Amorphous solid

Last updated July 15, 2024

In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid) is a solid that lacks the long-range order that is characteristic of a crystal. The terms "glass" and "glassy solid" are sometimes used synonymously with amorphous solid; however, these terms refer specifically to amorphous materials that undergo a glass transition.^[1] Examples of amorphous solids include glasses, metallic glasses, and certain types of plastics and polymers.^[2]^[3]

Etymology

The term comes from the Greek a ("without"), and morphé ("shape, form").

Structure

Amorphous materials have an internal structure of molecular-scale structural blocks that can be similar to the basic structural units in the crystalline phase of the same compound.^[4] Unlike in crystalline materials, however, no long-range regularity exists: amorphous materials cannot be described by the repetition of a finite unit cell. Statistical mesures, such as the atomic density function and radial distribution function, are more useful in describing the structure of amorphous solids.^[1]^[3]

Although amorphous materials lack long range order, they exhibit localized order on small length scales.^[1] By convention, short range order extends only to the nearest neighbor shell, typically only 1-2 atomic spacings.^[5]Medium range order may extend beyond the short range order by 1-2 nm.^[5]

Fundamental properties of amorphous solids

Glass transition at high temperatures

The freezing from liquid state to amorphous solid - glass transition - is considered one of the very important and unsolved problems of physics.

Universal low-temperature properties of amorphous solids

At very low temperatures (below 1-10 K), large family of amorphous solids have various similar low-temperature properties. Although there are various theoretical models, neither glass transition nor low-temperature properties of glassy solids are well understood on the fundamental physics level.

Amorphous solids is an important area of condensed matter physics aiming to understand these substances at high temperatures of glass transition and at low temperatures towards absolute zero. From 1970s, low-temperature properties of amorphous solids were studied experimentally in great detail.^[6]^[7] For all of these substances, specific heat has a (nearly) linear dependence as a function of temperature, and thermal conductivity has nearly quadratic temperature dependence. These properties are conventionally called anomalous being very different from properties of crystalline solids.

On the phenomenological level, many of these properties were described by a collection of tunneling two-level systems.^[8]^[9] Nevertheless, the microscopic theory of these properties is still missing after more than 50 years of the research.^[10]

Remarkably, a dimensionless quantity of internal friction is nearly universal in these materials.^[11] This quantity is a dimensionless ratio (up to a numerical constant) of the phonon wavelength to the phonon mean free path. Since the theory of tunneling two-level states (TLSs) does not address the origin of the density of TLSs, this theory cannot explain the universality of internal friction, which in turn is proportional to the density of scattering TLSs. The theoretical significance of this important and unsolved problem was highlighted by Anthony Leggett.^[12]

Nano-structured materials

Amorphous materials will have some degree of short-range order at the atomic-length scale due to the nature of intermolecular chemical bonding.^{[lower-alpha 1]} Furthermore, in very small crystals, short-range order encompasses a large fraction of the atoms; nevertheless, relaxation at the surface, along with interfacial effects, distorts the atomic positions and decreases structural order. Even the most advanced structural characterization techniques, such as X-ray diffraction and transmission electron microscopy, can have difficulty distinguishing amorphous and crystalline structures at short size scales.^[13]

Characterization of amorphous solids

Due to the lack of long-range order, standard crystallographic techniques are often inadequate in determining the structure of amorphous solids.^[14] A variety of electron, X-ray, and computation-based techniques have been used to characterize amorphous materials. Multi-modal analysis is very common for amorphous materials.

X-ray and neutron diffraction

Unlike crystalline materials which exhibit strong Bragg diffraction, the diffraction patterns of amorphous materials are characterized by broad and diffuse peaks.^[15] As a result, detailed analysis and complementary techniques are required to extract real space structural information from the diffraction patterns of amorphous materials. It is useful to obtain diffraction data from both X-ray and neutron sources as they have different scattering properties and provide complementary data.^[16] Pair distribution function analysis can be performed on diffraction data to determine the probability of finding a pair of atoms separated by a certain distance.^[15] Another type of analysis that is done with diffraction data of amorphous materials is radial distribution function analysis, which measures the number of atoms found at varying radial distances away from an arbitrary reference atom.^[17] From these techniques, the local order of an amorphous material can be elucidated.

X-ray absorption fine-structure spectroscopy

X-ray absorption fine-structure spectroscopy is an atomic scale probe making it useful for studying materials lacking in long range order. Spectra obtained using this method provide information on the oxidation state, coordination number, and species surrounding the atom in question as well as the distances at which they are found.^[18]

Atomic electron tomography

The atomic electron tomography technique is performed in transmission electron microscopes capable of reaching sub-Angstrom resolution. A collection of 2D images taken at numerous different tilt angles is acquired from the sample in question, and then used to reconstruct a 3D image.^[19] After image acquisition, a significant amount of processing must be done to correct for issues such as drift, noise, and scan distortion.^[19] High quality analysis and processing using atomic electron tomography results in a 3D reconstruction of an amorphous material detailing the atomic positions of the different species that are present.

Fluctuation electron microscopy

Fluctuation electron microscopy is another transmission electron microscopy based technique that is sensitive to the medium range order of amorphous materials. Structural fluctuations arising from different forms of medium range order can be detected with this method.^[20] Fluctuation electron microscopy experiments can be done in conventional or scanning transmission electron microscope mode.^[20]

Computational techniques

Simulation and modeling techniques are often combined with experimental methods to characterize structures of amorphous materials. Commonly used computational techniques include density functional theory, molecular dynamics, and reverse Monte Carlo.^[14]

Uses and observations

Amorphous thin films

Amorphous phases are important constituents of thin films. Thin films are solid layers of a few nanometres to tens of micrometres thickness that are deposited onto a substrate. So-called structure zone models were developed to describe the microstructure of thin films as a function of the homologous temperature (T_h), which is the ratio of deposition temperature to melting temperature.^[21]^[22] According to these models, a necessary condition for the occurrence of amorphous phases is that (T_h) has to be smaller than 0.3. The deposition temperature must be below 30% of the melting temperature.^{[lower-alpha 2]}^{[ citation needed ]}

Superconductivity

Regarding their applications, amorphous metallic layers played an important role in the discovery of superconductivity in amorphous metals made by Buckel and Hilsch.^[23]^[24] The superconductivity of amorphous metals, including amorphous metallic thin films, is now understood to be due to phonon-mediated Cooper pairing. The role of structural disorder can be rationalized based on the strong-coupling Eliashberg theory of superconductivity.^[25]

Thermal protection

Amorphous solids typically exhibit higher localization of heat carriers compared to crystalline, giving rise to low thermal conductivity.^[26] Products for thermal protection, such as thermal barrier coatings and insulation, rely on materials with ultralow thermal conductivity.^[26]

Technological uses

Today, optical coatings made from TiO₂, SiO₂, Ta₂O₅ etc. (and combinations of these) in most cases consist of amorphous phases of these compounds. Much research is carried out into thin amorphous films as a gas separating membrane layer.^[27] The technologically most important thin amorphous film is probably represented by a few nm thin SiO₂ layers serving as isolator above the conducting channel of a metal-oxide semiconductor field-effect transistor (MOSFET). Also, hydrogenated amorphous silicon (Si:H) is of technical significance for thin-film solar cells.^{[lower-alpha 3]}^[28]

Pharmaceutical use

In the pharmaceutical industry, some amorphous drugs have been shown to offer higher bioavailability than their crystalline counterparts as a result of the higher solubility of the amorphous phase. However, certain compounds can undergo precipitation in their amorphous form in vivo , and can then decrease mutual bioavailability if administered together.^[29]^[30]

In soils

Amorphous materials in soil strongly influence bulk density, aggregate stability, plasticity, and water holding capacity of soils. The low bulk density and high void ratios are mostly due to glass shards and other porous minerals not becoming compacted. Andisol soils contain the highest amounts of amorphous materials.^[31]

Phase

The occurrence of amorphous phases turned out to be a phenomenon of particular interest for the studying of thin-film growth.^[32] The growth of polycrystalline films is often used and preceded by an initial amorphous layer, the thickness of which may amount to only a few nm. The most investigated example is represented by the unoriented molecules of thin polycrystalline silicon films.^{[lower-alpha 4]}^[33] Wedge-shaped polycrystals were identified by transmission electron microscopy to grow out of the amorphous phase only after the latter has exceeded a certain thickness, the precise value of which depends on deposition temperature, background pressure, and various other process parameters. The phenomenon has been interpreted in the framework of Ostwald's rule of stages^[34] that predicts the formation of phases to proceed with increasing condensation time towards increasing stability.^[24]^[33]^{[lower-alpha 5]}

Notes

↑ See the structure of liquids and glasses for more information on non-crystalline material structure.
↑ For higher values, the surface diffusion of deposited atomic species would allow for the formation of crystallites with long-range atomic order.
↑ In the case of a hydrogenated amorphous silicon, the missing long-range order between silicon atoms is partly induced by the presence of hydrogen in the percent range.
↑ An initial amorphous layer was observed in many studies of thin polycrystalline silicon films.
↑ Experimental studies of the phenomenon require a clearly defined state of the substrate surface—and its contaminant density, etc.—upon which the thin film is deposited.

Related Research Articles

Glass is an amorphous (non-crystalline) solid. Because it is often transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window panes, tableware, and optics. Some common objects made of glass like "a glass" of water, "glasses", and "magnifying glass", are named after the material.

<span class="mw-page-title-main">Melting</span> Material phase change

Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which increases the substance's temperature to the melting point. At the melting point, the ordering of ions or molecules in the solid breaks down to a less ordered state, and the solid melts to become a liquid.

In physics, chemistry, and other related fields like biology, a phase transition is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of matter: solid, liquid, and gas, and in rare cases, plasma. A phase of a thermodynamic system and the states of matter have uniform physical properties. During a phase transition of a given medium, certain properties of the medium change as a result of the change of external conditions, such as temperature or pressure. This can be a discontinuous change; for example, a liquid may become gas upon heating to its boiling point, resulting in an abrupt change in volume. The identification of the external conditions at which a transformation occurs defines the phase transition point.

Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid–liquid interfaces, solid–gas interfaces, solid–vacuum interfaces, and liquid–gas interfaces. It includes the fields of surface chemistry and surface physics. Some related practical applications are classed as surface engineering. The science encompasses concepts such as heterogeneous catalysis, semiconductor device fabrication, fuel cells, self-assembled monolayers, and adhesives. Surface science is closely related to interface and colloid science. Interfacial chemistry and physics are common subjects for both. The methods are different. In addition, interface and colloid science studies macroscopic phenomena that occur in heterogeneous systems due to peculiarities of interfaces.

$<span class="mw-page-title-main">Transmission electron microscopy</span> Imaging and diffraction using electrons that pass through samples$

Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. The image is then magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a detector such as a scintillator attached to a charge-coupled device or a direct electron detector.

An amorphous metal is a solid metallic material, usually an alloy, with disordered atomic-scale structure. Most metals are crystalline in their solid state, which means they have a highly ordered arrangement of atoms. Amorphous metals are non-crystalline, and have a glass-like structure. But unlike common glasses, such as window glass, which are typically electrical insulators, amorphous metals have good electrical conductivity and can show metallic luster.

A scanning transmission electron microscope (STEM) is a type of transmission electron microscope (TEM). Pronunciation is [stɛm] or [ɛsti:i:ɛm]. As with a conventional transmission electron microscope (CTEM), images are formed by electrons passing through a sufficiently thin specimen. However, unlike CTEM, in STEM the electron beam is focused to a fine spot which is then scanned over the sample in a raster illumination system constructed so that the sample is illuminated at each point with the beam parallel to the optical axis. The rastering of the beam across the sample makes STEM suitable for analytical techniques such as Z-contrast annular dark-field imaging, and spectroscopic mapping by energy dispersive X-ray (EDX) spectroscopy, or electron energy loss spectroscopy (EELS). These signals can be obtained simultaneously, allowing direct correlation of images and spectroscopic data.

Chalcogenide glass is a glass containing one or more chalcogens. Polonium is also a chalcogen but is not used because of its strong radioactivity. Chalcogenide materials behave rather differently from oxides, in particular their lower band gaps contribute to very dissimilar optical and electrical properties.

$<span class="mw-page-title-main">Powder diffraction</span> Experimental method in X-ray diffraction$

Powder diffraction is a scientific technique using X-ray, neutron, or electron diffraction on powder or microcrystalline samples for structural characterization of materials. An instrument dedicated to performing such powder measurements is called a powder diffractometer.

<span class="mw-page-title-main">Polyamorphism</span> Ability of a substance to exist in more than one distinct amorphous state

Polyamorphism is the ability of a substance to exist in several different amorphous modifications. It is analogous to the polymorphism of crystalline materials. Many amorphous substances can exist with different amorphous characteristics. However, polyamorphism requires two distinct amorphous states with a clear, discontinuous (first-order) phase transition between them. When such a transition occurs between two stable liquid states, a polyamorphic transition may also be referred to as a liquid–liquid phase transition.

Ernst G. Bauer is a German-American physicist known for his studies in the field of surface science, thin film growth and nucleation mechanisms and the invention in 1962 of the Low Energy Electron Microscopy (LEEM). In the early 1990s, he extended the LEEM technique in two directions by developing Spin-Polarized Low Energy Electron Microscopy (SPLEEM) and Spectroscopic Photo Emission and Low Energy Electron Microscopy (SPELEEM). He is currently Distinguished Research Professor Emeritus at the Arizona State University.

In materials science, paracrystalline materials are defined as having short- and medium-range ordering in their lattice but lacking crystal-like long-range ordering at least in one direction.

<span class="mw-page-title-main">Polydioctylfluorene</span> Chemical compound

Polydioctylfluorene (PFO) is an organic compound, a polymer of 9,9-dioctylfluorene, with formula (C₁₃H₆(C₈H₁₇)₂)_n. It is an electroluminescent conductive polymer that characteristically emits blue light. Like other polyfluorene polymers, it has been studied as a possible material for light-emitting diodes.

<span class="mw-page-title-main">Polymer characterization</span>

Polymer characterization is the analytical branch of polymer science.

The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials from a hard and relatively brittle "glassy" state into a viscous or rubbery state as the temperature is increased. An amorphous solid that exhibits a glass transition is called a glass. The reverse transition, achieved by supercooling a viscous liquid into the glass state, is called vitrification.

<span class="mw-page-title-main">Allotropes of boron</span> Materials made only out of boron

Boron can be prepared in several crystalline and amorphous forms. Well known crystalline forms are α-rhombohedral (α-R), β-rhombohedral (β-R), and β-tetragonal (β-T). In special circumstances, boron can also be synthesized in the form of its α-tetragonal (α-T) and γ-orthorhombic (γ) allotropes. Two amorphous forms, one a finely divided powder and the other a glassy solid, are also known. Although at least 14 more allotropes have been reported, these other forms are based on tenuous evidence or have not been experimentally confirmed, or are thought to represent mixed allotropes, or boron frameworks stabilized by impurities. Whereas the β-rhombohedral phase is the most stable and the others are metastable, the transformation rate is negligible at room temperature, and thus all five phases can exist at ambient conditions. Amorphous powder boron and polycrystalline β-rhombohedral boron are the most common forms. The latter allotrope is a very hard grey material, about ten percent lighter than aluminium and with a melting point (2080 °C) several hundred degrees higher than that of steel.

The structure of liquids, glasses and other non-crystalline solids is characterized by the absence of long-range order which defines crystalline materials. Liquids and amorphous solids do, however, possess a rich and varied array of short to medium range order, which originates from chemical bonding and related interactions. Metallic glasses, for example, are typically well described by the dense random packing of hard spheres, whereas covalent systems, such as silicate glasses, have sparsely packed, strongly bound, tetrahedral network structures. These very different structures result in materials with very different physical properties and applications.

Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs.

Jianwei (John) Miao is a Professor in the Department of Physics and Astronomy and the California NanoSystems Institute at the University of California, Los Angeles. He performed the first experiment on extending crystallography to allow structural determination of non-crystalline specimens in 1999, which has been known as coherent diffractive imaging (CDI), lensless imaging, or computational microscopy. In 2012, Miao applied the CDI method to pioneer atomic electron tomography (AET), enabling the first determination of 3D atomic structures without assuming crystallinity or averaging.

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