Quantum effects at surfaces and interfaces are mind-bending phenomena that arise when electrons are confined to small spaces. These effects lead to unique electronic and optical properties that differ from bulk materials, opening up exciting possibilities for new technologies.

Understanding and controlling quantum effects at surfaces is crucial for developing advanced materials and devices. From quantum computing to spintronics, these effects are driving innovation in fields that could revolutionize technology as we know it.

Quantum effects at surfaces and interfaces

Manifestation of quantum effects

• Quantum effects at surfaces and interfaces arise due to the reduced dimensionality and confinement of electrons, leading to distinct electronic and optical properties compared to bulk materials
• Surface states and interface states can emerge as a result of the abrupt termination of the periodic potential at the surface or interface, giving rise to localized electronic states with unique energy levels and wave functions
• Quantum tunneling can occur at surfaces and interfaces, allowing electrons to penetrate potential barriers and enabling phenomena such as scanning tunneling microscopy (STM) and resonant tunneling in heterostructures
• Quantum well states can form in thin films or layered structures due to the confinement of electrons in the direction perpendicular to the surface or interface, resulting in quantized energy levels and modified electronic properties

Impact of quantum effects on properties

• Spin-orbit coupling can be enhanced at surfaces and interfaces due to the broken inversion symmetry, leading to spin-dependent electronic states and the emergence of spin-polarized surface or interface states
• Electron-electron interactions can be modified at surfaces and interfaces due to the reduced screening and altered Coulomb potential, leading to phenomena such as surface plasmons and many-body effects
• Quantum effects at surfaces and interfaces can significantly alter the electronic, optical, magnetic, and transport properties of materials compared to their bulk counterparts
• The understanding and control of quantum effects at surfaces and interfaces are crucial for the development of advanced materials and devices with tailored functionalities (quantum dots, 2D materials)

Quantum confinement and its impact

Concept of quantum confinement

• Quantum confinement refers to the spatial confinement of electrons or holes in one or more dimensions, leading to the modification of their energy levels and wave functions compared to the bulk material
• In quantum wells, electrons are confined in one dimension, resulting in the formation of two-dimensional electron gases (2DEGs) with quantized energy levels and modified density of states
  • The energy levels in a quantum well are inversely proportional to the square of the well width, leading to size-dependent electronic and optical properties
• Quantum wires and quantum dots exhibit confinement in two and three dimensions, respectively, resulting in one-dimensional and zero-dimensional electronic systems with discrete energy levels

Impact on electronic and optical properties

• Quantum confinement can lead to the blue-shift of the bandgap in semiconductor nanostructures, enabling the tuning of optical properties such as absorption and emission spectra
• The modified density of states in quantum-confined systems can enhance the efficiency of light-emitting devices, such as quantum well lasers and quantum dot light-emitting diodes (LEDs)
• Quantum confinement can also influence the transport properties of electrons, leading to phenomena such as ballistic transport and conductance quantization in nanoscale systems
• The control of quantum confinement effects allows for the engineering of electronic and optical properties in nanomaterials and nanodevices (quantum cascade lasers, single-photon emitters)

Probing quantum effects at surfaces

Scanning tunneling microscopy and spectroscopy

• Scanning tunneling microscopy (STM) is a powerful technique for imaging and probing the electronic structure of surfaces with atomic resolution, based on the quantum tunneling of electrons between a sharp tip and the sample surface
• Scanning tunneling spectroscopy (STS) is an extension of STM that measures the local density of states (LDOS) of a surface by recording the differential conductance (dI/dV) as a function of the applied bias voltage
  • STS can provide information about the energy-resolved electronic structure, including the presence of surface states, bandgaps, and quantum well states
• Scanning tunneling microscopy/spectroscopy (STM/S) can be combined with magnetic fields to study the spin-dependent electronic states and magnetism at surfaces and interfaces, enabling the investigation of spin-polarized surface states and spin-orbit coupling effects

Other experimental techniques

• Angle-resolved photoemission spectroscopy (ARPES) is a technique that measures the energy and momentum distribution of electrons emitted from a surface upon excitation by photons, allowing the mapping of the electronic band structure and Fermi surface
• Low-temperature STM/S measurements can provide enhanced energy resolution and enable the study of superconductivity, Kondo effect, and other many-body phenomena at surfaces and interfaces
• Inelastic electron tunneling spectroscopy (IETS) can probe the vibrational and magnetic excitations at surfaces and interfaces by measuring the inelastic tunneling channels in the differential conductance spectra
• Scanning probe techniques, such as atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM), can complement STM/S by providing information about the surface topography, work function, and local electrostatic properties

Applications of quantum effects in surface science

Quantum computing

• Quantum computing leverages the principles of quantum mechanics, such as superposition and entanglement, to perform computations that are intractable for classical computers. Surfaces and interfaces play a crucial role in the realization of quantum bits (qubits) and quantum gates
  • Superconducting qubits, such as Josephson junctions, rely on the quantum tunneling of Cooper pairs across insulating barriers, where the interface quality and surface properties are critical for qubit performance
  • Spin qubits, based on the manipulation of electron spins in quantum dots or donor atoms, require precise control over the surface and interface properties to achieve long coherence times and efficient qubit operations
• Topological insulators, which possess conducting surface states with spin-momentum locking, offer potential applications in quantum computing due to their robustness against backscattering and disorder

Spintronics and quantum devices

• Spintronics exploits the spin degree of freedom of electrons for information processing and storage, offering the potential for low-power, high-speed, and non-volatile devices
  • Giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) effects, which rely on the spin-dependent transport across magnetic/non-magnetic interfaces, have revolutionized the field of magnetic data storage and sensing
  • Spin injection and detection at surfaces and interfaces are crucial for the development of spin-based logic devices and spin transistors, where the efficient transfer and manipulation of spin-polarized currents are essential
• Quantum metamaterials, engineered structures with tailored quantum properties, can be designed by controlling the surface and interface properties, enabling novel applications in quantum sensing, quantum communication, and quantum simulation
• Quantum cryptography, based on the principles of quantum key distribution (QKD), relies on the quantum properties of single photons or entangled photon pairs to ensure secure communication. Surfaces and interfaces play a role in the generation, manipulation, and detection of quantum states of light