Doped Semiconductor Nanostructures Research Articles

Direct Determination of Carrier Parameters in Indium Tin Oxide Nanocrystals.

We develop here a comprehensive experimental approach to independently determine charge carrier parameters, namely, carrier density and mass, in plasmonic indium tin oxide nanocrystals. Typically, in plasmonic nanocrystals, only the ratio between these two parameters is accessible through optical absorption experiments. The multitechnique methodology proposed here combines single particle and ensemble optical and magneto-optical spectroscopies, also using 119Sn solid-state nuclear magnetic resonance spectroscopy to probe the surface depletion layer. Our methodology overcomes the limitations of standard fitting approaches based on absorption spectroscopy and ultimately gives access to carrier effective mass directly on the NCs, discarding the use of literature value based on bulk or thin film materials. We found that mass values depart appreciably from those measured on thin films; consequently, we found carrier density values that are different from reported literature values for similar systems. The effective mass was found to deviate from the parabolic approximation at a high carrier density. Finally, the dopant activation and defect diagram for ITO NCs for tin doping between 2.5 and 15% are determined. This approach can be generalized to other plasmonic heavily doped semiconductor nanostructures and represents, to the best of our knowledge, the only method to date to characterize the full Drude parameter space of 0-D nanosystems.

Charge-Transfer-Induced Self-Assembly of Doped Conjugated Block Copolymer Nanofibers.

Here, we report charge-transfer-driven self-assembly of conjugated block copolymers (BCP) into highly doped conjugated polymer nanofibers. The ground-state integer charge transfer (ICT) between a BCP composed of poly(3-hexylthiophene) and poly(ethylene oxide) (P3HT-b-PEO) and electron-deficient 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) induced spontaneous self-assembly of the donor and the acceptor into well-defined one-dimensional nanofibers. The presence of the PEO block plays an important role for the self-assembly by providing a polar environment that can stabilize nanoscale charge transfer (CT) assemblies. The doped nanofibers were responsive to various external stimuli such as heat, chemical, and light and exhibited efficient photothermal properties in the near-IR region. The CT-driven BCP self-assembly reported here provides a new platform for the fabrication of highly doped semiconductor nanostructures.

Broadening the absorption bandwidth based on heavily doped semiconductor nanostructures.

Broadband light absorption is a basis for the proper functionality of various materials, microstructures, and devices. Despite numerous studies, however, many aspects of broadband absorption remain uncovered. In this paper, we demonstrate an inverse-problem approach to designing nanostructures with a very low optical reflection and high absorption through a frequency band. Particular emphasis is made on a subwavelength transparent film as a top layer and anisotropic substrate. The polarization-dependent metamaterial absorber based on a subwavelenth semiconductor multicomponent multilayer structure is proposed and numerically investigated. For an illustration, we consider a four-component heavily doped silicon lattice with a thin undoped silicon top layer. The dielectric response of the structure is engineered by controlling the free carrier density and filling factor of each layer. A simulation study reveals a power law dependence of the bandwidth on the maximum reflectivity within the band.

Advanced Semiconductor Alloy Alxin1-Xp for Engineering and Medicine

Doped Advanced semiconductor materials with different properties are useful for early diagnosis and improved treatment in medical research. This is essential for advanced medical technology and lower mortality rates. New research on impurity-doped nano crystals is important. These dopants can directly affect electron transport in semiconductors, tune the optical properties of nano materials in desirable ways, and impart specific properties to the host. In this research report, we first discuss the factors that need to be considered to systematically control the production of these doped semiconductor materials, then describe various doped materials and typical synthetic approaches and techniques. Innovations in nanotechnology and materials design and their application in early diagnosis and treatment are believed to minimize the number of new cases of related diseases and reduce mortality.1,2,3 From natural to man-made materials, Doped semiconductor nanostructures, including inorganic and organic semiconductors, are increasingly attracting the attention of researchers and scientists worldwide

Efficient degradation of doxycycline and ofloxacin in an aqueous environment using Fe and Cu doped TiO2-SiO2 photocatalyst under sunlight

Several drugs have sparked interest as potential COVID-19 treatment options. Doxycycline (DOX) has been widely used with other potential agents to reduce COVID-19-induced inflammation. DOX and OFLX, both well-known antimicrobial and anti-inflammatory drugs, were chosen as model pollutants. Fe, Cu-codoped TiO2-SiO2 was synthesised as a novel photocatalyst active under sunlight irradiation to treat model pollutants. The synthesised catalyst samples were meticulously characterised using various techniques to evaluate their morphological, optical, and structural properties. The results of BET analysis showed that the TSFC1 sample has a large specific surface area of 288 m2g-1. Maximum degradation of DOX and OFLX (about 98%) was achieved with the TSFC1 catalyst. The photocatalytic reusability was investigated for up to seven successive cycles, and the composite particles maintained their high photodegradation activity for DOX and OFLX. TFSC1 composite, in particular, demonstrated high catalytic activity as well as excellent recovery potential, and its combination with solar light, silica, and dopants can be introduced as a promising strategy for efficiently destroying both DOX and OFLX antibiotics. This study highlights the feasibility of hybridising doped dual semiconductor nanostructures in implementing solar light-powered pharmaceutical wastewater degradation.

Contactless Measurements of Carrier Concentrations in InGaAs Layers for Utilizing in InP-Based Quantum Cascade Lasers by Employing Optical Spectroscopy.

The precise determination of carrier concentration in doped semiconductor materials and nanostructures is of high importance. Many parameters of an operational device are dependent on the proper carrier concentration or its distribution in both the active area as well as in the passive parts as the waveguide claddings. Determining those in a nondestructive manner is, on the one hand, demanded for the fabrication process efficiency, but on the other, challenging experimentally, especially for complex multilayer systems. Here, we present the results of carrier concentration determination in In0.53Ga0.47As layers, designed to be a material forming quantum cascade laser active areas, using a direct and contactless method utilizing the Berreman effect, and employing Fourier-transform infrared (FTIR) spectroscopy. The results allowed us to precisely determine the free carrier concentration versus changes in the nominal doping level and provide feedback regarding the technological process by indicating the temperature adjustment of the dopant source.

Theory of plasmonic properties of hyper-doped silicon nanostructures

The presence of a Localized Surface Plasmon Resonance in doped semiconductor nanostructures opens a new field for plasmonics and metasurfaces. Semiconductor nanostructures can be easily processed, have weak dissipation losses, and the plasmon resonance can be tuned from the mid- to the near-infrared spectral range by changing the dopant concentration (in complement to the constituent material, the size and shape of the nanostructure). We present in this paper an extension of the Green Dyadic Method applied to the case of doped silicon nanostructures of arbitrary shape on a planar silica substrate. The method is used to compute both far- and near-field optical properties, such as the extinction efficiency and the electromagnetic near-field intensity inside and around any doped silicon nanostructure, respectively. This theoretical approach provides an important tool for active dopant characterization in doped semiconductor nanostructures, for near-field imaging of complex nanoantennas produced by electron beam lithography, and for the definition of doped semiconductor-based metasurfaces.

Quantification of Carrier Density Gradients along Axially Doped Silicon Nanowires Using Infrared Nanoscopy

Doped semiconductor nanostructures are interesting for the fabrication of nanoscale electronic and photonic devices. Here, we use scattering-type scanning near-field optical microscopy (s-SNOM) to ...

Near-Field Imaging of Free Carriers in ZnO Nanowires with a Scanning Probe Tip Made of Heavily Doped Germanium

Although scattering-type scanning near-field infrared microscopy can provide access to key parameters of nanomaterials, presently it is performed with unoptimized gold-coated cantilever probes, mainly due to their commercial availability. This work demonstrates that heavily doped germanium, with a midinfrared plasma frequency, is also a good choice for such probes. The peculiarities of IR plasmonic resonances of doped semiconductor nanostructures are highlighted with the aid of multiple-scattering models. This development in coating-free monolithic tips for scanning-probe microscopy could facilitate a lot of nanotechnology research, particularly in plasmonics and photonics.

Localized surface plasmon enhanced photothermal conversion in Bi2Se3 topological insulator nanoflowers.

Localized surface plasmons (LSP), the confined collective excitations of electrons in noble metal and doped semiconductor nanostructures, enhance greatly local electric field near the surface of the nanostructures and result in strong optical response. LSPs of ordinary massive electrons have been investigated for a long time and were used as basic ingredient of plasmonics and metamaterials. LSPs of massless Dirac electrons, which could result in novel tunable plasmonic metamaterials in the terahertz and infrared frequency regime, are relatively unexplored. Here we report for first time the observation of LSPs in Bi2Se3 topological insulator hierarchical nanoflowers, which are consisted of a large number of Bi2Se3 nanocrystals. The existence of LSPs can be demonstrated by surface enhanced Raman scattering and absorbance spectra ranging from ultraviolet to near-infrared. LSPs produce an enhanced photothermal effect stimulated by near-infrared laser. The excellent photothermal conversion effect can be ascribed to the existence of topological surface states, and provides us a new way for practical application of topological insulators in nanoscale heat source and cancer therapy.

Magnetic Properties of N- and (Cr, N)-Doped TiO<sub>2</sub> Nanoparticles

The effect of N and Cr doping on the magnetic properties of TiO2 nanoparticles is analyzed in this paper. N- and (Cr, N)-doped TiO2 nanoparticles were synthesized by the sol–gel method using titanium tetraisopropoxide as metallic precursor, and urea and chromium nitrate as N and Cr doping precursors, respectively. The calcined nanoparticles were analyzed through X-ray diffraction and Rietveld refinement. The samples display an anatase structure with mean grain sizes $\sim5$ nm. A distortion of the anatase cell is detected (increase and decrease of $a$ and $c$ lattice parameters, respectively) with respect to the reported anatase bulk values. The room-temperature hysteresis loops of the samples display the coexistence of the paramagnetic and the ferromagnetic contributions. An enhancement in the ferromagnetic component is observed upon Cr and N codoping. The analysis of the temperature dependence of the magnetization confirms the coexistence of both the paramagnetic and the ferromagnetic contributions. Although the existence of a secondary phase cannot be completely excluded in the (Cr, N)-doped sample with the highest nitrogen content, the results confirm the achievement of intrinsic room temperature ferromagnetism in these doped semiconductor nanostructures.

Picosecond Dynamics of Excitonic Magnetic Polarons in Colloidal Diffusion-Doped Cd(1-x)Mn(x)Se Quantum Dots.

Spontaneous magnetization is observed at zero magnetic field in photoexcited colloidal Cd(1-x)Mn(x)Se (x = 0.13) quantum dots (QDs) prepared by diffusion doping, reflecting strong Mn(2+)-exciton exchange coupling. The picosecond dynamics of this phenomenon, known as an excitonic magnetic polaron (EMP), are examined using a combination of time-resolved photoluminescence, magneto-photoluminescence, and Faraday rotation (TRFR) spectroscopies, in conjunction with continuous-wave absorption, magnetic circular dichroism (MCD), and magnetic circularly polarized photoluminescence (MCPL) spectroscopies. The data indicate that EMPs form with random magnetization orientations at zero external field, but their formation can be directed by an external magnetic field. After formation, however, external magnetic fields are unable to reorient the EMPs within the luminescence lifetime, implicating anisotropy in the EMP potential-energy surfaces. TRFR measurements in a transverse magnetic field reveal rapid (<5 ps) spin transfer from excitons to Mn(2+) followed by coherent EMP precession at the Mn(2+) Larmor frequency for over a nanosecond. A dynamical TRFR phase inversion is observed during EMP formation attributed to the large shifts in excitonic absorption energies during spontaneous magnetization. Partial optical orientation of the EMPs by resonant circularly polarized photoexcitation is also demonstrated. Collectively, these results highlight the extraordinary physical properties of colloidal diffusion-doped Cd(1-x)Mn(x)Se QDs that result from their unique combination of strong quantum confinement, large Mn(2+) concentrations, and relatively narrow size distributions. The insights gained from these measurements advance our understanding of spin dynamics and magnetic exchange in colloidal doped semiconductor nanostructures, with potential ramifications for future spin-based information technologies.

Photoluminescence and photocatalytic studies of metal ions (Mn and Ni) doped ZnS nanoparticles

The present study deals with the structural, optical and photocatalytic studies of thioglycerol capped doped ZnS nanoparticles (NPs). Effect of two dopant metal ions (Ni and Mn) on photoluminescence emission and photocatalytic properties have been studied in detail. Zn1−xMxS; M=Ni or Mn; x=0, 0.01, 0.02, 0.03 and 0.04 NPs have been synthesized using simple chemical precipitation route. Structural and morphological studies have been done by using X-ray diffraction (XRD) technique and high resolution transmission electron microscopy (HRTEM). Capping of thioglycerol on the surface of doped ZnS has been confirmed by Fourier transform infrared (FTIR) studies. UV–Vis and photoluminescence studies have been carried out to study the effect of doping on optical properties of synthesized materials. Degradation of crystal violet has been carried out with the aim to investigate the effect of Ni or Mn doping on photocatalytic activity of ZnS. It has been observed that both the metal ions have decreased the photocatalytic activity of ZnS. The effect of photocatalytic reaction temperature on photocatalytic properties of one of the doped sample has also been investigated. It has been interpreted from the results that photocatalytic activity of doped semiconductor nanostructures is strongly dependant on their photoluminescence properties as well as on photocatalytic reaction temperature.

Electron-phonon interaction and Raman scattering in doped GaAs/AlAs superlattices

The Raman scattering spectroscopy method is used to study the interaction of phonons and free charge carriers in doped semiconductor nanostructures (superlattices). In doped superlattices based on polar semiconductors, the collective vibrational modes of free charge carriers (plasmons) shield the long-range Coulomb interaction of cations and anions, which leads to the formation of mixed phononplasmon modes. The angular dispersion (anisotropy) of phonon-plasmon modes in doped GaAs/AlAs superlattices is studied. The observed anisotropy is due to the anisotropy of dielectric permeability in superlattices.

Spin dynamics inp-doped semiconductor nanostructures subject to a magnetic field tilted from the Voigt geometry

We develop a theoretical description of the spin dynamics of resident holes in a p-doped semiconductor quantum well (QW) subject to a magnetic field tilted from the Voigt geometry. We find the expressions for the signals measured in time-resolved Faraday rotation (TRFR) and resonant spin amplification (RSA) experiments and study their behavior for a range of system parameters. We find that an inversion of the RSA peaks can occur for long hole spin dephasing times and tilted magnetic fields. We verify the validity of our theoretical findings by performing a series of TRFR and RSA experiments on a p-modulation doped GaAs/Al_{0.3}Ga_{0.7}As single QW and showing that our model can reproduce experimentally observed signals.

Nanocrystal Diffusion Doping

A diffusion-based synthesis of doped colloidal semiconductor nanocrystals is demonstrated. This approach involves thermodynamically controlled addition of both impurity cations and host anions to preformed seed nanocrystals under equilibrium conditions, rather than kinetically controlled doping during growth. This chemistry allows thermodynamic crystal compositions to be prepared without sacrificing other kinetically trapped properties such as shape, size, or crystallographic phase. This doping chemistry thus shares some similarities with cation-exchange reactions, but proceeds without the loss of host cations and excels at the introduction of relatively unreactive impurity ions that have not been previously accessible using cation exchange. Specifically, we demonstrate the preparation of Cd(1-x)Mn(x)Se (0 ≤ x ≤ ∼0.2) nanocrystals with narrow size distribution, unprecedentedly high Mn(2+) content, and very large magneto-optical effects by diffusion of Mn(2+) into seed CdSe nanocrystals grown by hot injection. Controlling the solution and lattice chemical potentials of Cd(2+) and Mn(2+) allows Mn(2+) diffusion into the internal volumes of the CdSe nanocrystals with negligible Ostwald ripening, while retaining the crystallographic phase (wurtzite or zinc blende), shape anisotropy, and ensemble size uniformity of the seed nanocrystals. Experimental results for diffusion doping of other nanocrystals with other cations are also presented that indicate this method may be generalized, providing access to a variety of new doped semiconductor nanostructures not previously attainable by kinetic routes or cation exchange.

Quantum-interference transport through surface layers of indium-doped ZnO nanowires

We have fabricated indium-doped ZnO (IZO) nanowires (NWs) and carried out four-probe electrical-transport measurements on two individual NWs with geometric diameters of ≈70 and ≈90 nm in a wide temperature T interval of 1–70 K. The NWs reveal overall charge conduction behavior characteristic of disordered metals. In addition to the T dependence of resistance R, we have measured the magnetoresistance (MR) in magnetic fields applied either perpendicular or parallel to the NW axis. Our R(T) and MR data in different T intervals are consistent with the theoretical predictions of the one- (1D), two- (2D) or three-dimensional (3D) weak-localization (WL) and the electron–electron interaction (EEI) effects. In particular, a few dimensionality crossovers in the two effects are observed. These crossover phenomena are consistent with the model of a ‘core–shell-like structure’ in individual IZO NWs, where an outer shell of thickness t (≃15–17 nm) is responsible for the quantum-interference transport. In the WL effect, as the electron dephasing length Lφ gradually decreases with increasing T from the lowest measurement temperatures, a 1D-to-2D dimensionality crossover takes place around a characteristic temperature where Lφ approximately equals d, an effective NW diameter which is slightly smaller than the geometric diameter. As T further increases, a 2D-to-3D dimensionality crossover occurs around another characteristic temperature where Lφ approximately equals t (<d). In the EEI effect, a 2D-to-3D dimensionality crossover takes place when the thermal diffusion length LT progressively decreases with increasing T and approaches t. However, a crossover to the 1D EEI effect is not seen because LT < d even at T = 1 K in our IZO NWs. Furthermore, we explain the various inelastic electron scattering processes which govern Lφ. This work demonstrates the complex and rich nature of the charge conduction properties of group-III metal-doped ZnO NWs. This work also strongly indicates that the surface-related conduction processes are essential to doped semiconductor nanostructures.

Kelvin probe force microscopy for characterizing doped semiconductors for future sensor applications in nano- and biotechnology

Kelvin probe force microscopy (KPFM) is one of the most promising non-contact electrical nanometrology techniques to characterize doped semiconductors. By applying a recently introduced explanation of measured KPFM signals, we show the applicability of KPFM to determine and control surface-near electrostatic forces in planar doped silicon and in doped silicon nanostructures. Surface-near electrostatic forces may be used for the immobilization of nano- and biomaterials in future sensor applications in nano- and biotechnology. Additionally, the influence of the electrostatic potential distribution in doped semiconductor nanostructures, e.g. in horizontal Si nanowires, and its influence on the surface-near electrostatic forces are discussed. It is explained how drift and diffusion of injected electrons and holes in intrinsic electric fields influence the detected KPFM signal. For example KPFM is successfully employed to locate p+p and n+p junctions along B-doped and As-doped p-Si nanowires, respectively. As an outlook the physical immobilization and the transport of biomaterials above arrays of separately addressable doped semiconductor cells will be discussed.

Ferromagnetism in Nd-doped ZnO nanowires and the influence of oxygen vacancies: ab initio calculations

Room-temperature ferromagnetism (RTFM) has been achieved in rare earth (RE) element doped ZnO nanostructures, while the mechanism of RTFM is still under debate. In this work, we investigated the electronic structure and magnetic properties of Nd-doped ZnO nanowires, and the influence of oxygen vacancies by using ab initio calculations. The impurity Nd atoms prefer to substitute the surface Zn sites and be ferromagnetically coupled with a moment of ~3.0 μB per Nd in the doped ZnO nanowires, through a superexchange interaction mediated by the oxygen ions. More interestingly, the surface oxygen vacancy can greatly enhance the stability of the ferromagnetic state via the "local carriers" of Nd- and Zn-s electrons, which accounts for the room-temperature ferromagnetism in the Nd-doped ZnO nanowires. This mechanism and analysis can be applied to other rare earth element doped semiconductor nanostructures.

Influence of a depletion layer on localized surface waves in doped semiconductor nanostructures

We report THz polarized reflectivity measurements on a doped GaAs grating made of thin walls with subwavelength dimensions and periodicity. A clear dip appears in the TM-polarized reflectivity spectrum, due to the excitation of a localized surface plasmon-phonon mode along the grating walls. The theoretical model used to describe the electromagnetic response of the grating shows the importance of the depletion layer at the GaAs surface and indicates a high sensitivity of the reflectivity dip frequency to the surface potential.