Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices

Modulation spectroscopy is a powerful method for the study and characterization of a large number of semiconductor configurations, including bulk/thin film, microstructures (heterojunctions, quantum wells, superlattices, quantum dots), surfaces/interfaces and actual device structures in addition to semiconductor growth/processing. Furthermore, the influence of external perturbations such as temperature, electric fields, hydrostatic pressure, uniaxial stress, etc. can be investigated. This optical technique utilizes a very general principle of experimental physics, in which a periodically applied perturbation (either to the sample or probe) leads to sharp, derivative-like spectral features in the optical response of the system. Because of the richness of the derivative-like spectra, the information in the lineshape fits, room temperature performance and relative simplicity of operation this method is becoming increasingly more important as a tool to study these materials and structures. This article will review developments in the field during the last decade.     

Magnetic nanoparticles and nanoparticle assemblies from metallorganic precursors

Magnetic nanoparticles of -Fe₂O₃, Fe₂O₃SiO₂ composite and magnetite Fe₃O₄ have been prepared using novel metallorganic precursors Fe[NC(C₆H₄)C(NSiMe₃)₂]₂Cl, Fe₂[O₂Si(C₆H₅)₂]₃ and [Fe(OBu^t)₃Na(THF)]₂) by hydrolysis, sol-gel condensation and further ultrasound and thermal treatment of the samples. The nanoparticles have been investigated by X-ray powder diffraction, TEM, SEM and AFM.     

Non-stoichiometry and electronic properties of interfaces

The paper gives an overview on the influence of point defects on electronic properties of interfaces including band alignment (barrier heights) and transport properties. As examples interfaces between metals and the II–VI semiconductors CdTe and ZnTe are presented. In addition untypical phenomena at semiconductor heterocontact formation at In₂S₃/ZnO and CuInSe₂/CdS interfaces is described. It is suggested that the barrier heights as well as the transport properties at both interfaces are strongly affected by defects, which are either present because of non-stoichiometry of the materials or introduced by contact formation due to chemical interactions.     

Controlling surfaces and interfaces of semiconductors using organic molecules

Control over semiconductor surface energetics can be achieved using different chemisorbed organic molecules with diverse electronic properties. We find evidence of such control over CdTe upon adsorption of dicarboxylic acid derivatives with different substituted phenyl rings. FT-IR measurements show that the dicarboxylic acid derivatives bind as carboxylates to form approximately one monolayer. Such chemisorption modifies both the band bending and the electron affinity (up to 500 and 700 mV, respectively), as measured by contact potential difference WPM Changes in band bending result from a coupling between molecular orbitals and surface states close to the valence band and depend on the withdrawing character of the phenyl substituent. A model is presented to interpret and explain the data.     

Two-dimensional gradient Ag nanoparticle assemblies: multiscale fabrication and SERS applications

We report a novel method for fabricating silver nanoparticle assemblies with a featured gradient of spatial organizations. The unique step is to generate a gradient of deposit mass by dynamical deposition on a mask-covered substrate with a collimated cluster beam in oblique incidence. Then such gradient can be translated to the gradients of sizes or number densities of the nanoparticles separately, depending on the nature of the substrate surface. Multiscale gradients are implemented from mesoscopic to macroscopic. One-chip rapid detection of the optimal structure for surface enhanced Raman scattering (SERS) is achieved on the gradient assembly with particle number densities.     

Tunable morphologies, structure and unusual responsive thermochromism of polydiacetylene supramolelular assemblies

Azobenzene mesogen-substituted polydiacetylene (PDA) which self-assembled into highly ordered lamellar structure and micrometer-sized aggregates structures were investigated in detail by transmission electron microscopy (TEM) as well as X-ray diffraction (XRD). The adjustable molecular structure of diacetylenic monomers allows control of molecular architecture of the supramolecular assembly, resulting in modification of morphology and stacking structure of PDA films. The films form blue phase at room temperature but undergo the well-known thermochromic transition which produce red phase. Due to the strong intermolecular interaction (π-π stacking), azobenzene-substituted PDA assemblies shows the enhanced thermal stability, and a reversible chromatic transition at a much higher critical temperature of chromatic transition (105 °C for completely reversible thermochromic transition and 150 °C for partially reversible thermochromic transition).     

Localized photovoltaic investigations on organic semiconductors and bulk heterojunction solar cells

Newly synthesized organic electronics materials are often available in submicrogram amounts only. Photoelectrochemical scanning droplet cell microscopy is a powerful method that allows a comprehensive characterisation of such small amounts including oxidation, reduction potentials, doping, determination of charge carriers, band gap, charge capacity, over-oxidation sensitivity and many more. Localized photoelectrochemical characterization of the poly[4,8-bis-substituted-benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl-alt-4-substituted-thieno [3,4-b] thiophene-2,6-diyl] (PBDTTT-c) and PBDTTT-c:PCBM bulk heterojunction was performed using photoelectrochemical scanning droplet cell microscopy (PE-SDCM). The optical properties and the real and imaginary part of the dielectric function, of the polymer were determined using spectroscopic ellipsometry. The photoelectrochemical characterizations were performed in a three and two electrode configuration of PE-SDCM under laser and white light illumination. The effect of illumination was characterized using dark/illumination sequences. The stability of the photocurrent was studied using longer term (600 s) illumination. Finally the effect of cell configuration and illumination conditions on the photovoltage was studied.     

Polypeptide folding-mediated tuning of the optical and structural properties of gold nanoparticle assemblies

Responsive hybrid nanomaterials with well-defined properties are of significant interest for the development of biosensors with additional applications in tissue engineering and drug delivery. Here, we present a detailed characterization using UV-vis spectroscopy and small angle X-ray scattering of a hybrid material comprised of polypeptide-decorated gold nanoparticles with highly controllable assembly properties. The assembly is triggered by a folding-dependent bridging of the particles mediated by the heteroassociation of immobilized helix-loop-helix polypeptides and a complementary nonlinear polypeptide present in solution. The polypeptides are de novo designed to associate and fold into a heterotrimeric complex comprised of two disulfide-linked four-helix bundles. The particles form structured assemblies with a highly defined interparticle gap (4.8±0.4 nm) that correlates to the size of the folded polypeptides. Transitions in particle aggregation dynamics, mass-fractal dimensions and ordering, as a function of particle size and the concentration of the bridging polypeptide, are observed; these have significant effects on the optical properties of the assemblies. The assembly and ordering of the particles are highly complex processes that are affected by a large number of variables including the number of polypeptides bridging the particles and the particle mobility within the aggregates. A fundamental understanding of these processes is of paramount interest for the development of novel hybrid nanomaterials with tunable structural and optical properties and for the optimization of nanoparticle-based colorimetric biodetection strategies.     

Optical and electronic properties of some semiconductors from energy gaps

II-VI and III-V tetrahedral semiconductors have significant potential for novel optoelectronic applications. In the present work, some of the optical and electronic properties of these groups of semiconductors have been studied using a recently proposed empirical relationship for refractive index from energy gap. The calculated values of these properties are also compared with those calculated from some well known relationships. From an analysis of the calculated electronic polarisability of these tetrahedral binary semiconductors from different formulations, we have proposed an empirical relation for its calculation. The predicted values of electronic polarisability of these semiconductors agree fairly well with the known values over a wide range of energy gap. The proposed empirical relation has also been used to calculate the electronic polarisability of some ternary compounds.     

Ceramic nanoparticle assemblies with tailored shapes and tailored chemistries via biosculpting and shape-preserving inorganic conversion

A novel biosynthetic paradigm is introduced for fabricating three-dimensional (3-D) ceramic nanoparticle assemblies with tailored shapes and tailored chemistries: biosculpting and shape-preserving inorganic conversion (BaSIC). Biosculpting refers to the use of biomolecules that direct the precipitation of ceramic nanoparticles to form a continuous 3-D structure with a tailored shape. We used a peptide derived from a diatom (a type of unicellular algae) to biosculpt silica nanoparticle based assemblies that, in turn, were converted into a new (nonsilica) composition via a shape-preserving gas/silica displacement reaction. Interwoven, microfilamentary silica structures were prepared by exposing a peptide, derived from the silaffin-1A protein of the diatom Cylindrotheca fusiformis, to a tetramethylorthosilicate solution under a linear shear flow condition. Subsequent exposure of the silica microfilaments to magnesium gas at 900 degrees C resulted in conversion into nanocrystalline magnesium oxide microfilaments with a retention of fine (submicrometer) features. Fluid(gas or liquid)/silica displacement reactions leading to a variety of other oxides have also been identified. This hybrid (biogenic/synthetic) approach opens the door to biosculpted ceramic microcomponents with multifarious tailored shapes and compositions for a wide range of environmental, aerospace, biomedical, chemical, telecommunications, automotive, manufacturing, and defense applications.     

Electrostatic and capillary force directed tunable 3D binary micro- and nanoparticle assemblies on surfaces

We report a simple, rapid and cost-effective method based on evaporation induced assembly to grow 3D binary colloidal assemblies on a hydrophobic/hydrophilic substrate by simple drop casting. The evaporation of a mixed colloidal drop results in ring-like or uniform area deposition depending on the concentration of particles, and thus assembly occurs at the periphery of a ring or uniformly all over the drop area. Binary colloidal assemblies of different crystal structure are successfully prepared over a wide range of size ratios (γ = small/large) from 0.06 to 0.30 by tuning the γ of the micro- and nanoparticles used during assembly. The growth mechanism of 3D binary colloidal assemblies is investigated and it is found that electrostatic forces facilitate assembly formation until the end of the evaporation process, with capillary forces also playing a role. In addition, the effects of solvent type, humidity, and salt concentration on crystal formation and ordering behaviour are also examined. Furthermore, long range, highly ordered binary colloidal assemblies can be fabricated by the choice of a low conducting solvent combined with evaporation induced assembly.     

Optical electronegativity, bulk modulus and electronic polarizability of materials

Correlations between the energy gap, optical electronegativity and bulk modulus are given for compound and ternary chalcopyrite semiconductors. Bulk modulus and electronic polarizability values have been evaluated from the optical electronegativity. The estimated values of these parameters are in good agreement with the available experimental data.     

A review of computational phononics: the bulk,interfaces, and surfaces

Broad-based interest in microscale heat transport in novel materials, engineered phononic materials, metamaterials, and their relevant systems has created significant demand for computational approaches to aid in investigation and design of materials that support phonons. This review describes the significant improvements that have been made and new needs that have emerged for capabilities associated with the computability of phonons. The technical scope encompasses issues, especially relevant to bulk, interface, and surface effects. Traditional approaches such as molecular dynamics, lattice dynamics, and Boltzmann transport equation continue to advance the field but are frequently extended to the limits of their physical or numerical validity. New materials beyond traditional group-IV, III–V, and II–VI semiconductors, phenomena that critically depend on scattering, such as in low-dimensional nanostructures, materials with interior surfaces and defects, and in high-temperature environments, continue to push these limits. The basis for the traditional calculation methods shares their origins with the earliest theories for thermal transport, acoustic waves in solids, spectroscopy and dynamical crystal lattices. These will remain in wide use in the future. But computing methods and the accompanying advances in microprocessor technologies have enabled growth of phonon computing models and methods in sophistication, accuracy, fidelity and complexity that will lead to fundamental impacts beyond the classic types of problems for which they were developed. With their increasingly integrated use for design and research, the myriad developments that presently exist must be understood for their suitability for certain applications and their ability to aid in the pursuit of new technologies.     

Raman scattering by electronic excitations in semiconductors and in highT c superconductors

Raman scattering by electronic excitations in normal and superconducting metals with complex Fermi surfaces is discussed. The theoretical results are applied to the case of high T_c superconductors. The possible implications of measurements of this scattering concerning the symmetry of the gap function is dealt with.     

Wide-gap layered oxychalcogenide semiconductors: Materials, electronic structures and optoelectronic properties

Applying the concept of materials design for transparent conductive oxides to layered oxychalcogenides, several p-type and n-type layered oxychalcogenides were proposed as wide-gap semiconductors and their basic optical and electrical properties were examined. The layered oxychalcogenides are composed of ionic oxide layers and covalent chalcogenide layers, which bring wide-gap and conductive properties to these materials, respectively. The electronic structures of the materials were examined by normal/inverse photoemission spectroscopy and energy band calculations. The results of the examinations suggested that these materials possess unique features more than simple wide-gap semiconductors. Namely, the layered oxychalcogenides are considered to be extremely thin quantum wells composed of the oxide and chalcogenide layers or 2D chalcogenide crystals/molecules embedded in an oxide matrix. Observation of step-like absorption edges, large band gap energy and large exciton binding energy demonstrated these features originating from 2D density of states and quantum size effects in these layered materials.     

Tunable plasmonic coupling between silver nano-cubes and silver nano-hole arrays

A quasi-three-dimensional (quasi-3D) system composed of Ag nano-cubes and Ag nano-hole arrays was fabricated through a low cost chemical process. The coupling of localized surface plasmons (LSPs) in the cube-hole array system has been investigated through surface-enhanced Raman scattering (SERS) from Rhodamine 6G (R6G) molecules. A SERS enhancement factor as large as 1.1 × 10(8) can be achieved due to this plasmonic coupling effect, and is highly sensitive to geometrical parameters, such as cube-hole array distance, hole diameter, inter-hole spacing and Ag film thickness.     

Thermal boundary resistance at interfaces between sapphire and indium

The Kapitza thermal boundary resistanceR _K has been measured above 1 K on several sapphire-indium boundaries prepared with different methods. By vapor-deposition of indium on sapphire and subsequent cold-welding with bulk indium, reproducible results were obtained. With the indium superconducting, we foundR _KT ^–3 within a certain temperature range, andR _K(1K)=42–44 and 30–36 cm² K/W for polished and rough sapphire surfaces, respectively. The calculation according to the acoustic mismatch theory yieldsR _K(1K)20 cm² K/W. Samples prepared by ultrasonic soldering also follow the relationR _KT ^–3 approximately, and giveR _K(1K)=14–17 cm² K/W. However, it is doubtful whether the calculation presuming a smooth boundary can be applied to the latter samples. Furthermore, we found that the method of vapor deposition and subsequent pouring on molten indium does not give good contacts. Moreover, the electronic contribution to the heat transfer across the boundary has been proved by ruling out other effects.     

Tuning magnetoresistance between positive and negative values in organic semiconductors

Magnetic-field-dependent injection current, namely magnetoresistance, is readily observable in organic semiconductor devices. This provides a non-contact approach to tune organic optoelectronic properties by using a magnetic field. Here, we demonstrate that this magnetoresistance can be changed between positive and negative values by adjusting the dissociation and charge reaction in excited states through changing the bipolar charge injection in organic light-emitting diodes. This finding reveals that the magnetic-field-dependent generation of secondary charge carriers from the dissociation and charge reaction affects the injection current by forming further space charges at the organic-electrode interfaces and therefore accounts for the tunable magnetoresistance. Furthermore, the dissociation and charge reaction have opposite dependences on magnetic field in the generation of secondary charge carriers, consequently leading to negative and positive magnetoresistance, respectively. As a result, adjusting the dissociation and charge reaction in excited states provides a convenient pathway to tune the magnetoresistance in organic semiconductors.