Dictionary of Terms in Organic Electronics. Specific Terms: M.
Basic concepts necessary to know for understanding of magnetic behavior of organic materials. Magnetization: Extent of a magnetic response of a material to the action of an external magnetic field. Materials by types of magnetization: Diamagnetic: Ordering magnetic particles are electron pairs or magnetic nuclei. They are ordered in one direction under external magnetic field. Materials exhibit magnetization opposing to the applied magnetic field. Magnetic field is weakened in a material. Week repelling effect to the magnetic field. Magnetization disappears when the field is removed. Example - most objects around. Wood. Paramagnetic: Ordering magnetic particles are unpaired electrons (spins). They are ordered in one direction under external magnetic field. Materials exhibit magnetization reinforcing (parallel, therefore - para) to the applied magnetic field. Magnetic field is strengthened in a material. Week attracting effect to the magnetic field. Magnetization disappears when the field is removed. Example - aluminum metal (Al). Ferromagnetic: Ordering magnetic particles are unpaired electrons (spins). They are ordered in one direction under external magnetic field. Below Curie point (see below), ferromagnets exhibit magnetization without of any external magnetic field (spontaneous magnetization) due to self-ordering of spins. Exhibit reinforcing magnetization to the applied magnetic field that usually remains when the field is removed (field-induced magnetization). Magnetic field is strengthened in a material. Strong attracting effect to the magnetic field. Example - iron metal (Fe). Antiferromagnetic: Ordering magnetic particles are unpaired electrons (spins). In contrast to paramagnetic and ferromagnetic, they are ordered in opposite directions in equal quantities under the action of magnetic field. The ordering usually remains when the field is removed. Below Neel point (see below), self-ordering can be observed. Magnetic field is weakened in a material. Week repelling effect to the magnetic field. Example - nickel oxide (NiO). Ferrimagnetic: Ordering magnetic particles are unpaired electrons (spins). They are ordered in opposite directions in unequal quantities under the action of magnetic field. Behave as week ferromagnets. Magnetic field is strengthened in a material. Week to strong attracting effect to the magnetic field. Example - magnetite (Fe3O4). Metamagnetic: Materials that can change their magnetic properties depending on the strength of applied magnetic field. For example, paramagnets transform to ferromagnets or vice versa when the field is increased or reduced. Magnetic properties of materials: Magnetic susceptibility: M = cvH M = Magnetization A/m (Ampere/meter), difference in magnetic field strength between applied external magnetic field and resulting field in a material. Magnetic susceptibility is used to characterize diamagnets and paramagnets. In the case of ferromagnets, so-called static magnetic susceptibility is more commonly used. Static susceptibility is the one that is measured at the constant field strengths as the function of such physical parameters as temperature, pressure, irradiation. If the static susceptibility of paramagnetic materials is constant at different temperatures, this kind of paramagnetism is known as Pauli paramagnetism and it is characteristic of paramagnetic metals. Magnetic permeability: B = mH B = Magnetic field (flux density) T (Tesla) (A/m is also used). It is strength of magnetic field inside of a material, when external field is applied. Relation between B, M, and H: B = m0(H + M) = m0(1 + cv)H = mH Magnetic saturation and coercivity: Magnetic saturation is characteristic of nonlinear magnetic materials that are ferromagnets, antiferromagnets, and ferrimagnets. Saturation of a material is a state, when magnetic flax density B inside of the material is not longer increased along with the further increase of an external magnetic field H. Coercivity is a constant of a ferromagnetic material, therefore it is used to characterize them. At the same time, magnetic susceptibility and permeability are constant and characteristic of diamagnets and paramagnets. Ferromagnets with low Hc are known as soft magnets, whereas the ones with high Hc are hard or permanent magnets. Both types of magnets are technologically important. Phase transfer points: Curie temperature (Tc): temperature above which ferromagnets transfer to paramagnets. Neel temperature (TN): temperature above which antiferromagnets transfer to paramagnets.
Blocking temperature (TB): temperature above which magnetic behavior of single molecule magnets (SMMs) may no longer be observed due to intermolecular interactions.
Magnetic materials - structural types Diagram below classifies modern development in both inorganic and organic magnetic materials Relatively recently, the science of so called molecular magnetism branched out from traditional magnetism that explored mainly magnetic behavior of compounds at the level of atoms (in metals). Molecular magnetism researches magnetic behavior of molecules rather than atoms. Magnetic behavior of organic compounds belongs to the field of molecular magnetism. Molecules as well as atoms may form bulk magnets (traditional type of magnets), that are also known as 3D magnets (see below). At the same time, two new types of molecular magnets recently evolved: single molecule magnets (SMMs) and nanostructured magnets. SMMs are fully functional magnets on a molecular level. They possess some unique properties, such as quantum tunneling of magnetization. SMMs include: polynuclear metal complexes and clusters, single-chain magnets, spin rings and some others. Nanostructured magnets built of self-organized or self-assembled crystallites. They may possess many unique properties in addition to magnetic ones, (such as anisotropy, transparency, electrical conductance, porosity etc.). Low dimensionality (0D, 1D, 2D) (see below) is characteristic of self-aggregated molecular magnets. Bulk molecular (and atomic) magnets usually contain materials that may be either paramagnetic, ferromagnetic, antiferromagnetic, or ferrimagnetic. Recently, a new type of molecular magnetic materials evolved, that may change magnetic properties under the action of different external factors, such as light, temperature, pressure, etc. They are known as multifunctional magnetic materials, which include so-called spin-crossover compounds, Prussian Blue analogues, cyanide-bridged clusters, chiral magnets. The other type of new functional molecular magnetic materials are dual-function materials. They possess one or more independent physical properties in addition to the magnetic ones, such as electrical conductivity, porosity, or optical properties. Magnetic molecular conductors are the most widely studied organic materials of this type.
Anisotropy and dimensionality of molecular magnets. If a monocrystal of a magnetic material undergo varying degree of magnetization depending on an angle between the crystal axis and a vector of a magnetic field applied, these magnets possess anisotropy of magnetization. It is also known as magnetocrystalline anisotropy. Materials with anisotropy of magnetization are also known as low-dimensional (1D, 2D) magnetic materials. Anisotropy and dimensionality of magnetization is determined how all the spins of a crystal interact (couple) with each other. The coupling may be short-range (between nearby spins) and long-range (between distant spins) and it is determined by 1) anisotropy of magnetic spins, 2) type of spin-spin interaction, and 3) structure of the crystal lattice of a material. Conventional or bulk atomic or ionic metallic or inorganic magnets usually do not possess noticeable anisotropy and equal magnetization of these materials in three directions 3D is normally observed. Molecular magnets are often 3D as well, however lower dimensionality is also very common for them. Very often, crystallization (aggregation) occurs in a linear manner to afford prevailing magnetization in one direction (1D networks). Materials, consisting of homogeneous plains aligned in parallel to each other may have prevailing equal magnetization in two directions (2D networks). When the spins of neighboring molecules do not couple or negligible weakly couple with each other, e.g. the molecules behave as magnetically isolated species, these materials are recognized to possess zero dimensionality (0D). Among organic magnetic materials some charged complexes, polyradicals, and single molecule magnets (SMMs) behave this way.
Molecular magnetic materials may undergo phase transition when subjected to the temperature change. Phase transition is usually gives rise to reorganization of spins, crystal lattice and change in both dimensionality and magnetic properties (ferromagnets transform to paramagnets), which is usually equivalent of Curie (Tc) and Neel (TN) temperatures well known for inorganic magnets. The primary methods of determining of dimensionality and phase transition of molecular magnets are heat capacity calorimetry-methods, which were summarized recently in a comprehensive review: ref. 2.
Magnetic organic materials may be classified as follows: 1. Compounds with 'all-organic' spins contain magnetic spins as unpaired electrons on only 'organic' elements, such as carbon, oxygen, nitrogen, sulfur, phosphorus. Unpaired electrons of these materials can be only p-electrons with isotropic spins of 1/2. Due to isotropy of spins, they may be 3D magnets in the cases of simple crystal lattices. However, crystal symmetry of lattices is anisotropic in majority of cases due to complexity of structure of organic radicals, and all types of lower-dimensional magnets are observed in reality.
In general, organic radicals are highly-energetic, unstable species, that react easily with solvents, surrounding media, and especially with each other to form dimers. Therefore, only long-living, specially stabilized organic radicals may possess useful magnetic properties. Stabilization is achieved usually by applying the combination of two approaches: 1. Protecting of an unpaired electron with bulk surrounding, like four methyl groups and cyclohexyl ring in TEMPO type of radicals (one of them, CATMP, 2 on the scheme above). 2. Delocalization of unpaired electron by it's conjugation with neighboring p- or p-electron-containing atoms or bonds. Among 'stabilizing' neighboring atoms, nitrogen (nitronyl-nitroxides: p-NPPN-1, TEMPO-type-2) and sulfur (thiazyl radicals: 4) are the most effective. Stabilizing p-bond may be represented as C=N bond in verdazyl group of radicals (structure similar to DPTOV, 3). 2. Compounds with both metal and organic spins include organic complexes and salts containing both magnetic metal ions and organic radicals. Thus, metallocenium salts of unsaturated nitriles were found to be efficient ferromagnets, and among them decamethylferrocenium tetracyanoethenide ([DMFs][TCNE] or [FeCp*2][TCNE], 5 on the scheme below) was one of the first organic ferromagnets discovered. It is necessary to point, that many metallic complexes containing TCNQ, a structural analog of TCNE, are also ferromagnets. However, TCNE as a smaller, more compact molecule, was found to be preferable over TCNQ in terms of enhancement of magnetic properties due to higher density of spins. At the same time, TCNQ anion is better electroconductor over TCNE due to higher degree of delocalization of the negative charge.
TCNE complexes of some ferromagnetic metals, such as vanadium (V(TCNE)x, where X ~ 2, 6 on the scheme below) were found to be first organic molecular room temperature ferromagnets. Manganese(III) - tetraphenylporphyrin ferromagnetic complex [MnTPP][TCNE] 7 is interesting due to relatively high Curie temperature as well as possibility to tune dimensionality and magnetic properties by simple varying the stoichiometry and nature of a solvated solvent. Mixed metal-organic radical magnets are mainly low-dimensional because of both anisotropy of spins of metal ions, where unpaired electrons are located on d and f atomic orbitals and common anisotropy of crystal lattice of organometallic complexes. 3. Metal complexes with only metal spins coordinated to organic ligands. Vast majority of organic molecular magnets belong to this type of magnets. Magnetic properties are due to metals, however ligands may play key roles in determining of a type of magnetization (such as ferro, or antiferromagnets), dimensionality of materials, Curie temperature, etc. Classification of organic ligands: 1). Classification by substance types. 1. Carbonic acids and anions: acetates (common abbreviation in complexes: Ac), acylates (RCO2-), dithioacetates (dta), oxalates (ox), dithiooxalates (dto), ethylenediaminotetraacetates (EDTA). 2. Oxamides and oxamates, are N,N'-oxamidobis(benzoate) (obbz), o-phenylenebis(oxamate) (opba), and pbaOH. 3. 1,3-Diketones, such as acetylacetone (acac) and hexafluoro acetylacetone (hfac). 4. Amines and diamines: ethylenediamine (en), EDTA; dicyanamide N(CN)2. Nitrogen-containing heterocycles: pyridine (py), 2-aminomethylpyridine (2-pic), 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen). Heterocycles with two or more nitrogens: pyrazole (pyz), 2,2'-bipyrimidine (bpym), 4-ethyl-1,2,4-triazole (Ettrz), 4,4'-bi-1,2,4-triazole (btr), phtalocyanine (Pc). 5. Carbanions: some of the most widely used carbanions are cyclopentadienyl (Cp), pentamethylcyclopentadienyl (Cp*), and ppz.
2). Classification by ligand charge, denticity, and hapticity: By charge: By denticity: Monodentate: contain only one atom that binds to a metal, example - pyridine (py); abbreviation k1. Bidentate: contain two noncontiguous atoms that can bind to a metal, example thioacetate (dta); abbreviation k2. Ambidentate: contain two atoms that can bind to a metal, but only one of them actually binds in a particular complex, example - thiocyanate (SCN-). Three and polydentate: contain three or more atoms that can bind to a metal, example obbz, EDTA; abbreviation k3, k4, kn. Chelating: contain two or more atoms that tend to bind to a single metal atom, example - acetylacetonate (acac); abbreviation usually k2. Bridging: contain one, two, or more atoms that tend to bind to two or more different metal atoms; abbreviation in complexes: Greek symbol m. Example - obbz, abbreviation in a bridged complex with two metals will be m-k2:k2-obbz, e.g. k2 in respect to one metal and k2 in respect to the other metal. By hapticity: 3). Classification by ligand field strength. A simplified electronic scheme of interaction of iron (III) cation with ligands is shown below. A neutral 'naked' atom of iron possesses five partially filled with electrons d-orbitals all of equal energy (degenerate). The same - all other d-elements. Oxidation of the neutral iron atom gives rise to cation Fe3+, also with degenerate d-orbitals, each contains one unpaired electron. When ligands approach to the iron (III) cation, the field of ligands causes 'splitting' of five d-orbitals in two groups of orbitals possessing different symmetry and energy. Depending on how strong the field of a ligand is, the difference in energy of two split groups of orbitals may be higher or lower. If approaching ligands possess low field strengths (roughly - 'soft' nucleophiles), difference in energy (D) is minimal and electrons of a metal shell occupy these orbitals according to Hund's rule, e.g. one electron per orbital. Thus we have five unpaired electrons in the resulting complex which is called a high spin (HS) complex with a total spin of 5/2 for this particular case. The nature of a ligand (and to some extend - a metal) determine the kind of symmetry, which the split d-orbitals adopt. Two atomic d-orbitals of lower energy participate in tetrahedral coordination (each orbital per two bonds) to form complexes such as FeCl4-. And vise versa, three atomic d-orbitals of lower energy participate in octahedral coordination (each orbital per two bonds) to form complexes such as [Fe(CN)6]3-. If approaching ligands possess high field strengths (roughly - 'hard' nucleophiles), the difference in energy (D) is high and electrons of the metal shell occupy the lower energy set of orbitals first until it is 'full' of electrons. Therefore the set may contain either one or two electrons per orbital, depending on total quantity of electrons. Thus we have only one unpaired electron in the resulting iron (III) complex which is called a low spin (LS) complex with a total spin of 1/2 for this case. Depending on a metal nature and oxidation state, low spin complexes may have also S = 0 (diamagnets) and 1.
Generally, ligand field strength increases when passing from 'soft' bonding atoms-nucleophiles to 'hard' ones, and from monodentate ligands to chelating ones (see above). Ligand field strength may be adjusted in the way, when a complex becomes 'bistable', e.g. a low spin state may easily transform to high spin state, and vice versa, under the action of light, pressure, or temperature. This is known as spin crossover phenomena, practical use of which may offer fine control over magnetic properties of a material potentially valuable for spintronic applications. 4). General classification of metal-coordinated magnetic complexes: Quantity of elements: quantity of magnetic elements included in complexes - monometallic (for example, the complex contains only manganese (Mn) nuclei), bimetallic (the complex contains both manganese (Mn), and copper (Cu) nuclei), trimetallic etc. Charge and charge compensation: neutral - no charge and charged possess positive or negative charge. Charge compensation is a counterion that is used to compensate the charge of a complex. A charge compensation counterpart often can be a magnetic complex itself. Charge transfer complexes: are the ones where reversible electron transfer inside of a complex (between nuclei) or between counterparts may occur. Charge transfer may occur as one electron transfer or two electron transfer. One electron transfer usually gives rise to charge transfer salts, example: TTF-TCNQ, whereas two-electron transfer gives s-complexes, example: complex of aluminum trichloride and benzene C6H6•AlCl3. Often both mechanisms of electron transfer in the same complex may realize and all forms exist in equilibrium.
Mobility
Charge carrier mobility Hole mobility: This term defines a property of a material (p-transporting semiconductor), it defines drift velocity of holes (positive polarons) in a semiconductor. Electron mobility: This term defines a property of an n-transporting semiconductor, it defines drift velocity of electrons (negative polarons) in a semiconductor. Field effect (FET) mobility: This term defines a property of a channel semiconductor in a fully assembled field effect transistor, often referred as a property of a transistor itself. It determines carrier behavior in a particular device that includes channel semiconductor carrier mobility, source-drain interface effects, and gate dielectric insulator - channel semiconductor interface effects. Time of flight (TOF) mobility: is a property of a material determined by time of flight method. TOF mobility is less dependent on interface effects than FET mobility and therefore is considered being closer to intrinsic carrier mobility of a semiconductor. m = vd/E = cm/E · s = cm2/V · s For relation mobility - conductivity, see conductivity electrical For relation mobility - charge transfer rate - molecular reorganization energy, see Markus-Hush and Einstein equations Charge carrier drift velocity and, correspondingly, mobility is usually reduced by disorder in crystalline or amorphous structure of a semiconductor. Defects normally give rise to carrier scattering and trapping that slow down carrier's overall speed along the direction of electric field. Charge carrier mobility is the most fundamental property of semiconductors that eventually determines operational speed of devices. It is especially important for the efficiency of transistors (how quickly they can switch) and photovoltaics (how quickly the separated charges can "run away" from each other). Charge carrier drift length (Ld) is a highly important derivative of mobility that eventually determines effective size or thickness of electronic devices, especially photovoltaics and light-emitting diodes. Carrier drift length is limited by the carrier recombination lifetime (t): Ld = m t E Mobility strongly depends on the nature, structure, and purity of a material. Mobility of electrons is usually much higher than that of holes in inorganic semiconductors. The opposite is true for organic semiconductors: mobility of positive polarons is much higher than that of negative ones. Conductivity of a semiconductor is determined by mobility of dominant charge carrier.
Typical mobility of highly ordered mono- or polycrystalline inorganic, silicon, arsenic, or germanium-based semiconductors ranges from 100 to 10000 cm2/V · s. Disordered amorphous silicon (a-silicon) for TFT applications possesses much lower mobility of 0.7-1 cm2/V · s. Crystals of organic semiconductors differ strongly from that of inorganic ones in organization of crystal structure. The crystal lattice of organic compounds includes molecules, whereas inorganic semiconductors built of atoms. The charge carrier transport in organic compounds occurs within molecules, between molecules, as well as between crystal planes and grains. These multiple 'barriers' strongly slow down charge carriers; hence, mobility of organic semiconductors is much lower than that of inorganic ones. Thus, the values from 0.1 to 1 cm2/V · s are considered to be good for organic semiconductors. Even for the compound possessing highest mobility reported to date, pentacene, the values have never been reported to exceed 100 cm2/V · s. In addition, some reports on high mobility of pentacene films exceeding 10 cm2/V · s could not be reproduced. One of the recognized ways to improve the carrier mobility of organic semiconductors is to increase so-called p-p-stacking of organic molecules in crystalline or film structure, e.g. interaction of p-electrons of adjacent p-conjugated molecular planes. This may be achieved through p-extension of the conjugated planes. The other way is to improve regularity and morphology of polymers, or crystalline organization of oligomers through a variety of modern crystallization, deposition, and film cast techniques. For recent advances in improving of mobility of organic semiconductors see also:  |
