Molecular Structure 3D Viewer & Encyclopedia

Molecular Structure Encyclopedia

UV Excitation Mechanisms

Long Wave UV (LWUV - 365 nm): Long-wave ultraviolet light with a wavelength of approximately 365 nanometers excites defect centers in crystalline structures primarily through:

• Nitrogen-Vacancy (N-V) Centers: In diamond, UV photons promote electrons from the ground state (³A₂) to excited states (³E). The electron absorbs a photon with energy E = hν = hc/λ, where λ = 365 nm corresponds to ~3.4 eV. Following excitation, the electron undergoes vibronic relaxation and emits visible light through fluorescence or transitions to metastable states for phosphorescence.
• Boron Acceptors: In boron-doped diamonds, UV light excites electrons from boron acceptor levels (located ~0.37 eV above the valence band) to higher energy states. The subsequent relaxation results in blue fluorescence emission.
• Structural Defects: Dislocations and plastic deformation sites in pink diamonds can create localized energy states that absorb UV light and emit in the visible spectrum.

Short Wave UV (SWUV - 254 nm): Short-wave ultraviolet light with a wavelength of approximately 254 nanometers (higher energy, ~4.9 eV) causes:

• Higher Energy Transitions: SWUV photons have sufficient energy to excite electrons to higher energy levels, including conduction band transitions in semiconductors like silicon-carbide.
• Ionization Events: Higher photon energy can cause direct ionization of defect centers, creating electron-hole pairs that recombine with delayed emission (phosphorescence).
• Multi-Photon Processes: Can induce two-photon absorption processes in certain defect configurations, leading to stronger fluorescence or persistent phosphorescence.
• Band Gap Excitation: In silicon-carbide (band gap ~3.0 eV for 3C-SiC), 254 nm photons can directly excite electrons across the band gap, creating charge carriers that recombine through defect-mediated processes.

Fluorescence Physics

Fluorescence is a three-stage process:

1. Absorption (Excitation): A UV photon with energy E = hc/λ is absorbed, promoting an electron from the ground state S₀ to an excited singlet state S₁ or triplet state T₁.
2. Vibronic Relaxation: The excited electron rapidly loses vibrational energy (10⁻¹² to 10⁻¹⁴ seconds) through collisions, relaxing to the lowest vibrational level of the excited state.
3. Emission: The electron transitions back to the ground state, emitting a photon with lower energy (longer wavelength) than the absorbed photon (Stokes shift). For N-V centers in diamond, absorption at 365 nm can result in emission around 575 nm (red) or 637 nm (deep red), depending on the charge state.

Phosphorescence Mechanism

Phosphorescence occurs when excited electrons undergo intersystem crossing to a triplet state (T₁), where they are "forbidden" from directly returning to the singlet ground state. The electron remains trapped until:

• Thermal energy causes transition back to S₁, followed by emission
• Direct triplet-to-ground-state transition (forbidden but possible with spin-orbit coupling)

This results in delayed emission that persists after the UV source is removed. Phosphorescence lifetimes in diamonds can range from milliseconds to hours, depending on the defect center and temperature.

Fluorescence Quantum Yield (Φ_F)

Definition: Fluorescence quantum yield (Φ_F) is the efficiency of fluorescence, defined as the ratio of emitted photons to absorbed photons:

Φ_F = Photons Emitted / Photons Absorbed

Quantum yield is a dimensionless value between 0 and 1, indicating the probability that an excited molecule will fluoresce instead of losing energy non-radiatively through processes such as:

• Internal conversion (vibrational relaxation)
• Intersystem crossing to triplet states
• Collisional quenching
• Energy transfer to other molecules

Measurement Methods:

• Absolute Method: Using an integrating sphere to measure all emitted photons relative to absorbed photons
• Relative Method: Comparing fluorescence intensity to a known standard (e.g., fluorescein in 0.1 M NaOH, Φ_F ≈ 0.92)

Typical Quantum Yield Values by Material:

• Nitrogen-Vacancy (N-V) Centers in Diamond: Φ_F ≈ 0.30-0.40 (highly efficient, making N-V centers valuable for quantum applications)
• Boron-Doped Diamonds: Φ_F ≈ 0.25-0.30 (moderate efficiency, blue fluorescence)
• Pure Diamond: Φ_F ≈ 0.01-0.05 (very low, minimal defect centers)
• Structural Defect Diamonds (Pink): Φ_F ≈ 0.10-0.20 (variable, depends on defect concentration)
• Silicon-Carbide Defect Centers: Φ_F ≈ 0.10-0.15 (moderate efficiency, depends on polytype and defects)
• Carbon-60 (C₆₀): Φ_F ≈ 0.001-0.01 (very weak fluorescence in solution, can be enhanced with functionalization)

Factors Affecting Quantum Yield:

• Defect Concentration: Higher defect density can increase or decrease Φ_F depending on quenching effects
• Temperature: Generally decreases with increasing temperature due to enhanced non-radiative processes
• UV Wavelength: Different excitation wavelengths can access different energy levels, affecting quantum yield
• Environment: Solvent, matrix, or crystal environment can significantly influence quantum yield
• Impurity Interactions: Competing energy transfer pathways can reduce quantum yield

Scientific Significance: Quantum yield is crucial for quantifying fluorescence efficiency in analytical chemistry, materials science, and quantum applications. High quantum yield materials (Φ_F > 0.5) are particularly valuable for fluorescence-based sensors, biomarkers, and quantum information processing.

Diamond (Crystalline Carbon)

Diamond is a metastable allotrope of carbon where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. The diamond lattice belongs to the cubic crystal system and has space group Fd3m (no. 227).

Pure Diamond Structure: In a pure diamond, each carbon atom is bonded to four other carbon atoms in a tetrahedral geometry with C-C bond lengths of 1.54 Å. The cubic unit cell contains 8 carbon atoms.

Isotope Varieties:

• Carbon-12 (C-12): The most abundant isotope (98.93%), containing 6 protons and 6 neutrons. Stable and most common form.
• Carbon-13 (C-13): Contains 6 protons and 7 neutrons (1.07% abundance). Stable isotope used in NMR spectroscopy.
• Carbon-14 (C-14): Radioactive isotope with 6 protons and 8 neutrons. Trace amounts found in natural diamonds due to cosmic ray interactions.

Colored Diamond Variants

Nitrogen-Doped Diamonds (Type Ib): Yellow or brown diamonds containing isolated nitrogen atoms substituting carbon atoms in the lattice. Each nitrogen atom creates an unpaired electron that affects the crystal's optical properties. Under LWUV, N centers absorb photons and promote electrons, leading to characteristic yellow/blue fluorescence. Typical quantum yield (Φ_F): 0.30-0.40 for N-V centers, making them highly efficient fluorophores.

Pink Diamonds: The pink coloration is often caused by structural defects in the crystal lattice, particularly plastic deformation along certain crystal planes. The lattice distortion causes selective absorption and scattering of light, resulting in pink appearance. UV excitation can enhance visibility of these defects. Typical quantum yield (Φ_F): 0.10-0.20, variable depending on defect concentration and type.

Green Diamonds: Natural green diamonds typically result from exposure to natural radiation or contain nitrogen-vacancy centers. Artificial green coloration can occur from irradiation treatments. Typical quantum yield (Φ_F): 0.20-0.25, depending on the specific defect centers present.

Clear D Color Diamonds with Green Fluorescence: D color represents the highest grade of colorless diamonds. When containing trace amounts of uranium or other radioactive elements, these diamonds exhibit green fluorescence under ultraviolet light due to the interaction between radioactive decay products and the crystal structure. Typical quantum yield (Φ_F): 0.15-0.20, lower efficiency due to competing non-radiative processes from radioactive decay.

Blue Diamonds (Boron-Doped, Type IIb): Blue color results from boron atoms substituting for carbon atoms in the crystal lattice. Boron has one fewer electron than carbon, creating electron-deficient centers (p-type semiconductor properties). Under UV excitation, boron acceptors emit characteristic blue fluorescence. Typical quantum yield (Φ_F): 0.25-0.30, moderate efficiency for blue emission.

Carbon-60 (C₆₀) - Buckminsterfullerene

Carbon-60, also known as buckminsterfullerene or "buckyball," is a spherical molecule composed of 60 carbon atoms arranged in a truncated icosahedron structure. It consists of 12 pentagonal rings and 20 hexagonal rings, forming a closed cage structure resembling a soccer ball.

Molecular Structure: C60 has icosahedral symmetry (Iₕ point group) and is the smallest and most stable fullerene. Each carbon atom is sp² hybridized and forms three covalent bonds with neighboring carbon atoms, creating a delocalized π-electron system across the entire molecule.

Key Properties:

• Molecular Formula: C₆₀
• Molecular Weight: 720.66 g/mol
• Diameter: ~7.1 Å (0.71 nm)
• Bond Lengths: 1.39 Å (hexagon-hexagon), 1.45 Å (pentagon-hexagon), average 1.42 Å
• HOMO-LUMO Gap: ~1.6-1.9 eV
• Symmetry: Icosahedral (Iₕ) - 120 symmetry operations
• Point Group: Iₕ

Electronic Structure: C60 has a closed-shell electronic configuration with a HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) gap of approximately 1.6-1.9 eV. The molecule exhibits strong electron-accepting properties and can form complexes with various metals and other molecules.

UV Excitation: Under UV light, C60 molecules can undergo:

• π-π* Transitions: Electronic transitions between π orbitals in the delocalized electron system
• Long Wave UV (365 nm, 3.4 eV): Can excite electrons from HOMO to lower-energy excited states
• Short Wave UV (254 nm, 4.9 eV): Higher energy transitions, potentially leading to electron transfer or fullerene cage modification
• Fluorescence: C60 exhibits weak fluorescence in solution, with emission typically in the visible to near-IR range. Typical quantum yield (Φ_F): 0.001-0.01 in solution, very low due to efficient non-radiative decay pathways. Functionalization or encapsulation can enhance quantum yield significantly.
• Singlet Oxygen Generation: C60 is an efficient photosensitizer, converting UV energy to produce singlet oxygen (¹O₂) through energy transfer

Discovery and Significance: C60 was discovered in 1985 by Harry Kroto, Richard Smalley, and Robert Curl, earning them the 1996 Nobel Prize in Chemistry. This discovery opened the field of fullerene chemistry and led to the development of carbon nanotechnology.

Applications: C60 and other fullerenes have applications in:

• Organic photovoltaics and solar cells
• Drug delivery systems
• Superconducting materials (when doped)
• Antioxidants (due to radical-scavenging properties)
• Catalysis
• Materials science research

Isotope Varieties: Carbon-60 can be synthesized with different carbon isotope compositions, including pure C-12, C-13 enrichment, and mixed isotopes. Isotope-labeled C60 is used in scientific research for tracking, NMR studies, and understanding molecular dynamics.

Silicon-Carbide (SiC)

Silicon-Carbide is a compound semiconductor material with exceptional properties. It exists in multiple polytypes, with the most common being:

• 3C-SiC (Beta): Cubic zinc blende structure, space group F43m, band gap ~2.3-3.0 eV
• 4H-SiC: Hexagonal structure with 4 Si-C bilayers per unit cell, band gap ~3.26 eV
• 6H-SiC: Hexagonal structure with 6 Si-C bilayers per unit cell, band gap ~3.02 eV

Under SWUV (254 nm, 4.9 eV), silicon-carbide can exhibit direct band-to-band excitation, where photons have sufficient energy to promote electrons across the band gap, creating electron-hole pairs that recombine radiatively or through defect centers, resulting in visible fluorescence or phosphorescence. Typical quantum yield (Φ_F): 0.10-0.15 for defect-mediated fluorescence, with higher values possible for optimized defect centers.

Crystal Lattice Properties

Diamond Cubic Structure:

• Unit cell parameter: a = 3.567 Å
• Density: 3.515 g/cm³
• Hardness: 10 on Mohs scale (highest)
• Refractive index: 2.42
• Thermal conductivity: Very high (~2000 W/m·K)
• Band gap: 5.5 eV (indirect)

Bonding: Each carbon atom forms four strong covalent bonds (sp³ hybridization) with neighboring atoms, creating an extremely rigid three-dimensional network structure.