4.1 Coherent and incoherent transport mechanisms
2 min read•august 7, 2024
Electrons in molecules can travel in two ways: coherently or incoherently. keeps the electron's wave-like nature intact, allowing for quantum effects like tunneling. involves electrons losing their wave properties through or hopping between sites.
The way electrons move affects how well electricity flows through molecules. Coherent transport often leads to better conductivity, while incoherent transport can limit it. Understanding these mechanisms helps scientists design more efficient molecular electronic devices.
Coherent Transport Mechanisms
Quantum Tunneling and Ballistic Transport
Top images from around the web for Quantum Tunneling and Ballistic Transport
Exploring coherent transport through π-stacked systems for molecular electronic devices ... View original
Is this image relevant?
Exploring coherent transport through π-stacked systems for molecular electronic devices ... View original
Is this image relevant?
Exploring coherent transport through π-stacked systems for molecular electronic devices ... View original
Is this image relevant?
Exploring coherent transport through π-stacked systems for molecular electronic devices ... View original
Is this image relevant?
1 of 2
Top images from around the web for Quantum Tunneling and Ballistic Transport
Exploring coherent transport through π-stacked systems for molecular electronic devices ... View original
Is this image relevant?
Exploring coherent transport through π-stacked systems for molecular electronic devices ... View original
Is this image relevant?
Exploring coherent transport through π-stacked systems for molecular electronic devices ... View original
Is this image relevant?
Exploring coherent transport through π-stacked systems for molecular electronic devices ... View original
Is this image relevant?
1 of 2
- Coherent transport preserves the phase information of the electron wave function as it moves through a material or molecule
- Quantum tunneling allows electrons to pass through potential barriers that they classically could not surmount, based on the wave-particle duality of quantum mechanics
- Occurs when the electron wavelength is larger than the barrier width, enabling the electron to tunnel through (α-Hemihydrate crystals)
- involves electrons traveling through a material with minimal scattering, maintaining their momentum
- Happens when the electron is longer than the dimensions of the material (carbon nanotubes, graphene nanoribbons)
Phase Coherence and Quantum Interference
- refers to the electrons maintaining a fixed phase relationship as they propagate through a material
- arises from the wave nature of electrons, allowing them to interfere constructively or destructively
- Constructive interference enhances electron transmission, while destructive interference suppresses it (double-slit experiment, Aharonov-Bohm effect)
- Coherent transport is highly sensitive to external factors such as temperature, electric fields, and magnetic fields, which can disrupt the phase coherence
Incoherent Transport Mechanisms
Hopping and Scattering
- Incoherent transport involves the loss of phase information as electrons move through a material, typically due to scattering events
- is a transport mechanism where electrons jump between localized states, losing phase coherence in the process
- Occurs in materials with disorder or localized electronic states (organic , disordered polymers)
- Scattering events, such as electron-phonon interactions or , randomize the electron momentum and destroy phase coherence
- Scattering can be elastic, conserving energy, or inelastic, involving energy exchange (electron-phonon scattering, electron-electron scattering)
Mean Free Path and Transport Regime
- The mean free path is the average distance an electron travels between scattering events
- Determines the dominant : ballistic (coherent) or diffusive (incoherent)
- When the mean free path is much longer than the system size, coherent transport dominates (quantum dots, nanowires)
- As the system size becomes larger than the mean free path, incoherent transport takes over, and the electron motion becomes diffusive (bulk materials, long molecules)
- The transition from coherent to incoherent transport depends on factors such as temperature, disorder, and electron-phonon coupling strength
Key Terms to Review (23)
Ballistic transport: Ballistic transport refers to the movement of charge carriers, such as electrons, through a conductor without scattering or losing energy over relatively short distances. This phenomenon is significant in nanoscale materials, where the dimensions are comparable to the mean free path of the charge carriers, allowing them to traverse the material without collisions. Understanding ballistic transport is essential for grasping coherent and incoherent transport mechanisms as well as the principles underlying conductance quantization in mesoscopic systems.
Coherent transport: Coherent transport refers to the quantum mechanical phenomenon where charge carriers, such as electrons, maintain their phase relationship while moving through a conductor. This phase coherence allows for interference effects that can lead to distinct transport properties in molecular and nanoscale systems, contrasting sharply with classical transport mechanisms observed in traditional electronics.
Conductance Measurement: Conductance measurement refers to the process of quantifying how easily electric current can flow through a material or device. It is an important aspect of understanding the electronic properties of materials, especially in contexts like coherent and incoherent transport mechanisms, where the nature of charge carrier movement significantly influences overall conductivity.
Conjugated System: A conjugated system refers to a series of connected p-orbitals with delocalized electrons that participate in resonance, resulting in enhanced stability and unique electronic properties. These systems often play a crucial role in molecular electronics, as the ability of electrons to move freely between adjacent p-orbitals influences both coherent and incoherent transport mechanisms, impacting conductivity and charge transport.
Electron hopping: Electron hopping refers to the process by which an electron moves between localized states in a material, often influenced by thermal energy. This mechanism is particularly relevant in molecular electronics, as it enables charge transport through disordered systems where coherent conduction may not be feasible. Understanding electron hopping helps to explain transport properties in materials with different structural and electronic characteristics.
Impurity Scattering: Impurity scattering refers to the process where charge carriers, like electrons, are deflected from their original paths due to interactions with impurities or defects within a material. This phenomenon significantly impacts how well charge carriers can move through a material, influencing its conductivity. Understanding impurity scattering is essential when examining coherent and incoherent transport mechanisms, as it plays a role in determining the degree of order or disorder in charge transport.
Incoherent transport: Incoherent transport refers to the movement of charge carriers, such as electrons, through a material where the wave-like properties of these particles are lost, resulting in a random and unpredictable flow. This type of transport typically occurs when there is significant scattering of charge carriers due to defects, impurities, or thermal vibrations in the lattice structure of a material. As a result, incoherent transport can limit the efficiency of electronic devices and plays a crucial role in understanding how charge moves in disordered systems.
Landauer Theory: Landauer Theory is a fundamental principle in quantum mechanics and information theory that relates the minimum possible energy cost of irreversible computation to the entropy produced during the process. This theory highlights how information loss is fundamentally tied to energy dissipation, offering insights into coherent and incoherent transport mechanisms, the effects of molecular length and conjugation on conductance, and the integration of molecular systems with traditional electronic systems.
Mean Free Path: Mean free path is the average distance a particle travels between collisions with other particles in a medium. This concept is crucial in understanding transport mechanisms, as it influences how particles move through materials, either coherently or incoherently, and affects the overall conductivity and electronic properties of materials.
Metals: Metals are elements that typically have high electrical and thermal conductivity, malleability, and ductility. These properties are essential in understanding coherent and incoherent transport mechanisms, as they allow for the flow of electrons and influence how charge carriers move through materials. Metals form the backbone of many electronic devices, making their behavior under various transport conditions crucial for applications in molecular electronics.
Molecular Junction: A molecular junction is a nanoscale structure formed between two conductive electrodes and a single molecule, enabling the study of electronic transport properties at the molecular level. These junctions are critical in understanding how molecules can function as electronic components, bridging the gap between traditional electronics and molecular electronics. They play a pivotal role in investigating charge transport mechanisms, which can be coherent or incoherent, and in manipulating individual molecules using scanning probe microscopy (SPM) techniques.
Molecular switches: Molecular switches are molecules that can reversibly change their conformations or electronic states in response to external stimuli such as light, voltage, or chemical changes. This ability to toggle between different states allows them to perform functions similar to traditional electronic components, making them crucial for advancements in molecular electronics and related fields.
Nanoelectronic devices: Nanoelectronic devices are small-scale electronic components that operate at the nanoscale, typically involving structures that are less than 100 nanometers in size. These devices leverage quantum mechanical effects to enhance functionality and performance, making them vital for the development of advanced technologies in areas like computing, communication, and sensing.
Phase coherence: Phase coherence refers to the property of waves or particles being in sync with each other in terms of their phase, which plays a crucial role in determining the behavior of electrons during transport. When phase coherence is maintained, the wave-like nature of electrons allows for coherent transport, leading to phenomena like quantum interference and enhanced conductivity. Conversely, loss of phase coherence can result in incoherent transport, where the electron's wave properties are lost, leading to scattering and reduced mobility.
Quantum interference: Quantum interference refers to the phenomenon where quantum states combine in such a way that the probability amplitudes of different paths lead to either constructive or destructive interference. This concept is crucial in understanding how particles, such as electrons, behave in systems where multiple pathways are available, impacting their transport properties. It plays a significant role in coherent transport mechanisms and also highlights how environmental factors can influence single-molecule conductance.
Scanning Tunneling Microscopy: Scanning tunneling microscopy (STM) is a powerful technique used to visualize surfaces at the atomic level by measuring the tunneling current between a sharp conducting tip and the sample surface. This method allows scientists to obtain detailed images of surfaces, providing insights into the electronic properties and behavior of materials, especially at the nanoscale.
Scattering: Scattering refers to the process where particles, such as electrons, deviate from their original trajectory due to interactions with other particles or obstacles. This deviation can significantly influence the transport of charge carriers in materials, especially in the context of coherent and incoherent transport mechanisms, where the behavior of these carriers can either maintain their phase relationship or lose it due to scattering events.
Semiconductors: Semiconductors are materials that have electrical conductivity between conductors and insulators, making them essential in electronic devices. Their unique properties allow for control over electrical current, enabling applications in transistors, diodes, and other electronic components. The behavior of semiconductors is influenced by their energy levels, which can be altered through quantum confinement, transport mechanisms, and advanced fabrication techniques.
Superposition: Superposition is a fundamental principle in quantum mechanics that states a quantum system can exist in multiple states at once until it is measured or observed. This concept is crucial in various applications, including quantum computing and molecular electronics, where the ability of particles to be in multiple states simultaneously leads to enhanced computational capabilities and innovative designs for electronic devices.
Temperature Dependence: Temperature dependence refers to how the properties and behavior of materials, especially in the context of electronic systems, change with varying temperatures. In molecular electronics, temperature can significantly influence charge transport, coherence of electron waves, and the noise levels in systems, affecting their efficiency and reliability. Understanding these variations is crucial for optimizing device performance across different operational conditions.
Tight-Binding Model: The tight-binding model is a theoretical framework used to describe the electronic properties of solids, particularly in crystalline materials. It focuses on how electrons can hop between neighboring atomic sites, allowing for the calculation of energy bands and electronic states. This model is crucial for understanding quantum confinement effects, transport mechanisms, conductance quantization, and how molecular structures impact electronic properties.
Transport regime: Transport regime refers to the different mechanisms and conditions that dictate how charge carriers move through a material, which can be categorized primarily into coherent and incoherent transport. This concept is vital for understanding the efficiency and functionality of electronic devices at the molecular level, as it influences factors such as current flow, energy dissipation, and the overall performance of materials used in molecular electronics.
Wavefunction: A wavefunction is a mathematical description of the quantum state of a particle or system, representing the probability amplitude for the position and momentum of the particle. This concept is central to quantum mechanics, as it encapsulates all the information about a quantum system and allows for the calculation of observable properties through the use of the Schrödinger equation. The wavefunction plays a critical role in understanding how particles behave, especially in contexts where both wave-like and particle-like characteristics are observed.