capacitance change read by electronics.

NAN-260 · What is a comb-drive actuator?

Interdigitated fixed and movable fingers. Applied voltage creates a lateral electrostatic force F = (nε₀t ÷ g)V², nearly independent of displacement, giving a large linear travel range.

NAN-261 · Why is vacuum packaging used for MEMS gyroscopes and resonators?

Air damping lowers quality factor. Vacuum packaging raises Q, increasing gyroscope sensitivity and resonator frequency stability, while accelerometers often use some gas for damping.

NAN-262 · How do piezoresistive MEMS sensors work?

Strain changes the resistivity of doped silicon resistors placed at high-stress locations, such as membrane edges. A Wheatstone bridge converts the resistance change to voltage, as in MEMS pressure sensors.

NAN-263 · How does scaling affect MEMS forces?

Forces scaling with area (electrostatic, surface tension) or length dominate over volume-scaled forces (inertia, gravity, magnetic in some cases) as size shrinks. Electrostatic actuation therefore dominates in MEMS.

NAN-264 · What is a digital micromirror device (DMD)?

A MEMS chip with up to millions of aluminium micromirrors, each tilting about ±12° by electrostatic actuation to reflect light toward or away from projection optics, forming pixels in DLP projectors.

Nanomedicine & Drug Delivery

NAN-265 · Why is a size of about 10–200 nm preferred for intravenous nanocarriers?

Particles below about 5–6 nm are cleared quickly by the kidneys, while particles above about 200 nm are captured by the liver and spleen. The intermediate range gives longer circulation and allows EPR accumulation.

NAN-266 · How does surface charge affect nanoparticle fate in the body?

Cationic particles adhere to negatively charged cell membranes and are taken up more, but also cause more toxicity and protein adsorption. Neutral or slightly negative particles circulate longer.

NAN-267 · What are the main cellular uptake pathways for nanoparticles?

Clathrin-mediated endocytosis, caveolae-mediated endocytosis, macropinocytosis and phagocytosis (by immune cells). Pathway depends on size, shape, charge and surface ligands.

NAN-268 · What is endosomal escape and why is it important?

After endocytosis, carriers are trapped in endosomes that mature into degradative lysosomes. Cargo such as mRNA or siRNA must escape to the cytosol; ionizable lipids and proton-sponge polymers promote membrane disruption.

NAN-269 · How do ionizable lipids in LNPs work?

They are nearly neutral at blood pH (reducing toxicity) but become positively charged in acidic endosomes (pKa about 6–7), interacting with anionic endosomal lipids to disrupt the membrane and release nucleic acid.

NAN-270 · What is the mononuclear phagocyte system (MPS)?

Phagocytic cells mainly in the liver (Kupffer cells), spleen and lymph nodes that capture opsonized nanoparticles from blood, limiting circulation time and delivery to target tissues.

NAN-271 · What is opsonization?

Adsorption of blood proteins such as complement and immunoglobulins onto nanoparticles, marking them for recognition and clearance by phagocytes. PEGylation reduces it.

NAN-272 · How does pH-responsive drug release work?

Carriers include acid-labile bonds (hydrazone, acetal) or ionizable polymers that swell or degrade at the lower pH of tumours (about 6.5–7.0) or endosomes (about 5–6), releasing the drug there.

NAN-273 · What is drug loading capacity versus encapsulation efficiency?

Loading capacity is drug mass ÷ total nanoparticle mass. Encapsulation efficiency is the fraction of drug added that ends up encapsulated. A formulation can have high efficiency but low loading.

NAN-274 · Why are gold nanoparticles useful in diagnostics?

Their intense plasmon colour enables visual tests such as lateral-flow pregnancy and antigen tests. Colour shifts on aggregation and easy thiol functionalization support sensitive colorimetric assays.

NAN-275 · What is a nanocarrier's biodistribution?

The pattern of where the carrier accumulates in organs over time, typically measured by radiolabelling, fluorescence imaging or ICP-MS of tissues. Most systemically injected nanoparticles accumulate in the liver and spleen.

NAN-276 · How do superparamagnetic iron oxide nanoparticles act as MRI contrast agents?

They create local magnetic field inhomogeneities that shorten T₂ (and T₂*) relaxation of nearby water protons, darkening regions where they accumulate (negative contrast).

Nanotech for Energy & Environment

NAN-277 · Why does nano-silicon improve lithium-ion anodes, and what is the challenge?

Silicon's theoretical capacity is about 3,600–4,200 mAh/g versus 372 mAh/g for graphite. But it expands about 300% on lithiation; nanoparticles and nanowires accommodate strain without fracture, though unstable SEI growth remains a problem.

NAN-278 · How is supercapacitor energy calculated?

E = ½CV². For a 100 F cell at 2.7 V, E = 0.5 × 100 × 2.7² ≈ 365 J, about 0.1 Wh.

NAN-279 · Why is TiO₂ photocatalysis limited to UV light?

Anatase TiO₂ has a bandgap of about 3.2 eV, corresponding to light below about 387 nm, which is only around 4–5% of solar energy. Doping, sensitization and plasmonic co-catalysts extend activity into the visible.

NAN-280 · How do plasmonic nanoparticles enhance solar cells?

Metal nanoparticles scatter light into longer paths within the absorber and concentrate near-field intensity, increasing absorption in thin films. Hot-electron injection is another proposed mechanism.

NAN-281 · What is multiple exciton generation (MEG) in quantum dots?

Absorption of a photon with energy at least twice the bandgap produces two or more electron-hole pairs. Quantum confinement relaxes momentum conservation and enhances MEG, potentially exceeding the single-junction efficiency limit.

NAN-282 · What is the thermoelectric figure of merit ZT?

ZT = S²σT ÷ κ, where S is the Seebeck coefficient, σ electrical conductivity, κ thermal conductivity and T absolute temperature. Nanostructuring lowers lattice κ while preserving σ, raising ZT.

NAN-283 · How do nanomaterial catalysts help fuel cells?

Carbon-supported Pt nanoparticles of about 2–5 nm maximize active surface per gram for the oxygen reduction reaction, reducing Pt loading. Core-shell and alloy nanoparticles further increase activity.

NAN-284 · What is electrochemically active surface area (ECSA)?

The catalyst area that participates in electrochemical reactions, measured from hydrogen adsorption charge or CO stripping. For Pt, hydrogen adsorption charge is taken as about 210 µC/cm².

NAN-285 · How do nanomaterials enable carbon capture?

MOFs, zeolites and amine-functionalized mesoporous silicas offer high surface area and tunable pores and chemistry for selective CO₂ adsorption with lower regeneration energy than liquid amine scrubbing.

NAN-286 · How do nanofiltration membranes work?

They have pores of roughly 1 nm and charged surfaces, rejecting multivalent ions and organic molecules by size exclusion and Donnan exclusion, while passing water and many monovalent ions.

NAN-287 · How does nZVI degrade chlorinated solvents?

Fe⁰ oxidizes to Fe²⁺, donating electrons that reductively dechlorinate compounds like trichloroethylene to ethene and chloride. The high surface area of nanoparticles speeds reaction.

NAN-288 · What are nanosensors used for in environmental monitoring?

Detecting heavy metals, pesticides, toxic gases and pathogens at very low concentrations using nanomaterials whose electrical, optical or plasmonic properties change on binding target species.

Nanotoxicology, Safety & Ethics

NAN-289 · Why can inhaled nanoparticles reach deep into the lungs?

Particles around 10–100 nm deposit efficiently in the alveolar region by diffusion. Some can translocate across the air-blood barrier, and ultrafine particles may reach the brain via the olfactory nerve.

NAN-290 · What makes long, rigid CNTs a particular concern?

They resemble asbestos fibres. Macrophages cannot fully engulf fibres longer than about 15–20 µm, causing frustrated phagocytosis, chronic inflammation and potential mesothelioma in animal studies.

NAN-291 · How do nanoparticles generate reactive oxygen species?

Through surface catalytic reactions, photoactivation (e.g. TiO₂), Fenton chemistry with released transition metal ions (Fe, Cu), and by disrupting mitochondria in cells.

NAN-292 · Why does silver nanoparticle toxicity depend on dissolution?

Much of the toxicity comes from released Ag⁺ ions. Dissolution rate depends on size, coating, oxygen and chloride or sulfide in the medium, so the same particles can behave very differently across environments.

NAN-293 · What is the "Trojan horse" mechanism?

Nanoparticles are taken into cells by endocytosis and then dissolve in acidic lysosomes, releasing toxic ions inside the cell at higher concentrations than would enter from dissolved ions alone.

NAN-294 · What is a dose metric, and which are used for nanoparticles?

The quantity used to express exposure. For nanoparticles mass, particle number and surface area concentrations are used, and surface area often correlates best with inflammatory response for low-solubility particles.

NAN-295 · How are workplace nanoparticle exposures measured?

Using condensation particle counters for number concentration, scanning mobility particle sizers for size distribution, diffusion chargers for surface area, and filter sampling with electron microscopy for identification.

NAN-296 · What engineering controls are used for nanomaterial handling?

Local exhaust ventilation, fume hoods, HEPA-filtered enclosures and glove boxes, handling materials wet or in pastes, and closed transfer systems.

NAN-297 · How does REACH address nanomaterials in the EU?

Since 2020, REACH annexes require registrants to provide nanoform-specific information, such as particle size distribution, shape and surface chemistry, and safety data for each nanoform.

NAN-298 · What are the in vitro assays commonly used in nanotoxicology?

Viability assays (MTT, LDH release), ROS assays (DCFH-DA), inflammation markers (cytokines), genotoxicity (comet assay, micronucleus) and uptake studies. Nanoparticles can interfere with optical readouts, so controls are essential.

NAN-299 · Why do nanoparticles interfere with toxicity assays?

They can adsorb dyes or nutrients, absorb or scatter light, catalyse dye reactions or quench fluorescence, producing false positive or negative results. Particle-only controls and orthogonal assays are needed.

NAN-300 · What is the environmental fate of engineered nanomaterials?

Release from products leads to aggregation, dissolution, sulfidation, adsorption of natural organic matter and sedimentation. Much ends up in wastewater sludge, which can be spread on soils.

Other levels: Basic · Advanced · Tricky · All Nanotechnology Engineering questions