9.5 Surface states

Surface states are electronic states that exist at material boundaries, altering properties and enabling new technologies. These states, including Tamm, Shockley, and topological varieties, arise from broken symmetry and unique band structures at surfaces.

Understanding surface states is crucial for designing devices that rely on interface phenomena. They influence electronic properties like band structure, density of states, and work function. Various experimental techniques allow researchers to probe and characterize these states.

Types of surface states

• Surface states play a crucial role in condensed matter physics by altering the electronic properties of materials at their boundaries
• Understanding different types of surface states provides insights into material behavior and enables the development of novel technologies

Tamm states

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• Localized electronic states occurring at abrupt terminations of periodic crystal potentials
• Arise from the breaking of translational symmetry at the surface
• Exhibit wave functions that decay exponentially into both the vacuum and bulk crystal
• Often found in ionic crystals and semiconductors (NaCl, GaAs)

Shockley states

• Intrinsic surface states originating from the band structure of the bulk material
• Form when bulk bands are split by the surface potential, creating new states within the band gap
• Characterized by wave functions that extend deeper into the bulk compared to Tamm states
• Commonly observed in metals and some semiconductors (Cu, Au, Si)

Topological surface states

• Unique electronic states protected by time-reversal symmetry and topology of the bulk band structure
• Exhibit linear dispersion and spin-momentum locking, resulting in robust conduction channels
• Immune to backscattering from non-magnetic impurities, leading to dissipationless transport
• Found in topological insulators and Weyl semimetals (Bi2Se3, Cd3As2)

Electronic properties

• Surface states significantly influence the electronic properties of materials at interfaces
• Understanding these properties is essential for designing and optimizing devices that rely on surface phenomena

Surface band structure

• Describes the energy-momentum relationship of electronic states at the surface
• Differs from bulk band structure due to broken symmetry and surface reconstruction
• Can exhibit unique features such as surface bands, resonances, and hybridization with bulk states
• Visualized using techniques like angle-resolved photoemission spectroscopy (ARPES)

Density of states

• Represents the number of available electronic states per unit energy interval at the surface
• Often shows distinct peaks or features corresponding to surface states
• Influences various surface properties including reactivity and electron emission
• Can be probed experimentally using scanning tunneling spectroscopy (STS)

Work function

• Minimum energy required to remove an electron from the surface to vacuum level
• Depends on surface composition, structure, and electronic properties
• Affects electron emission processes and catalytic activity
• Can be modified by surface treatments or adsorbates (Cs coating lowers work function)

Experimental techniques

• Various experimental methods are employed to study surface states and their properties
• These techniques provide complementary information about surface electronic structure and morphology

Angle-resolved photoemission spectroscopy

• Powerful technique for mapping the electronic band structure of surfaces
• Utilizes the photoelectric effect to eject electrons from the sample
• Measures the kinetic energy and emission angle of photoelectrons to reconstruct the band structure
• Capable of resolving surface states, bulk bands, and their spin polarization

Scanning tunneling microscopy

• Provides atomic-scale imaging and spectroscopy of surfaces
• Operates based on quantum tunneling of electrons between a sharp tip and the sample surface
• Allows direct visualization of surface topography and local density of states
• Can probe individual surface states and their spatial distribution

Low-energy electron diffraction

• Technique for determining the surface structure and symmetry of crystalline materials
• Utilizes elastic scattering of low-energy electrons from surface atoms
• Produces diffraction patterns that reveal information about surface periodicity and reconstruction
• Complements other surface-sensitive techniques by providing structural information

Surface reconstruction

• Surfaces often undergo structural changes to minimize their energy
• Understanding reconstruction mechanisms is crucial for predicting and controlling surface properties

Mechanisms of reconstruction

• Driven by the need to minimize surface free energy and dangling bonds
• Involves rearrangement of surface atoms to form new bonding configurations
• Can lead to changes in surface symmetry, periodicity, and electronic structure
• Influenced by factors such as temperature, pressure, and surface composition

Common reconstruction patterns

• Observed in various materials systems with specific nomenclature
• Include simple adatom structures, missing row reconstructions, and complex rearrangements
• Examples include Si(111) 7x7, Au(110) 1x2, and Pt(100) hex reconstructions
• Often described using Wood's notation to indicate surface unit cell changes

Energy considerations

• Reconstruction occurs when the energy gain from new bonding configurations outweighs the strain energy
• Surface stress plays a crucial role in determining the stability of different reconstructions
• Temperature-dependent phase transitions between different reconstructed surfaces can occur
• Understanding energy considerations helps predict and control surface structures

Adsorption on surfaces

• Adsorption of atoms or molecules on surfaces is fundamental to many technological processes
• Studying adsorption phenomena provides insights into surface reactivity and catalysis

Physisorption vs chemisorption

• Physisorption involves weak van der Waals interactions between adsorbates and surfaces
• Chemisorption involves the formation of chemical bonds between adsorbates and surface atoms
• Physisorption typically has lower binding energies and longer adsorbate-surface distances
• Chemisorption often leads to significant changes in the electronic structure of both adsorbate and surface

Binding sites

• Specific locations on the surface where adsorbates preferentially attach
• Include high-symmetry sites such as on-top, bridge, and hollow positions
• Determined by the surface structure, electronic properties, and adsorbate characteristics
• Can be identified using techniques like scanning tunneling microscopy and density functional theory calculations

Adsorbate-induced states

• New electronic states that arise from the interaction between adsorbates and surface atoms
• Can significantly modify the surface electronic structure and reactivity
• Include bonding and antibonding states, as well as resonances with surface bands
• Play a crucial role in determining the strength and nature of adsorbate-surface interactions

Surface plasmons

• Collective oscillations of electrons at metal-dielectric interfaces
• Important for various applications in optics, sensing, and energy conversion

Surface plasmon polaritons

• Electromagnetic waves coupled to electron oscillations propagating along metal-dielectric interfaces
• Exhibit strong field confinement and enhancement near the surface
• Characterized by dispersion relations that depend on the dielectric properties of both media
• Enable subwavelength light manipulation and waveguiding (metal nanowires)

Localized surface plasmons

• Non-propagating excitations of conduction electrons in metallic nanostructures
• Result in strong light scattering and absorption at specific resonance frequencies
• Highly sensitive to the size, shape, and composition of nanoparticles
• Utilized in applications such as surface-enhanced Raman spectroscopy and colorimetric sensing

Applications in sensing

• Surface plasmons enable highly sensitive detection of chemical and biological species
• Surface plasmon resonance (SPR) sensors detect refractive index changes near metal surfaces
• Localized surface plasmon resonance (LSPR) sensors utilize shifts in nanoparticle resonances
• Plasmonic enhancement of spectroscopic techniques improves detection limits (SERS, SEF)

Quantum well states

• Electronic states confined in thin films or layered structures
• Arise from quantum confinement effects in one or more dimensions

Confinement effects

• Occur when the film thickness becomes comparable to the electron wavelength
• Lead to quantization of electronic energy levels perpendicular to the film
• Result in discrete energy levels instead of continuous bands in the confinement direction
• Influence various properties including optical absorption, conductivity, and magnetism

Quantum size effects

• Thickness-dependent oscillations in material properties due to quantum confinement
• Observed in thin films, nanoparticles, and quantum dots
• Include variations in electronic structure, work function, and superconducting critical temperature
• Enable tuning of material properties by controlling system dimensions (quantum dots)

Thin film properties

• Quantum well states significantly influence the properties of ultrathin films
• Electronic structure evolves from discrete atomic-like states to bulk-like bands with increasing thickness
• Surface and interface states can hybridize with quantum well states, modifying film properties
• Quantum size effects in thin films impact growth modes, stability, and reactivity

Surface magnetism

• Magnetic properties at surfaces often differ from bulk due to reduced coordination and symmetry breaking
• Understanding surface magnetism is crucial for developing spintronic devices and magnetic data storage

Magnetic anisotropy

• Preferential alignment of magnetic moments along specific crystallographic directions
• Often enhanced at surfaces due to reduced symmetry and modified electronic structure
• Influences the stability of magnetic domains and determines easy and hard magnetization axes
• Can be tailored by surface engineering and adsorption (Co/Pt multilayers)

Exchange coupling

• Interaction between magnetic moments of neighboring atoms or layers
• Can be significantly modified at surfaces due to changes in atomic coordination
• Determines magnetic ordering and Curie temperature of surface layers
• Enables phenomena such as exchange bias in ferromagnetic/antiferromagnetic interfaces

Spin-polarized surface states

• Electronic states at surfaces with imbalanced spin populations
• Arise from exchange splitting of surface bands in ferromagnetic materials
• Can be probed using spin-resolved photoemission spectroscopy
• Play a crucial role in spin-dependent transport and tunneling phenomena (magnetic tunnel junctions)

Surface superconductivity

• Superconducting properties at surfaces can differ significantly from bulk behavior
• Understanding surface superconductivity is essential for optimizing superconducting devices

Proximity effect

• Induction of superconducting correlations in non-superconducting materials near interfaces
• Enables creation of Josephson junctions and superconducting heterostructures
• Decay length of induced superconductivity depends on material properties and temperature
• Utilized in superconducting quantum interference devices (SQUIDs)

Surface superconducting gap

• Energy gap in the electronic density of states characteristic of superconducting state
• Can be modified at surfaces due to changes in electron-phonon coupling and dimensionality
• Measurable using scanning tunneling spectroscopy and point-contact spectroscopy
• Understanding surface gap behavior is crucial for optimizing superconducting devices

Vortex states

• Quantized magnetic flux lines penetrating type-II superconductors
• Exhibit unique behavior near surfaces due to boundary conditions and pinning effects
• Influence critical currents and magnetic field penetration in superconducting materials
• Can be directly visualized using techniques like scanning SQUID microscopy

Technological applications

• Surface states and related phenomena find numerous applications in various technological fields
• Understanding and controlling surface properties enables development of novel devices and materials

Catalysis

• Surface states play a crucial role in determining catalytic activity and selectivity
• Tailoring surface electronic structure can optimize adsorption energies and reaction barriers
• Surface reconstruction and defects often serve as active sites for catalytic reactions
• Applications include fuel cells, chemical synthesis, and environmental remediation (Pt nanoparticles)

Nanoelectronics

• Surface states and quantum confinement effects enable novel electronic device concepts
• Topological surface states in quantum spin Hall insulators offer potential for low-power electronics
• Two-dimensional materials like graphene utilize unique surface properties for high-mobility devices
• Surface engineering allows tuning of Schottky barriers and contact resistance in nanoelectronic devices

Surface acoustic wave devices

• Utilize acoustic waves propagating along material surfaces for signal processing and sensing
• Surface states and reconstruction influence the propagation and interaction of surface acoustic waves
• Applications include filters, oscillators, and chemical sensors in telecommunications and automotive industries
• Piezoelectric materials like lithium niobate are commonly used for SAW device fabrication