Showing papers in "Journal of Physics: Condensed Matter in 2006"

Magnetic particle hyperthermia : nanoparticle magnetism and materials development for cancer therapy

Abstract: Loss processes in magnetic nanoparticles are discussed with respect to optimization of the specific loss power (SLP) for application in tumour hyperthermia. Several types of magnetic iron oxide nanoparticles representative for different preparation methods (wet chemical precipitation, grinding, bacterial synthesis, magnetic size fractionation) are the subject of a comparative study of structural and magnetic properties. Since the specific loss power useful for hyperthermia is restricted by serious limitations of the alternating field amplitude and frequency, the effects of the latter are investigated experimentally in detail. The dependence of the SLP on the mean particle size is studied over a broad size range from superparamagnetic up to multidomain particles, and guidelines for achieving large SLP under the constraints valid for the field parameters are derived. Particles with the mean size of 18 nm having a narrow size distribution proved particularly useful. In particular, very high heating power may be delivered by bacterial magnetosomes, the best sample of which showed nearly 1 kW g −1 at 410 kHz and 10 kA m −1 . This value may even be exceeded by metallic magnetic particles, as indicated by measurements on cobalt particles.

Nanoemulsions: formation, structure, and physical properties

Abstract: We summarize procedures for producing 'nanoemulsions' comprised of nanoscale droplets, methods for controlling the droplet size distribution and composition, and interesting physical properties of nanoemulsions. In contrast to more common microscale emulsions, nanoemulsions exhibit optical transparency at high droplet volume fractions, , surprisingly strong elasticity at low , and enhanced diffusive transport and shelf stability. For these reasons, nanoemulsions have great potential in a wide range of industries including pharmaceuticals, foods, and personal care products.

Effects of confinement on freezing and melting.

Abstract: We present a review of experimental, theoretical, and molecular simulation studies of confinement effects on freezing and melting We consider both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials) The most commonly used molecular simulation, theoretical and experimental methods are first presented We also provide a brief description of the most widely used porous materials The current state of knowledge on the effects of confinement on structure and freezing temperature, and the appearance of new surface-driven and confinement-driven phases are then discussed We also address how confinement affects the glass transition

TOPICAL REVIEW: Nanoemulsions: formation, structure, and physical properties

Abstract: We summarize procedures for producing 'nanoemulsions' comprised of nanoscale droplets, methods for controlling the droplet size distribution and composition, and interesting physical properties of nanoemulsions. In contrast to more common microscale emulsions, nanoemulsions exhibit optical transparency at high droplet volume fractions, , surprisingly strong elasticity at low , and enhanced diffusive transport and shelf stability. For these reasons, nanoemulsions have great potential in a wide range of industries including pharmaceuticals, foods, and personal care products.

Processing quantum information in diamond

Abstract: Quantum computing is an attractive and multidisciplinary field, which became a focus for experimental and theoretical research during the last decade. Among other systems, such as ions in traps and superconducting circuits, solid state based qubits are considered to be promising candidates for use in first experimental tests of quantum hardware. Here we report recent progress in quantum information processing with point defects in diamond. Qubits are defined as single spin states (electron or nuclear). This allows exploration of long coherence times (up to seconds for nuclear spins at cryogenic temperatures). In addition, the optical transition between ground and excited electronic states allows coupling of spin degrees of freedom to the state of the electromagnetic field. Such coupling gives access to spin state read-out via spin-selective scattering of photons. This also allows the use of spin states as robust memory for flying qubits (photons).

Single-molecule experiments in biological physics: methods and applications.

Abstract: I review single-molecule experiments (SMEs) in biological physics. Recent technological developments have provided the tools to design and build scientific instruments of high enough sensitivity and precision to manipulate and visualize individual molecules and measure microscopic forces. Using SMEs it is possible to manipulate molecules one at a time and measure distributions describing molecular properties, characterize the kinetics of biomolecular reactions and detect molecular intermediates. SMEs provide additional information about thermodynamics and kinetics of biomolecular processes. This complements information obtained in traditional bulk assays. In SMEs it is also possible to measure small energies and detect large Brownian deviations in biomolecular reactions, thereby offering new methods and systems to scrutinize the basic foundations of statistical mechanics. This review is written at a very introductory level, emphasizing the importance of SMEs to scientists interested in knowing the common playground of ideas and the interdisciplinary topics accessible by these techniques. The review discusses SMEs from an experimental perspective, first exposing the most common experimental methodologies and later presenting various molecular systems where such techniques have been applied. I briefly discuss experimental techniques such as atomic-force microscopy (AFM), laser optical tweezers (LOTs), magnetic tweezers (MTs), biomembrane force probes (BFPs) and single-molecule fluorescence (SMF). I then present several applications of SME to the study of nucleic acids (DNA, RNA and DNA condensation) and proteins (protein-protein interactions, protein folding and molecular motors). Finally, I discuss applications of SMEs to the study of the nonequilibrium thermodynamics of small systems and the experimental verification of fluctuation theorems. I conclude with a discussion of open questions and future perspectives.

A practical guide to giant vesicles. Probing the membrane nanoregime via optical microscopy

Abstract: Research on giant vesicles is becoming increasingly popular. Giant vesicles provide model biomembrane systems for systematic measurements of mechanical and rheological properties of bilayers as a function of membrane composition and temperature, as well as hydrodynamic interactions. Membrane response to external factors (for example electric fields, ions and amphiphilic molecules) can be directly visualized under the microscope. In this paper we review our current understanding of lipid bilayers as obtained from studies on giant unilamellar vesicles. Because research on giant vesicles increasingly attracts the interest of scientists from various backgrounds, we also try to provide a concise introduction for newcomers in the field. Finally, we summarize some recent developments on curvature effects induced by polymers, domain formation in membranes and shape transitions induced by electric fields.

Second-generation photocatalytic materials : anion-doped TiO2

Abstract: Previous experimental studies describe an efficient photoresponse in the visiblelight region for anion-doped TiO2 .D oping with carbon, nitrogen, as well as sulfur, yields promising second-generation photocatalysis with TiO2 .W e present a theoretical investigatio no fs ubstitutional anion doping in TiO2 and discuss doping effects on the electronic structure, and subsequently the photoactivity. The resulting bandgap narrowing predicted in this work is consistent with experimental observations. Furthermore, we discuss the effects of doping concentration on the localization properties of the valence band edge. Ou rs ystematic study of anion-doped TiO2 implies that the carbon-doped TiO2 is the most promising due to a significant overlap between the O 2p state and the carbon states near the valence band edge. Additionally, carbon dopants produce th el argest valence band red shift of the three anion-doped TiO2 studied.

Oxygen-defect-induced magnetism to 880 K in semiconducting anatase TiO2−δ films

Abstract: We demonstrate a semiconducting material, TiO2??, with ferromagnetism up to 880?K, without the introduction of magnetic ions. The magnetism in these films stems from the controlled introduction of anion defects from both the film?substrate interface as well as processing under an oxygen-deficient atmosphere. The room-temperature carriers are n-type with n~3 ? 1017?cm?3. The density of spins is ~1021?cm?3. Magnetism scales with conductivity, suggesting that a double exchange interaction is active. This represents a new approach in the design and refinement of magnetic semiconductor materials for spintronics device applications.

Drug loaded magnetic nanoparticles for cancer therapy

Abstract: Magnetic nanoparticles have been investigated for biomedical applications for more than 30 years. In medicine they are used for several approaches such as magnetic cell separation or magnetic resonance imaging (MRI). The development of biocompatible nanosized drug delivery systems for specific targeting of therapeutics is the focus of medical research, especially for the treatment of cancer and diseases of the vascular system. In an experimental cancer model, we performed targeted drug delivery and used magnetic iron oxide nanoparticles, bound to a chemotherapeutic agent, which were attracted to an experimental tumour in rabbits by an external magnetic field (magnetic drug targeting). Complete tumour remission could be achieved. An important advantage of these carriers is the possibility for detecting these nanoparticles after treatment with common imaging techniques (i.e. x-ray-tomography, magnetorelaxometry, magnetic resonance imaging), which can be correlated to histology.

An explanation of the power-law decay of luminescence

Abstract: Luminescence decay with time often shows a power-law dependence of the form intensity , where t is time and k is usually in the range 1–1.5. It is shown here that this power law can result from the tunnelling of trapped electrons to recombination centres that are randomly distributed, and that the range of exponents matches that of the observations. The explanation accounts for the most extreme case of an observed t−1.06 dependence extending over nine decades of time.

The structure of amorphous, crystalline and liquid GeO2

Abstract: Germanium dioxide (GeO2) is a chemical analogue of SiO2. Furthermore, it is also to some extent a structural analogue, as the low- and high-pressure short-range order (tetrahedral and octahedral) is the same. However, a number of differences exist. For example, the GeO2 phase diagram exhibits a smaller number of polymorphs, and all three GeO2 phases (crystalline, glass, liquid) have an increased sensitivity to pressure, undergoing pressure-induced changes at much lower pressures than their equivalent SiO2 analogues. In addition, differences exist in GeO2 glass in the medium-range order, resulting in the glass transition temperature of germania being much lower than for silica. This review highlights the structure of amorphous GeO2 by different experimental (e.g., Raman and NMR spectroscopy, neutron and x-ray diffraction) and theoretical methods (e.g., classical molecular dynamics, ab initio calculations). It also addresses the structures of liquid and crystalline GeO2, that have received much less attention. Furthermore, we compare and contrast the structures of GeO2 and SiO2, as well as along the GeO2–SiO2 join. It is probably a very timely review, as interest in this compound, that can be investigated in the liquid state at relatively low temperatures and pressures, continues to increase.

Valence electron rules for prediction of half-metallic compensated-ferrimagnetic behaviour of Heusler compounds with complete spin polarization

Abstract: In this work, a rule for predicting half-metallic compensated-ferrimagnets in the class of Heusler compounds will be described. This concept results from combining the well-known Slater–Pauling rule with the Kubler rule. The Kubler rule states that Mn on the Y position in Heusler compounds tends to a highly localized magnetic moment. When strictly following this new rule, some candidates in the class of Heusler compounds are expected to be completely compensated-ferrimagnetic but with a spin polarization of 100% at the Fermi energy. This rule is applied to three examples within the class of Heusler compounds. All discussion of the materials is supported by electronic structure calculations.

Rubber friction: role of the flash temperature

Abstract: When a rubber block is sliding on a hard rough substrate, the substrate asperities will exert time-dependent deformations of the rubber surface resulting in viscoelastic energy dissipation in the rubber, which gives a contribution to the sliding friction. Most surfaces of solids have roughness on many different length scales, and when calculating the friction force it is necessary to include the viscoelastic deformations on all length scales. The energy dissipation will result in local heating of the rubber. Since the viscoelastic properties of rubber-like materials are extremely strongly temperature dependent, it is necessary to include the local temperature increase in the analysis. At very low sliding velocity the temperature increase is negligible because of heat diffusion, but already for velocities of order 10(-2) m s(-1) the local heating may be very important. Here I study the influence of the local heating on the rubber friction, and I show that in a typical case the temperature increase results in a decrease in rubber friction with increasing sliding velocity for v>0.01 m s(-1). This may result in stick-slip instabilities, and is of crucial importance in many practical applications, e.g. for tyre-road friction and in particular for ABS braking systems.

Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition

Abstract: We report on an inelastic (Raman) light scattering study of the local structure of amorphous GeTe (a-GeTe) films. A detailed analysis of the temperature-reduced Raman spectra has shown that appreciable structural changes occur as a function of temperature. These changes involve modifications of atomic arrangements such as to facilitate the rapid amorphous to crystal transformation, which is the major advantage of phase-change materials used in optical data storage media. A particular structural model, supported by polarization analysis, is proposed which is compatible with the experimental data as regards both the structure of a-GeTe and the crystallization transition. The remarkable difference between the Raman spectrum of the crystal and the glass can thus naturally be accounted for.

Surface characterization of sulfur and alkanethiol self-assembled monolayers on Au(111)

Abstract: In the last two decades surface science techniques have decisively contributed to our present knowledge of alkanethiol self-assembled monolayers (SAMs) on solid surfaces. These organic layers have been a challenge for surface scientists, in particular because of the soft nature of the organic material (which can be easily damaged by irradiation), the large number of atoms present in the molecules, and the complex physical chemistry involved in the self-assembly process. This challenge has been motivated by the appealing technological applications of SAMs that cover many fields of the emerging area of nanotechnology. Sulfur (S) is closely related to alkanethiols and can be used to understand basic aspects of the surface structure of SAMs. In this review we focus on the atomic/molecular structures of S-containing SAMs on Au(111). Particular emphasis is given to the substrate, adsorption sites, chemical state of the S–metal bond and also to the experimental and theoretical tools used to study these structures at the atomic or molecular levels.

Nanoferroelectrics: statics and dynamics

Abstract: A topical review is given of the physics of submicron ferroelectrics, describing the application considerations for memory devices (both as switching memory elements for ferroelectric nonvolatile random access memories, FRAMs, and as passive capacitors for volatile dynamic random access memories, DRAMs) as well as the fundamental physics questions regarding both the thickness and lateral size of present interest.

Amorphous ices: experiments and numerical simulations

Abstract: Polyamorphism, i.e. the presence of more than one amorphous state, was observed for the first time in amorphous ice or glassy water. In addition to LDA (low-density amorphous ice), a second amorphous state, HDA (high-density amorphous ice), was discovered ∼20 years ago. Since then, polyamorphism has been observed in many other substances, such as SiO2 ,G eO 2, Si, and Ge. Five years ago, experimental results suggesting the existence of a third amorphous state, VHDA (very high-density amorphous ice), were reported, opening the possibility that more than two amorphous states could also be observed in other substances. A consistent phase diagram of glassy water does not yet exist. Such a phase diagram is necessary if one also wants to understand the anomalous behaviour of supercooled liquid water. Since the discovery of HDA, a large amount of work based on experiments and computer simulations has appeared. It is the purpose of this work to review such studies with special emphasis in comparing the experimental and simulation results. In particular, we review the recent studies concerning VHDA and its nature, and discuss the main open questions relating to the phase diagram of glassy water. (Some figures in this article are in colour only in the electronic version)

Density functional theory for hard-sphere mixtures: the White Bear version mark II

Abstract: In the spirit of the White Bear version of fundamental measure theory we derive a new density functional for hard-sphere mixtures which is based on a recent mixture extension of the Carnahan-Starling equation of state. In addition to the capability to predict inhomogeneous density distributions very accurately, like the original White Bear version, the new functional improves upon consistency with an exact scaled-particle theory relation in the case of the pure fluid. We examine consistency in detail within the context of morphological thermodynamics. Interestingly, for the pure fluid the degree of consistency of the new version is not only higher than for the original White Bear version but also higher than for Rosenfeld's original fundamental measure theory.

Inverse lyotropic phases of lipids and membrane curvature

Abstract: In recent years it has become evident that many biological functions and processes are associated with the adoption by cellular membranes of complex geometries, at least locally. In this paper, we initially discuss the range of self-assembled structures that lipids, the building blocks of biological membranes, may form, focusing specifically on the inverse lyotropic phases of negative interfacial mean curvature. We describe the roles of curvature elasticity and packing frustration in controlling the stability of these inverse phases, and the experimental determination of the spontaneous curvature and the curvature elastic parameters. We discuss how the lyotropic phase behaviour can be tuned by the addition of compounds such as long-chain alkanes, which can relieve packing frustration. The latter section of the paper elaborates further on the structure, geometric properties, and stability of the inverse bicontinuous cubic phases.

Membrane protein simulations with a united-atom lipid and all-atom protein model: lipid-protein interactions, side chain transfer free energies and model proteins.

Abstract: We have reparameterized the dihedral parameters in a commonly used united-atom lipid force field so that they can be used with the all-atom OPLS force field for proteins implemented in the molecular dynamics simulation software GROMACS. Simulations with this new combination give stable trajectories and sensible behaviour of both lipids and protein. We have calculated the free energy of transfer of amino acid side chains between water and 'lipid-cyclohexane', made of lipid force field methylene groups, as a hydrophobic mimic of the membrane interior, for both the OPLS-AA and a modified OPLS-AA force field which gives better hydration free energies under simulation conditions close to those preferred for the lipid force field. The average error is 4.3 kJ mol−1 for water–'lipid-cyclohexane' compared to 3.2 kJ mol−1 for OPLS-AA cyclohexane and 2.4 kJ mol−1 for the modified OPLS-AA water–'lipid-cyclohexane'. We have also investigated the effect of different methods to combine parameters between the united-atom lipid force field and the united-atom protein force field ffgmx. In a widely used combination, the strength of interactions between hydrocarbon lipid tails and proteins is significantly overestimated, causing a decrease in the area per lipid and an increase in lipid ordering. Using straight combination rules improves the results. Combined, we suggest that using OPLS-AA together with the united-atom lipid force field implemented in GROMACS is a reasonable approach to membrane protein simulations. We also suggest that using partial volume information and free energies of transfer may help to improve the parameterization of lipid–protein interactions and point out the need for accurate experimental data to validate and improve force field descriptions of such interactions.

Effect of cobalt substitution on the properties of nickel-copper ferrite

Abstract: Cobalt-substituted nickel–copper ferrite samples having the chemical formula Ni0.95−xCoxCu0.05Fe2O4, where x varies as 0.01, 0.02 and 0.03, were prepared by the standard double sintering ceramic technique. The formation of the ferrite phase was confirmed by x-ray diffraction (XRD) studies. Resistivity and thermo-emf variation with temperature were studied in the temperature range from room temperature to 773 K as a function of cobalt content. As the cobalt content increases, the resistivity of the ferrites decreases. AC conductivity measurements made in the frequency range 100 Hz–1 MHz show that conduction in these ferrites is due to small polaron hopping. The dielectric constant and loss tangent (tanδ) were measured at room temperature as a function of frequency in the range 20 Hz to 1 MHz. The room-temperature dielectric measurements show dispersion behaviour with increasing frequency. To understand the conduction mechanism, complex impedance measurements were carried out. The variation in saturation magnetization (Ms) with variation of cobalt content is also studied.

Raman spectroscopy study of heavy-ion-irradiated α-SiC

Abstract: Raman spectroscopy was used to investigate the structure of ion-irradiated α-SiC single crystals at room temperature and 400 °C. Irradiations induce a decrease of the Raman line intensities related to crystalline SiC, the appearance of several new Si–C vibration bands attributed to the breakdown of the Raman selection rules, and the formation of homonuclear bonds Si–Si and C–C within the SiC network. For low doses, the overall sp3 bond structure and the chemical order may be almost completely conserved. By contrast, the amorphous state shows a strong randomization of the Si–Si, Si–C and C–C bonds. The relative Raman intensity decreases exponentially versus increasing dose due to the absorption of the irradiated layer. The total disorder follows a sigmoidal curve, which is well fitted by the direct impact/defect stimulated model. The chemical disorder expressed as the ratio of C–C bonds to Si–C bonds increases exponentially versus the dose. A clear correlation is established between the total disorder and the chemical disorder. The increase of temperature allows the stabilization of a disordered/distorted state and a limitation of damage accumulation owing to the enhancement of the dynamic annealing.

The effect of field parameters, nanoparticle properties and immobilization on the specific heating power in magnetic particle hyperthermia

Abstract: Magnetic nanoparticles (MNP) are intended for utilization in cancer therapy as they produce damaging heat in the presence of AC magnetic fields. In order to reach the required temperature with minimum particle concentration in tissue the specific heating power (SHP) of MNP should be as high as possible. The aim was to clarify the influence of magnetic field parameters and nanoparticle properties on the SHP. As usual ferrofluids exhibit broad size distributions, a magnetic fractionation of a commercial iron oxide nanoparticle suspension was performed in order to obtain particles with varying properties. The fractions obtained were characterized by means of atomic force microscopy and magnetometry, among other techniques. Frequency spectra of the susceptibility show clear peaks at low frequencies related to the Brown relaxation. This effect vanishes after particle immobilization. Theoretical spectra considering experimentally determined size distributions are in agreement with experimental data. The SHP derived from AC susceptometry is in accordance with that directly determined by calorimetry. A maximum SHP of 160 W g−1 (400 kHz, 8 kA m−1) was detected for the largest particles, showing a behaviour in the transitional regime between superparamagnetic and stable ferromagnetic.

A mechanism for the 150 µC cm(-2) polarization of BiFeO(3) films based on first-principles calculations and new structural data.

Abstract: Following our experimental report of a giant ferroelectric polarization in the region of 150 µC cm−2 in BiFeO3 (BFO) films, we have performed first-principles calculations based on the local density approximation to density functional theory, aiming to clarify its mechanism. Upon optimization of lattice constants we have shown that the natural tetragonal structure of BFO has a giant tetragonality ratio of 1.26 and large ionic off-centring. Experimentally this structure has been detected in BFO films deposited on La-doped SrTiO3 substrates. The spontaneous polarization calculated ab initio for this structure is 143.5 µC cm−2, in agreement with the remanent polarization of hysteresis loops measured at 90 K. These results suggest that the giant polarization of our BFO films may occur upon stabilization of the optimal tetragonal phase with giant tetragonality. Future experimental effort aiming to routinely obtain such values of spontaneous polarization should concentrate on how to isolate this phase without compromising the insulating and switching properties of BFO.

First-principles study of elastic properties of CeO2, ThO2 and PoO2

Abstract: Using first-principles density functional calculations, the structural and elastic properties of fluorite type oxides CeO2, ThO2 and PoO2 were studied by means of the full-potential linear muffin-tin orbital method. Calculations were performed within the local density approximation (LDA) as well as generalized gradient approximation (GGA) to the exchange correlation potential. The calculated equilibrium lattice constants and bulk moduli are in good agreement with the experimental results, as are the computed elastic constants for CeO2 and ThO2. For PoO2 this is the first quantitative theoretical prediction of the ground state properties, and it still awaits experimental confirmation. The calculations find PoO2 to be a semiconductor with an indirect band gap and elastic constants similar in magnitude to those of CeO2 and ThO2.

Electronic structure and bonding in hexagonal boron nitride

Abstract: The bonding, cohesive, and electronic properties of hexagonal boron nitride were studied using density functional theory calculations. The properties of this system were calculated using three different exchange–correlation functionals (local density approximation and two forms of the generalized gradient approximation) to determine their relative predictive abilities for this system. In-plane and interplanar bonding was examined using band diagrams, the density of states, and the electron localization function. Different stackings, or arrangements of one basal plane with respect to another, were examined to determine how the bonding and electronic structure changed between different stackings. Calculated band gaps were in the 2.9–4.5 eV range and predominantly indirect, regardless of stacking or the exchange–correlation functional used. The calculated band gaps are in the low range of experimental band gap values, and do not explain the large range of experimental values.

Aggregation behaviour of magnetic nanoparticle suspensions investigated by magnetorelaxometry

Abstract: The aggregation behaviour of magnetic nanoparticles (MNP) is a decisive factor for their application in medicine and biotechnology. We extended the moment superposition model developed earlier for describing the Neel relaxation of an ensemble of immobilized particles with a given size distribution by including the Brownian relaxation mechanism. The resulting cluster moment superposition model is used to characterize the aggregation of magnetic nanoparticles in various suspensions in terms of mean cluster size, aggregate fraction, and size dispersion. We found that in stable ferrofluids 50%-80% of larger magnetic nanoparticles are organized in dimers and trimers. The scaling of the relaxation curves with respect to MNP concentration is found to be a sensitive indicator of the tendency of a MNP suspension to form large aggregates, which may limit the biocompatibility of the preparation. Scaling violation was observed in aged water based ferrofluids, and may originate from damaged MNP shells. In biological media such as foetal calf serum, bovine serum albumin, and human serum we observed an aggregation behaviour which reaches a maximum at a specific MNP concentration. We relate this to agglutination of the particles by macromolecular bridges between the nanoparticle shells. Analysis of the scaling behaviour helps to identify the bridging component of the suspension medium that causes agglutination.

Towards a fullerene-based quantum computer

Abstract: Molecular structures appear to be natural candidates for a quantum technology: individual atoms can support quantum superpositions for long periods, and such atoms can in principle be embedded in a permanent molecular scaffolding to form an array. This would be true nanotechnology, with dimensions of order of a nanometre. However, the challenges of realizing such a vision are immense. One must identify a suitable elementary unit and demonstrate its merits for qubit storage and manipulation, including input/output. These units must then be formed into large arrays corresponding to an functional quantum architecture, including a mechanism for gate operations. Here we report our efforts, both experimental and theoretical, to create such a technology based on endohedral fullerenes or 'buckyballs'. We describe our successes with respect to these criteria, along with the obstacles we are currently facing and the questions that remain to be addressed.

O− bound small polarons in oxide materials

Abstract: Holes bound to acceptor defects in oxide crystals are often localized by lattice distortion at just one of the equivalent oxygen ligands of the defect. Such holes thus form small polarons in symmetric clusters of a few oxygen ions. An overview on mainly the optical manifestations of those clusters is given. The article is essentially divided into two parts: the first one covers the basic features of the phenomena and their explanations, exemplified by several paradigmatic defects; in the second part numerous oxide materials are presented which exhibit bound small polaron optical properties. The first part starts with summaries on the production of bound hole polarons and the identification of their structure. It is demonstrated why they show strong, wide absorption bands, usually visible, based on polaron stabilization energies of typically 1 eV. The basic absorption process is detailed with a fictitious two-well system. Clusters with four, six and twelve equivalent ions are realized in various oxide compounds. In these cases several degenerate optically excited polaron states occur, leading to characteristic final state resonance splittings. The peak energies of the absorption bands as well as the sign of the transfer energy depend on the topology of the clusters. A special section is devoted to the distinction between interpolaron and intrapolaron optical transitions. The latter are usually comparatively weak. The oxide compounds exhibiting bound hole small polaron absorptions include the alkaline earth oxides (e.g. MgO), BeO and ZnO, the perovskites BaTiO3 and KTaO3, quartz, the sillenites (e.g. Bi12TiO20), Al2O3, LiNbO3, topaz and various other materials. There are indications that the magnetic crystals NiO, doped with Li, and LaMnO3, doped with Sr, also show optical features caused by bound hole polarons. Beyond being elementary paradigms for the properties of small polarons in general, the defect species treated can be used to explain radiation and light induced absorption especially in laser and non-linear oxide materials, the role of some defects in photorefractive compounds, the coloration of various gemstones, the structure of certain catalytic surface centres, etc. The relation to further phenomena is discussed: free small polarons, similar distorted centres in the sulfides and selenides, acceptor defects trapping two holes.