Unit-4: Materials for Memory and Display Systems

Memory Devices

Conventional semiconductor memories and their limitations

• Classical memory devices are fabricated on silicon-based integrated circuits using transistors and capacitors.
• Trend toward higher-performance gadgets demands higher-density, faster-access memory.
• Strategy: deliberately pack more components on one chip → transistor feature size shrank from $130\,\text{nm}$ (2000) to $32\,\text{nm}$ (present).
• Below $22\,\text{nm}$, Si transistors become unstable: reliability to store/read bits decreases, power consumption rises, unwanted heat increases.

Definition of a memory device

• Semiconductor device whose primary function is electronically storing information.

Basic concepts / requirements for any electronic memory

• Each memory cell must possess at least two stable states coded as $0$ and $1$.
• States must remain stable for the desired storage period.
• External stimulus must switch between states ("writing").
• A further external signal must allow the states to be distinguished ("reading").

Electrical bistability in organic materials

• Many organic/polymeric materials show two electrically stable states (ON/OFF) that can be switched by an external electric field → principle of organic memory.

Classification of electronic memories

• A computer employs several storage types; electronic memories divided chiefly by volatility:

1. Volatile memory

• Loses stored data without continuous power or periodic refresh.
• Dominant primary storage today.
• Random-Access Memory (RAM)
  • Requires refresh cycles.
  • Sub-classes: SRAM (static RAM) and DRAM (dynamic RAM).

2. Non-volatile memory

• Retains data without power.
• Sub-division:
  • Read-Only Memory (ROM)
  • Factory-programmable only.
  • Sub-types: WORM (write-once, read-many) and EPROM (erasable programmable ROM).
  • Hybrid memory (re-writeable)
  • Flash memory and EEPROM.

Organic memory materials

Organic memories exploit molecular bistability; three broad material classes:

I. Organic molecules

• Exhibit OFF→ON switching when threshold voltage applied.
• Key subclasses:
  1. Acene derivatives
  • Linearly fused benzene rings.
  • High charge-carrier mobility.
  • Examples:
    • Pentacene (five rings) → p-type semiconductor, high hole mobility.
    • Perfluoropentacene (all H replaced by F) → n-type semiconductor.
    • Their similar packing yet opposite polarity enables charge-transfer memories.
  1. Charge-transfer complexes
  • Consist of an organic electron donor + electron acceptor (organic or metal).
  • Two charge states arise from donor→acceptor electron transfer.
  • Example: $\text{TCNQ}$ (tetracyanoquinodimethane) complexes.

II. Polymeric molecules

Five polymer families exhibit memory behaviour:

1. Functional polyimides (PIs)
   • Phthalimide (acceptor) + triphenylamine (donor) → Donor-Acceptor (D-A) backbone.
   • High thermal & mechanical stability; solution-processable.
   • Two charge states via intramolecular electron transfer.
2. Conjugated polymers
   • $\pi$-rich backbones; acceptor groups induce charge-transfer channel controlling volatility.
   • D-A conjugated polymers used in DRAM, SRAM, WORM & Flash devices.
   • Example: polyacetylene.

III. Organic–inorganic hybrid materials

1. Organic–carbon allotrope hybrids
   • Donor polymers (thiophene, fluorene, carbazole, aniline) + fullerene acceptors.
   • Fullerenes can capture up to 6 e⁻ → WORM memories.
2. Organic–inorganic nanocomposites
   • Functional polymer blended with metal nanoparticles, oxides, quantum dots.
   • Example: 8-hydroxyquinoline polymer + Au nanoparticles between electrodes.

Advantages of organic/polymer memories

• Solution processing, chemically tailorable structures, simple architecture, facile miniaturization, low cost & low operating power.

Display Systems

Liquid crystals (LCs) – introduction

• Mesophase with properties intermediate between solids & liquids.
• Flow like liquids yet exhibit orientational order like solids.
• Definition: thermodynamically stable, anisotropic phase without 3-D lattice, existing between solid and isotropic liquid.
• Key for modern low-power displays (mobiles, cockpits, laptops, etc.).

Structure–property relation for LC behaviour

• Molecules should be elongated, contain both rigid & flexible segments, and have differential solubility along the chain.
• Example: n-alkanes show no LC, but inserting a C=C double bond can induce mesophase.

Classification of liquid crystals

1. Thermotropic (temperature dependent)
   • Phase changes driven solely by $T$.
   • Example: cholesteryl benzoate solid → LC from $145.5^{\circ}\text{C}$ to $178.5^{\circ}\text{C}$.
   • Sub-types: a. Nematic (thread-like)
     • Optically inactive; elongated molecules parallel; only orientational order.
     • Example: para-azoxyanisole (PAA) LC from $118^{\circ}\text{C}$–$135^{\circ}\text{C}$.
       b. Chiral (twisted) nematic
     • Derived from chiral molecules → helical arrangement.
       c. Smectic
     • Soap-like; molecules arranged in layers, possessing both slight orientational & positional order.
2. Lyotropic (concentration dependent)
   • Amphiphilic molecules form ordered phases when mixed with solvent; mesophase depends on concentration.
   • Examples: soap–water mixtures; phospholipid–water (basis of cell membranes).

Properties of liquid crystals

• Anisotropic physical properties scale with alignment to director.
• Exhibit fluidity, surface tension, viscosity (liquid-like) + ordered optical/electrical behaviour (solid-like).

Applications of liquid crystals

1. Display systems
   • Dashboard indicators, traffic signals, advert boards, fuel-pump displays.
   • Analytical instruments (pH meter, conductometer, colorimeter, potentiometer).
   • Consumer electronics: watches, TVs, calculators, phones, laptops, desktops.
2. Thermography
   a. Medical: early tumor/breast-cancer detection; arthritis/back-pain diagnostics via skin-temperature mapping.
   b. Radiation & pressure sensors (cholesteric LCs convert radiation → heat → color change).
3. Environmental monitoring: color shift indicates air pollutants.
4. Circuit diagnostics: locate potential failure points via electric-field sensitivity.
5. Research solvents: NMR, IR, UV spectroscopy; numerous analytical instruments.
6. Miscellaneous: vein/artery visualization, cosmetics, pharmaceuticals, fire-resistant LC polymers for fibre-optic cable coatings.

Jablonski diagram – electronic excitation & de-excitation

• Energy diagram depicting electronic states & transitions.

Energy levels

• $S_0$: ground singlet state.
• $S<em>1, S</em>2$: first & second excited singlet states.
• $T<em>1, T</em>2$: first & second excited triplet states.

Key processes

1. Absorption
   • Vertical upward arrow; electron promoted within $\sim10^{-15}\,\text{s}$.
2. Emission pathways a. Non-radiative
   • Internal Conversion (IC): $S<em>n \rightarrow S</em>{n-1}$, same spin, energy lost as heat.
   • Inter-System Crossing (ISC): $S$ ↔ $T$ transition, spin change, no photon.
     b. Radiative
   • Fluorescence: $S<em>1 \rightarrow S</em>0$ photon emitted; $10^{-9}$–$10^{-7}\,\text{s}$.
   • Phosphorescence: $T<em>1 \rightarrow S</em>0$ photon; $10^{-3}\,\text{s}$ or longer.

Uses

• Fundamental tool in molecular spectroscopy to interpret fluorescence/phosphorescence phenomena and energy dissipation pathways.

Photoactive & Electroactive Organic Materials

Motivation & advantages over inorganic semiconductors

• Enable lightweight, flexible, low-cost, chemically tailorable devices.
• Applicable in organic photovoltaic devices (OPVs), organic LEDs, organic FETs.

Opto-electronic phenomena exhibited

• Light absorption/emission (UV→NIR).
• Photogeneration, transport & electrode injection of charge carriers.
• Excellent nonlinear optical (NLO) responses.

Material categories & examples

• Small molecules: metal/metal-free phthalocyanines, porphyrins, anthracene, pentacene, fullerenes.
• Conjugated oligomers: oligothiophenes, extended pentacene derivatives — properties tune with $\pi$-length.
• Conducting polymers: polyacetylene, poly(p-phenylene), poly(p-phenylene vinylene), poly(9,9-dialkylfluorene), polythiophene, polypyrrole, polyaniline.

Nanomaterials for future miniaturized opto-electronics

• Carbon nanostructures: graphene, fullerenes, carbon nanotubes (CNTs).
• Quantum dots (QDs) with diameters $\le0.5\,\text{nm}$: Si, Ge, III–V (GaAs, GaN, InN, GaP, InP, AlN, InAs), II–VI (CdS, CdSe, ZnS, ZnSe), oxides (In$2$O$3$, ZnO, TiO$_2$).
• Support phenomena: surface plasmon resonance, spintronics, plasmonics for advanced photonic/electronic devices.

Organic Light-Emitting Devices & Emerging Display Technologies

Light-Emitting Electrochemical Cell (LEC)

• Solid-state electroluminescent device: two metal electrodes sandwich organic semiconductor containing mobile ions.
• Similar to OLED but less dependent on electrode work-function mismatch → identical electrodes (e.g. Au) viable.
• Can employ graphene or CNT/polymer blends as transparent electrodes → avoids indium-tin-oxide (ITO).
• Active layer thickness non-critical → enables inexpensive printing; planar architecture lets researchers visually track internal operation.

Quantum-Dot Light-Emitting Diodes (QLEDs)

• Market demand: higher color saturation & electrical stability beyond OLED.
• Construction: standard LCD panel + blue LED backlight + quantum-dot (QD) film.
• Working sequence
  1. Blue backlight excites QDs.
  2. QDs size-selectively convert blue photons to precise red or green wavelengths.
  3. LCD modulates resultant RGB light to form image.
• Advantages: wider color gamut, higher brightness, superior color accuracy vs. conventional LED TVs.

Organic Light-Emitting Diode (OLED)

• Replaces inorganic p–n junction with organic layers.
• Basic stack (bottom→top): substrate → cathode → emissive layer → conductive layer → anode → seal (top encapsulation);
  protective glass/plastic on either side.

Operating principle

1. Apply voltage across anode (+) & cathode (–).
2. Cathode injects electrons; anode injects holes.
3. Electrons accumulate in emissive layer; holes in conductive layer.
4. Holes drift into emissive layer; recombination with electrons produces photons (electroluminescence).
5. Continuous recombination yields steady light output while current flows.

Applications & real-world relevance

• Used anywhere LCDs appear: TVs, monitors, smartphones, wearables, automotive displays.
• Benefits: thin, flexible, high contrast, self-emitting (no backlight), lower power.
• Commercial milestones: 2015 Apple Watch (first Apple OLED); 2017 iPhone X (first OLED iPhone).