Doped Nanowires Research Articles

Spectroscopic properties and Judd–Ofelt analysis of Eu3+ doped GdPO4 nanoparticles and nanowires

Eu3+ doped GdPO4 orthophosphates with hexagonal and monoclinic crystal structures were prepared by a solvothermal process at 160°C. Nanoparticles (NPs) and nanowires (NWs) were obtained using a mixture of water and glycerol as solvents. Luminescence properties of 5% Eu3+ doped GdPO4 NPs and NWs were investigated. As-obtained luminescent NWs:Eu and NPs:Eu powders were post-annealed at 750°C since this temperature did not change neither their initial morphology nor their crystalline structure and led to luminescence efficiency intense enough to suit optical applications. A post-heating treatment of the samples at higher temperature (1000°C) resulted in a change in both the crystalline structure from hexagonal to monoclinic form and the morphology. Excitation and emission spectra as well as luminescence decays were recorded on both post-annealed luminescent NW:Eu and NP:Eu powders. A comparative study of the optical results obtained for the different morphologies and structures was carried out by calculating the asymmetry ratio (A21) of Eu3+ from emission spectra. The symmetry of the local environment of Eu3+ activators has been described in the frame of Judd–Ofelt theory, using Ω2 and Ω4 intensity parameters derived from emission spectra data analysis, adopting a standard procedure. It was observed that Ω2 value for the NPs is different from that of the NWs indicating that the environment surrounding the active ion changes depending on the sample morphologies. The radiative lifetimes of the 5D0 excited state and the corresponding branching ratios for different emission transitions were also estimated and compared to the recorded luminescence decay rates at room temperature.

One‐Step Facile Synthesis and Luminescence Properties of Eu3+‐Doped Silica Nanowires

AbstractUniformly sized and shaped Eu3+‐doped silica nanowires were conventionally fabricated by the microemulsion method accompanied with a sol–gel process for the first time. The products were characterized by different techniques, and their luminescence properties were investigated and compared. The scanning electron microscopy and transmission electron microscopy images demonstrated that the as‐synthesized Eu3+‐doped silica nanowires were formed in high yield with a smooth surface and a uniform diameter of about 130–150 nm. The results of FTIR spectroscopy, energy‐dispersive spectroscopy, and X‐ray diffraction showed that the Eu3+ ions were doped into the silica network structure in the form of Si–O–Eu bonds. The effect of the Eu3+ concentration on the luminescent intensity was investigated, and the highest emission intensity was found at the composition of 20 mol‐% Eu3+. The obtained Eu3+‐doped silica nanowires exhibited characteristic luminescent emission of Eu3+ at λ = 613 nm and apparent concentration quenching above the concentration of 20 mol‐% Eu3+. The growth processes of the silica nanowires were explained in detail on the basis of a modified vapor–liquid–solid mechanism.

Localized tip enhanced Raman spectroscopic study of impurity incorporated single GaN nanowire in the sub-diffraction limit

The localized effect of impurities in single GaN nanowires in the sub-diffraction limit is reported using the study of lattice vibrational modes in the evanescent field of Au nanoparticle assisted tip enhanced Raman spectroscopy (TERS). GaN nanowires with the O impurity and the Mg dopants were grown by the chemical vapor deposition technique in the catalyst assisted vapor-liquid-solid process. Symmetry allowed Raman modes of wurtzite GaN are observed for undoped and doped nanowires. Unusually very strong intensity of the non-zone center zone boundary mode is observed for the TERS studies of both the undoped and the Mg doped GaN single nanowires. Surface optical mode of A1 symmetry is also observed for both the undoped and the Mg doped GaN samples. A strong coupling of longitudinal optical (LO) phonons with free electrons, however, is reported only in the O rich single nanowires with the asymmetric A1(LO) mode. Study of the local vibration mode shows the presence of Mg as dopant in the single GaN nanowires.

Electrodeposition of Ni and Te‐doped Cobalt Triantimonide in Citrate Solutions

AbstractSkutterudite compounds form a new class of potential candidates for thermoelectric applications. Cobalt triantimonide (CoSb3) shows good thermoelectric properties at medium and high temperatures. Doping this system with substitution elements, for either Co or Sb or both, may result in an increase of the thermoelectric figure of merit (ZT). This work focused on the electrochemical doping and characterization of films and nanowires of Co‐Sb system in citrate solutions using gold‐coated PCTE templates. The electrodeposition was performed on gold surface that was pre‐treated electrochemically to ensure reproducible results. The electrochemical treatment acted as an annealing process for the surface, which resulted in an increase in Au(111) as demonstrated by XRD. Detailed electrochemical studies including deposition‐stripping experiments was performed in order to develop a better understanding of the co‐deposition kinetics and a better control over the composition of doped Co‐Sb system. Scanning electron microscopy (SEM/EDS) helped study the morphology and the composition of the doped and undoped Co‐Sb system. Co‐deposition of Co‐Sb showed that the amount of Co is higher in nanowires than in film or mushroom caps due to the slow Sb deposition rate dictated by slow Sb(III) complex diffusion. Doped nanowires have been also obtained. Both Ni and Te electrochemical doping of the Co‐Sb system affected the composition of the deposit but there was no effect on nanowire morphology.

(Invited) Defects and Dopants in Silicon and Germanium Nanowires

Several architectures for ultra-scaled devices targeting classical and quantum information processing as well as chemical sensing rely on silicon and germanium nanowires (SiNWs, GeNWs). Despite the efforts in the preparation and characterization of these nanostructures some fundamental issues remain relatively unexplored. In particular the investigation of defects at the interface between the semiconductor and its oxide or other semiconductors or oxides in core-shell structures represent an important challenge, as the NW diameter reduces and the surface-to-volume ratio increases [1,2]. The current experimental data and understanding of the n-type doping of silicon and germanium nanowires produced by different methods will be reviewed. The role of hydrogen and defects at the interface between the semiconductor and its oxide [3-5] in the donor de-activation mechanisms, and the effect of the confinement and dielectric mismatch in determining the donor electronic wave function will be discussed. The implications of the differences between the electronic properties of interfacial defects in silicon and germanium with their respective oxides will be also addressed.

Fabrication of ion doped WO3 photocatalysts through bulk and surface doping

Na+ doped WO3 nanowire photocatalysts were prepared by using post-treatment (surface doping) and in situ (bulk doping) doping methods. Photocatalytic degradation of Methyl Blue was tested under visible light irradiation, the results showed that 1wt.% Na+ bulk-doped WO3 performed better, with higher photoactivity than surface-doped WO3. Photoelectrochemical characterization revealed the differences in the photocatalytic process for surface doping and bulk doping. Uniform bulk doping could generate more electron–hole pairs, while minimizing the chance of electron–hole recombination. Some bulk properties such as the bandgap, Fermi level and band position could also be adjusted by bulk doping, but not by surface doping.

Using Activated Transport in Parallel Nanowires for Energy Harvesting and Hot-Spot Cooling

We study arrays of parallel doped semiconductor nanowires in a temperature range where the electrons propagate through the nanowires by phonon assisted hops between localized states. By solving the Random Resistor Network problem, we compute the thermopower $S$, the electrical conductance $G$, and the electronic thermal conductance $K^e$ of the device. We investigate how those quantities depend on the position -- which can be tuned with a back gate -- of the nanowire impurity band with respect to the equilibrium electrochemical potential. We show that large power factors can be reached near the band edges, when $S$ self-averages to large values while $G$ is small but scales with the number of wires. Calculating the amount of heat exchanged locally between the electrons inside the nanowires and the phonons of the environment, we show that phonons are mainly absorbed near one electrode and emitted near the other when a charge current is driven through the nanowires near their band edges. This phenomenon could be exploited for a field control of the heat exchange between the phonons and the electrons at submicron scales in electronic circuits. It could be also used for cooling hot spots.

Sensing Properties of Ga-doped ZnO Nanowire Gas Sensor

Pure ZnO and ZnO nanowires doped with 3 wt.% Ga (&#x2018;3GZO&#x2019;) were grown by pulsed laser deposition in a furnace system. The doping of Ga in ZnO nanowires was analyzed by observing the optical and chemical properties of the doped nanowires. The diameter and length of nanowires were under 200 nm and several <TEX>${\mu}m$</TEX>, respectively. Changes of significant resistance were observed and the sensitivities of ZnO and 3GZO nanowires were compared. The sensitivities of ZnO and 3GZO nanowire sensors measured at 300&#x2103; for 1 ppm of ethanol gas were 97% and 48%, respectively.

Defect-engineered GaN:Mg nanowire arrays for overall water splitting under violet light

We report that by engineering the intra-gap defect related energy states in GaN nanowire arrays using Mg dopants, efficient and stable overall neutral water splitting can be achieved under violet light. Overall neutral water splitting on Rh/Cr2O3 co-catalyst decorated Mg doped GaN nanowires is demonstrated with intra-gap excitation up to 450 nm. Through optimized Mg doping, the absorbed photon conversion efficiency of GaN nanowires reaches ∼43% at 375–450 nm, providing a viable approach to extend the solar absorption of oxide and non-oxide photocatalysts.

Fabrication and characterisation of suspended narrow silicon nanowire channels for low-power nano-electro-mechanical (NEM) switch applications

Suspended silicon nanowires with narrow (∼10nm) conduction channel are fabricated and characterised for further development of low power nano-electro-mechanical (NEM) switching devices using CMOS compatible fabrication. Double suspension fabrication process using an amorphous silicon sacrificial layer and xenon difluoride etching is employed for thermally-oxidised suspended Si nanowire channels. Device current–voltage characteristics demonstrate depletion mode operation of heavy doped nanowires with an on/off ratio of 105 and a threshold voltage of −1.8V. In plane electromechanical pull-in to side gate is demonstrated and confirmed to be consistent with finite element analysis.

In situ UHVEM irradiation study of intrinsic point defect behavior in Si nanowire structures

AbstractSi nanowire‐based Tunnel‐Field Effect Transistor (TFET) characteristics are intensively studied as function of nanowire diameter and doping. A significant reduction of B diffusion with decreasing nanowire diameter is e.g. observed and attributed to reduced transient enhanced diffusion close to the nanowire surface caused by the recombination and out‐diffusion of excess self‐interstitials. In an Ultra High Voltage Electron Microscope (UHVEM), the formation of self‐interstitial clusters can be studied in situ while varying e‐beam flux, irradiation temperature, impurity concentration and capping layers surrounding the nanowires.Results are presented on {113}‐defect formation in Si nanowires with diameters between 40 and 500 nm. The Si nanowires are embedded in SiO2 and are etched into an epitaxial Si stack on a heavily As doped Si substrate. The top layer of the epitaxial stack is in situ B doped or B implanted. In situ UHVEM studies are performed on focused ion beam prepared cross‐section samples, irradiating with different fluxes of 2 MeV electrons between room temperature and 375 °C. A strong dependence of {113}‐defect formation on nanowire radius and doping is observed. The observations are compared with simulations based on quasi‐chemical reaction rate theory. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Doping GaP Core-Shell Nanowire pn-Junctions: A Study by Off-Axis Electron Holography.

The doping process in GaP core-shell nanowire pn-junctions using different precursors is evaluated by mapping the nanowires' electrostatic potential distribution by means of off-axis electron holography. Three precursors, triethyltin (TESn), ditertiarybutylselenide, and silane are investigated for n-type doping of nanowire shells; among them, TESn is shown to be the most efficient precursor. Off-axis electron holography reveals higher electrostatic potentials in the regions of nanowire cores grown by the vapor-liquid-solid (VLS) mechanism (axial growth) than the regions grown parasitically by the vapor-solid (VS) mechanism (radial growth), attributed to different incorporation efficiency between VLS and VS of unintentional p-type carbon doping originating from the trimethylgallium precursor. This study shows that off-axis electron holography of doped nanowires is unique in terms of the ability to map the electrostatic potential and thereby the active dopant distribution with high spatial resolution.

One dimensional semiconductor nanostructures: An effective active-material for terahertz detection

One-dimensional (1D) nanostructure devices are at the frontline of studies on future electronics, although issues like massive parallelization, doping control, surface effects, and compatibility with silicon industrial requirements are still open challenges. The recent progresses in atomic to nanometer scale control of materials morphology, size, and composition including the growth of axial, radial, and branched nanowire (NW)-based heterostructures make the NW an ideal building block for implementing rectifying diodes or detectors that could be well operated into the Terahertz (THz), thanks to their typical achievable attofarad-order capacitance. Here, we report on our recent progresses in the development of 1D InAs or InAs/InSb NW-based field effect transistors exploiting novel morphologies and/or material combinations effective for addressing the goal of a semiconductor plasma-wave THz detector array technology. Through a critical review of material-related parameters (NW doping concentration, geometry, and/or material choice) and antenna-related issues, here we underline the crucial aspects that can affect detection performance across the THz frequency region.

Modulation doping of GaAs/AlGaAs core-shell nanowires with effective defect passivation and high electron mobility.

Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 ± 300 cm(2) V(-1) s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3 and 2.4 ± 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.

Doped GaN nanowires on diamond: Structural properties and charge carrier distribution

In this work, we present a detailed study on GaN nanowire doping, which is vital for device fabrication. The nanowires (NWs) are grown by means of molecular beam epitaxy on diamond (111) substrates. Dopant atoms are found to facilitate nucleation, thus an increasing NW density is observed for increasing dopant fluxes. While maintaining nanowire morphology, we demonstrate the incorporation of Si and Mg up to concentrations of 9× 1020cm−3 and 1 × 1020cm−3, respectively. The dopant concentration in the nanowire cores is determined by the thermodynamic solubility limit, whereas excess dopants are found to segregate to the nanowire surface. The strain state of the NWs is investigated by X-ray diffraction, which confirms a negligible strain compared to planar thin films. Doping-related emissions are identified in low-temperature photoluminescence spectroscopy and the temperature quenching yields ionization energies of Si donors and Mg acceptors of 17 meV and 167 meV, respectively. At room temperature, luminescence and absorption spectra are found to coincide and the sub-band gap absorption is suppressed in n-type NWs. The charge carrier distribution in doped GaN nanowires is simulated under consideration of surface states at the non-polar side facets. For doping concentrations below 1017cm−3, the nanowires are depleted of charge carriers, whereas they become highly conductive above 1019cm−3.

Monitoring the spreading stage of lung cells by silicon nanowire electrical cell impedance sensor for cancer detection purposes

We developed a silicon nanowire based electrical cell impedance sensor (SiNW-ECIS) as an instrument that detects cancerous cultured living lung cells by monitoring their spreading state at which the cells stretched and become extended on nanowires. Further current penetration into the extended membrane of malignant cells in respect to normal ones (In the first 6h after cells interaction with surface) are the key mechanism in our diagnosis procedure. The developed device applied to monitor the spreading-induced electrical differences between cancerous and normal lung cells in an integral fashion. Detection was performed so faster than the time required to complete cells mitosis. Morphology and architecture of doped Si nanowires covered microelectrodes observably enhance the contact area between cells and electrodes which support accurate signal recording from stretched cells as indicated by SEM and florescent images.

Tuning the polarization-induced free hole density in nanowires graded from GaN to AlN

We report a systematic study of p-type polarization-induced doping in graded AlGaN nanowire light emitting diodes grown on silicon wafers by plasma-assisted molecular beam epitaxy. The composition gradient in the p-type base is varied in a set of samples from 0.7%Al/nm to 4.95%Al/nm corresponding to negative bound polarization charge densities of 2.2 × 1018 cm−3 to 1.6 × 1019 cm−3. Capacitance measurements and energy band modeling reveal that for gradients greater than or equal to 1.30%Al/nm, the deep donor concentration is negligible and free hole concentrations roughly equal to the bound polarization charge density are achieved up to 1.6 × 1019 cm−3 at a gradient of 4.95%Al/nm. Accurate grading lengths in the p- and n-side of the pn-junction are extracted from scanning transmission electron microscopy images and are used to support energy band calculation and capacitance modeling. These results demonstrate the robust nature of p-type polarization doping in nanowires and put an upper bound on the magnitude of deep donor compensation.

Impact of isotopic disorders on thermal transport properties of nanotubes and nanowires

We present a one-dimensional lattice model to describe thermal transport in isotopically doped nanotubes and nanowires. The thermal conductivities thus predicted, as a function of isotopic concentration, agree well with recent experiments and other simulations. Our results display that for any given concentration of isotopic atoms in a lattice without sharp atomic interfaces, the maximum thermal conductivity is attained when isotopic atoms are placed regularly with an equal space, whereas the minimum is achieved when they are randomly inserted with a uniform distribution. Non-uniformity of disorder can further tune the thermal conductivity between the two values. Moreover, the dependence of the thermal conductivity on the nanoscale feature size becomes weak at low temperature when disorder exists. In addition, when self-consistent thermal reservoirs are included to describe diffusive nanomaterials, the thermal conductivities predicted by our model are in line with the results of macroscopic theories with an interfacial effect. Our results suggest that the disorder provides an additional freedom to tune the thermal properties of nanomaterials in many technological applications including nanoelectronics, solid-state lighting, energy conservation, and conversion.

The fabrication of innovative single crystal N,F-codoped titanium dioxide nanowires with enhanced photocatalytic activity for degradation of atrazine

N,F-codoped TiO2 nanowires were synthesized using titanium sulfate as a Ti precursor and isopropanol as a protective capping agent by a hydrothermal route. The obtained doped nanowires were characterized by XRD, SEM, TEM, HRTEM, SAED, XPS, porosimetry analysis and UV–vis diffuse reflectance spectroscopy. The synthesized N,F-codoped TiO2 nanowires calcined at 500°C have TiO2 (B) and anatase phase and then are completely transformed to anatase phase after calcination at 600°C. Photocatalytic activity measurements show that the N,F-codoped TiO2 nanowires calcined at 600°C possess the best photocatalytic performance for the degradation of atrazine under UV and visible light irradiation in comparison with N,F-codoped TiO2 nanoparticles and N-doped TiO2 nanowires, which was attributed to the relatively more quantity of active {001} crystal facet due to the addition of F− and the synergistic effect of N and F doping.

Thermal and electromechanical characterization of top-down fabricated p-type silicon nanowires**Invited talk at the 7th International Workshop on Advanced Materials Science and Nanotechnology IWAMSN2014, 2–6 November, 2014, Ha Long, Vietnam

In this paper we report thermal conductivity and piezoresistivity measurements of top-down fabricated highly boron doped (NA = 1.5 × 1019 cm−3) suspended Si nanowires. These measurements were performed in a cryogenic probe station respectively by using the 3 omega method and by in situ application of a longitudinal tensile stress to the nanowire under test with a direct four point bending of the Si nanowire die. Nanowires investigated have a thickness of 160 nm, a width in the 80–260 nm range and a length in the 2.5–5.2 μm range. We found that for these geometries, thermal conduction still obeys Fourier’s law and that, as expected, the thermal conductivity is largely reduced when the nanowires width is shrunk, but, to a lower extent than published values for nanowires grown by vapor–liquid–solid (VLS) processes. While a large giant piezoresistance effect was evidenced by various authors when a static stress is applied, we only observed a limited nanowire size dependence of the piezoresistivity in our experiments where a dynamical mechanical loading is applied. This confirms that the giant piezoresistance effect in unbiased Si nanowires is not an intrinsic bulk effect but is dominated by surface related effects in agreement with the piezopinch effect model.