High pressure is a powerful tool for modifying the electronic and optical properties of semiconductor nanostructures, particularly quantum dots (QDs). When external pressure is applied, the lattice structure of the material undergoes compression, leading to significant changes in band structure, carrier confinement, and energy level spacing. Cadmium telluride (CdTe) quantum dots serve as an excellent model system to study these effects due to their well-defined quantum confinement characteristics and pressure-sensitive electronic properties.
The application of hydrostatic pressure primarily alters the interatomic distances within the QD lattice, which in turn modifies the bandgap and confinement potential. In CdTe QDs, the pressure-induced reduction in lattice constant increases the overlap of atomic orbitals, leading to a widening of the valence and conduction bands. This results in a systematic increase in the bandgap energy, often quantified by the pressure coefficient (dEg/dP). For CdTe QDs, experimental studies report a pressure coefficient in the range of 80-100 meV/GPa, depending on dot size and surface passivation.
Quantum confinement effects are directly influenced by these pressure-induced bandgap changes. The energy levels of electrons and holes within the QD shift upward as the confinement potential becomes more pronounced due to lattice compression. The degree of this shift is size-dependent; smaller dots exhibit larger energy level displacements under pressure because of their stronger quantum confinement. For example, in CdTe QDs with diameters below 5 nm, the first excitonic transition energy can increase by over 200 meV at pressures of 5 GPa, whereas larger dots show a less pronounced shift.
Carrier localization is another critical aspect affected by high pressure. In nanostructures, carriers (electrons and holes) are confined within a small volume, but pressure can further localize them by enhancing the potential barriers at the QD boundaries. This effect is particularly evident in type-I quantum dots like CdTe, where both carriers are confined within the same spatial region. Under pressure, the increased bandgap and stronger confinement potential reduce carrier delocalization, leading to sharper emission lines and reduced inhomogeneous broadening in photoluminescence spectra.
Pressure also modifies the excitonic properties of QDs. The binding energy of excitons—the Coulombic attraction between electrons and holes—increases under compression due to reduced dielectric screening and enhanced quantum confinement. In CdTe QDs, exciton binding energies can rise by 20-30% under pressures of a few GPa. This strengthens the oscillator strength of optical transitions, making pressure-tuned QDs attractive for high-efficiency optoelectronic applications.
Strain effects play a crucial role in high-pressure studies of QDs. Unlike bulk materials, nanostructures experience non-uniform strain due to their finite size and surface effects. The core of the QD is subjected to nearly hydrostatic pressure, while the surface regions may undergo anisotropic strain. This strain gradient can lead to variations in the electronic structure across the dot, influencing carrier wavefunctions and transition probabilities. In CdTe QDs, this manifests as a pressure-dependent Stokes shift between absorption and emission peaks, which becomes more pronounced at higher pressures.
Phase transitions under extreme pressure further complicate the quantum confinement landscape. CdTe, for instance, undergoes a structural transition from the zinc-blende to the rocksalt phase at pressures around 3-4 GPa in bulk form. However, in QDs, the transition pressure can be significantly higher due to the additional energy required to overcome quantum confinement effects. The phase transition alters the band structure dramatically, often leading to a collapse of the bandgap and loss of luminescence. Studying these transitions in QDs provides insights into the stability of nanoscale materials under extreme conditions.
High-pressure techniques also enable the investigation of electron-phonon coupling in QDs. Compression reduces phonon frequencies due to increased atomic force constants, which in turn affects carrier relaxation pathways. In CdTe QDs, the longitudinal optical (LO) phonon energy decreases with pressure, altering non-radiative recombination rates and thermalization processes. This has direct implications for the performance of QDs in light-emitting devices, where efficient radiative recombination is desired.
The interplay between pressure and quantum confinement can be summarized in terms of key trends:
- Bandgap increases linearly with pressure, with a magnitude dependent on QD size.
- Exciton binding energy rises due to enhanced Coulomb interaction and reduced dielectric screening.
- Carrier localization strengthens, leading to sharper optical transitions.
- Strain gradients introduce inhomogeneities in electronic structure.
- Phase transition pressures are elevated compared to bulk materials.
These findings highlight the unique opportunities offered by high-pressure studies in tailoring the optoelectronic properties of quantum-confined systems. By systematically varying pressure, researchers can probe fundamental aspects of carrier dynamics, excitonic interactions, and structural stability in nanostructures, paving the way for advanced applications in tunable light sources, pressure sensors, and high-efficiency photovoltaics.
The ability to precisely control quantum confinement through external pressure opens new avenues for designing materials with customized electronic properties. Future research may explore the combined effects of pressure and other external stimuli, such as electric or magnetic fields, to further manipulate carrier behavior in nanostructures. Such studies will deepen our understanding of nanoscale phenomena and enable the development of next-generation quantum technologies.