5.4 Environmental influences on single-molecule conductance
3 min read•august 7, 2024
Environmental factors can make or break single-molecule conductance. Solvents, pH, and electrostatic surroundings all play a part in how electrons flow through tiny molecular bridges between electrodes.
Physical factors like and mechanical stress also shake things up. These external influences can change a molecule's shape, energy levels, and how it connects to electrodes, altering its conductance in surprising ways.
Solvent and Electrolyte Effects
Impact of Solvents on Single-Molecule Conductance
Top images from around the web for Impact of Solvents on Single-Molecule Conductance
The single-molecule electrical conductance of a rotaxane-hexayne supramolecular assembly ... View original
Is this image relevant?
Electronic conductance and thermopower of single-molecule junctions of oligo(phenyleneethynylene ... View original
Is this image relevant?
The single-molecule electrical conductance of a rotaxane-hexayne supramolecular assembly ... View original
Is this image relevant?
Electronic conductance and thermopower of single-molecule junctions of oligo(phenyleneethynylene ... View original
Is this image relevant?
1 of 2
Top images from around the web for Impact of Solvents on Single-Molecule Conductance
The single-molecule electrical conductance of a rotaxane-hexayne supramolecular assembly ... View original
Is this image relevant?
Electronic conductance and thermopower of single-molecule junctions of oligo(phenyleneethynylene ... View original
Is this image relevant?
The single-molecule electrical conductance of a rotaxane-hexayne supramolecular assembly ... View original
Is this image relevant?
Electronic conductance and thermopower of single-molecule junctions of oligo(phenyleneethynylene ... View original
Is this image relevant?
1 of 2
- Solvents can significantly influence the conductance of single molecules by altering their electronic structure and charge transport properties
- Polarity of the solvent affects the energy levels of the molecule and the charge injection barriers at the molecule-electrode interfaces
- Solvent-molecule interactions (hydrogen bonding, dipole-dipole interactions) can modify the molecular conformation and the electronic coupling between the molecule and the electrodes
- Solvent viscosity and dielectric constant play a role in determining the stability and lifetime of the
pH and Electrochemical Gating Effects
- pH of the solution can modulate the conductance of single molecules, especially those containing pH-sensitive functional groups (carboxylic acids, amines)
- Protonation or deprotonation of these groups alters the electronic structure and charge distribution within the molecule, leading to changes in conductance
- Electrochemical gating involves applying an external voltage to the solution to control the redox state of the molecule and tune its conductance
- Gating can shift the energy levels of the molecule relative to the Fermi level of the electrodes, enabling the study of charge transport mechanisms (hopping, )
Influence of Electrostatic Environment
- Electrostatic environment surrounding the molecule can significantly impact its conductance by modifying the energy landscape for charge transport
- Presence of ions, counterions, or charged species in the solution screens the electric field and affects the potential drop across the molecule
- Ionic strength and type of electrolyte determine the Debye length and the extent of electrostatic screening
- Electrostatic interactions between the molecule and the electrodes can alter the molecule-electrode coupling and the charge injection barriers
Physical Factors
Temperature Effects on Single-Molecule Conductance
- Temperature plays a crucial role in determining the conductance of single molecules by influencing the charge transport mechanisms and the stability of the molecular junctions
- Increasing temperature can enhance the thermal energy of the charge carriers, leading to increased conductance through thermally activated processes (hopping, thermionic emission)
- Temperature-dependent measurements can provide insights into the energy barriers for charge transport and the role of electron-phonon interactions
- High temperatures can also induce conformational changes or thermal breakage of the molecular junctions, affecting the conductance stability
Mechanical Stress and Strain Effects
- Mechanical stress or strain applied to the molecular junctions can modulate the conductance of single molecules by altering their conformation and electronic structure
- Stretching or compressing the molecule changes the bond lengths and angles, affecting the overlap of molecular orbitals and the electronic coupling between the molecule and the electrodes
- Mechanical deformation can also modify the energy level alignment between the molecule and the electrodes, leading to changes in conductance
- Measuring conductance under controlled mechanical stress provides insights into the relationship between molecular structure and charge transport properties
Molecular Conformation and State
Conformational Changes and Their Impact on Conductance
- Molecular conformation plays a significant role in determining the conductance of single molecules by influencing the electronic structure and charge transport pathways
- Conformational changes (rotations, twists, bends) can alter the conjugation length, the degree of π-orbital overlap, and the electronic coupling between different parts of the molecule
- Molecules with planar, conjugated structures often exhibit higher conductance compared to those with non-planar or twisted conformations
- Externally induced conformational changes (light, electric field, pH) can be used to control the conductance of single molecules and create molecular switches or
Redox State and Its Effect on Conductance
- Redox state of the molecule can significantly influence its conductance by modifying the electronic structure and the energy level alignment with the electrodes
- Oxidation or reduction of the molecule can add or remove electrons from specific molecular orbitals, changing the energy gap and the charge transport properties
- Redox-active molecules (ferrocene, quinones) can exhibit multiple stable conductance states depending on their oxidation state
- Electrochemical gating or chemical redox agents can be used to control the redox state of the molecule and switch between different conductance levels
Key Terms to Review (16)
Break-junction technique: The break-junction technique is a method used to investigate the electrical properties of single molecules by creating a nanogap in a metallic wire. This technique allows for precise control over the distance between electrodes, enabling researchers to measure conductance changes as molecules are introduced into the gap. It plays a crucial role in understanding how environmental factors influence molecular conductance, as variations in conditions can dramatically affect electronic behavior.
Charge Transfer: Charge transfer refers to the movement of electrical charge from one molecule to another, which is fundamental to understanding electronic properties in molecular systems. This process is key in influencing conductivity, as it dictates how effectively electrons can move through a material. Additionally, charge transfer plays a crucial role in enabling logic operations within molecular electronics, where the efficiency of signal processing relies on the successful transfer of charge between molecules.
Conjugated Polymers: Conjugated polymers are a class of materials characterized by alternating single and double bonds along their backbone, allowing for extended $ ext{\pi}$-electron delocalization. This unique structure gives them significant electrical, optical, and mechanical properties, making them essential in various applications like organic electronics and photovoltaics.
Electron transport: Electron transport is the movement of electrons through a material, typically in response to an electric field, which plays a crucial role in determining the electrical conductivity of that material. This process is fundamental in various systems, such as nanostructures and molecular devices, where the efficiency of electron transport directly influences their performance and functionality. Understanding how electrons travel through these systems allows researchers to manipulate conductance properties and develop advanced electronic applications.
Energy conversion devices: Energy conversion devices are systems or components that transform one form of energy into another, allowing for the utilization of energy in various applications. These devices play a crucial role in enhancing the efficiency of energy use, impacting areas like electronics and environmental sustainability, especially when considering how environmental factors affect their performance at the molecular level.
Landauer Formula: The Landauer formula is a fundamental equation in quantum transport theory that relates the electrical conductance of a quantum system to the transmission probability of electrons through that system. This formula highlights the quantization of conductance, emphasizing how the conductance can only take on discrete values based on the properties of the molecular junctions involved, and it plays a crucial role in understanding the interactions between molecules and electrodes as well as how environmental factors can influence these conductance properties.
Marcus Theory: Marcus Theory is a theoretical framework that explains the rates of electron transfer reactions in molecular systems, emphasizing the role of energy levels and the coupling between electronic states. This theory provides insights into how changes in molecular structure and interactions with surroundings influence charge transfer, making it crucial for understanding processes in molecular electronics and biological systems.
Metal-organic frameworks: Metal-organic frameworks (MOFs) are a class of materials made up of metal ions or clusters coordinated to organic ligands, forming a porous structure. These materials are highly versatile due to their tunable porosity and surface area, making them ideal for various applications including gas storage, separation, and catalysis. Their unique properties also enhance their potential for use in electronic devices and molecular memory storage.
Molecular alignment: Molecular alignment refers to the arrangement and orientation of molecules in a specific direction, which can significantly influence their electronic properties and behavior in various materials. This alignment is crucial in determining how well molecules can conduct electricity, as the orientation affects charge transport pathways and interactions between molecules. Proper molecular alignment can enhance device performance in applications like organic electronics, while misalignment may lead to increased resistance and instability.
Molecular junctions: Molecular junctions are nanoscale interfaces formed between molecules and conductive materials, enabling electron transport at the molecular level. They serve as the essential components in molecular electronics, where the flow of electrons through these junctions is critical for device functionality and performance.
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.
Sensors: Sensors are devices that detect and respond to physical stimuli from the environment, converting those stimuli into measurable signals. These signals can then be analyzed to provide valuable information about the surrounding conditions, making sensors crucial in various applications, including molecular electronics. They can influence the conductance of single molecules, interact with self-assembled monolayers, and enable the functionality of molecular switches based on redox reactions or light.
Solvent effects: Solvent effects refer to the influence that a solvent has on the physical and chemical properties of solutes, which can significantly impact molecular interactions and reactions. These effects are crucial in understanding how different environments affect the conductance of single molecules, as solvents can alter energy levels, electron transfer rates, and overall molecular stability.
Temperature: Temperature is a measure of the average kinetic energy of particles in a substance, indicating how hot or cold that substance is. In the context of molecular electronics, temperature influences how molecules conduct electricity, affecting their behavior in different environments and conditions. Understanding temperature is crucial because it plays a significant role in determining the stability and performance of electronic components at the molecular level.
Tunneling: Tunneling is a quantum mechanical phenomenon where particles can pass through a potential energy barrier, even if they seemingly do not have enough energy to overcome it. This effect is critical in molecular electronics, as it allows electrons to travel between molecules and electrodes without needing to go over an energy barrier, significantly influencing how devices operate at the nanoscale.