Uncapped Samples Research Articles

Ion-induced interdiffusion of surface GaN quantum dots

In the flourishing fields of quantum technology gallium nitride (GaN) quantum dots (QDs) have great appeal by providing high stability and room-temperature operation. Here, we report on the ion implantation of surface GaN QDs grown in the hexagonal crystal structure. An uncapped sample (S1) and two samples capped by 8ML (S2) and 16ML (S3) of AlN are subjected to a 100keV gallium (S1, S2) and a 210keV erbium (S3) ion beam. The fluence ranged from 5×1010cm−2 to 1×1015cm−2 (S1, S2) and from 5×1010cm−2 to 5×1013cm−2 (S3). QD characterization is performed by cathodoluminescence measurements at 77K and atomic force microscopy and scanning electron microscopy.Strong interdiffusion processes upon ion impact at the interfaces are evidenced leading besides other effects to a quenching of the quantum confined Stark effect. Moreover, a model for the QD morphology based on a fluence-dependent diffusion coefficient is developed.

Desorption induced GaN quantum dots on (0001) AlN by MOVPE

GaN quantum dots (QDs) are realized on (0001) AlN templates by growing a thin GaN layer on an AlN buffer layer and applying a subsequent desorption step without ammonia present. A growth interruption (GRI), which is commonly applied after the GaN growth allowing for QD formation, is systematically investigated regarding the temperature, duration and initial GaN coverage. Without GRI the initial GaN layer exhibits a two-dimensional nonuniform growth at the step edges. In this study, the surface morphology only changes significantly if the GRI is performed without ammonia exposure. Thus, an initial two-dimensional GaN layer can be shaped into three-dimensional nanostructures. Presented coverage studies by atomic force microscopy (AFM) show desorption as the main driving force for island evolution. By tailoring the growth parameters, GaN QDs can be achieved. Uncapped GaN samples exhibit QDs with 1.2 nm in height and 30 nm in diameter. Additionally, capped GaN QDs exhibit excitonic luminescence lines at about 4.3 eV with FWHM down to 2 meV and an excitonic fine structure splitting of 7 meV. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

The effect of storage conditions on sample stability in the routine clinical laboratory.

'Add-on' tests are often requested for samples already in the laboratory. This study was carried out to assess the stability of common analytes in uncapped samples stored in different conditions, and therefore their suitability for 'add-on' requests. Storage conditions evaluated included initial storage at room temperature for 16 hours, followed by storage at 4℃ (Group 1: current conditions employed in the laboratory), compared with storage at 4℃ immediately following analysis (Group 2: optimum storage conditions). Some analytes were not suitable for 'add-ons' when samples were stored in either storage condition, whereas some were suitable for 'add-ons' irrespective of storage condition. Storage conditions influenced the suitability of 'add-on' tests for a proportion of analytes including urea, amylase, total protein, alkaline phosphatase, adjusted calcium, lactate dehydrogenase, triglycerides, HDL cholesterol and total cholesterol; these analytes were stable in optimum conditions (Group 2) but not in current conditions (Group 1). 'Add-on' tests can only be safely performed on a proportion of routine analytes. For some analytes, storage conditions affect their suitability for delayed analysis.

Investigation of cubic GaN quantum dots grown by the Stranski‐Krastanov process

AbstractWe investigate the formation of cubic GaN quantum dots (QDs) on pseudomorphic strained cubic AlN layers on 3C‐SiC (001) substrates grown by means of molecular beam epitaxy. Surface morphologies of various QD sizes and densities were obtained from uncapped samples by atomic force microscopy. These results were correlated with similar but capped samples by photoluminescence experiments. The QD density varies by one order of magnitude from ∼1x1010 cm‐2 to ∼1x1011 cm‐2 as a function of the GaN coverage on the surface. The initial layer thickness for the creation of cubic GaN QDs on cubic AlN was obtained to 1.95 monolayers by a comparison between the experimental results and an analytical model. Our results reveal the strain‐driven Stranski‐Krastanov growth mode as the main formation process of the cubic GaN QDs. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of InAs/GaAs QDs Size with the Growth Rate: A Numerical Investigation

This paper investigates the impact of the deposition rate on the mean buried InAs/GaAs quantum dots’ (QDs) size by means of a coupled photoluminescence spectroscopy and numerical approach. The proposed method consists in tuning the theoretical transition energies by changing the QDs aspect ratio towards best fit of the photoluminescence emission energies arising from the state filling effect. The electron‐hole confined states are obtained by solving the single particle one band effective mass Schrödinger equation in cylindrical coordinates for a lens shaped QD by finite element method taking into account the strain effects. The obtained evolution is in agreement with morphological data taken from similar uncapped QDs samples.

Red shift of the band-edge photoluminescence emission and effects of annealing and capping agent on structural and optical properties of ZnO nanoparticles

Use of nontoxic and biodegradable capping agents during the synthesis of nanomaterials has drawn a lot of research attention due to their potential applicability in biophotonic and bio-imaging devices. However, here we have reported the synthesis of an uncapped and biodegradable polyvinyl alcohol (PVA) capped ZnO nanoparticles (NPs), by using a simple chemical precipitation method at room temperature, followed by isochronal annealing of the as-synthesized sample at 200, 400, 500 and 600°C for 2h in air. The effects of using PVA and thermal annealing on the structural and optical properties of synthesized ZnO NPs have been reported. From the X-ray diffraction data analyses it has been observed that the use of PVA caused tensile strain in the annealed samples, whereas the samples synthesized without PVA capping showed compressive strain. For an estimation of strain and sizes of the NPs, three different models namely, uniform deformation model (UDM), uniform stress deformation model (USDM) and uniform deformation energy density model (UDEDM) have been used. The photoluminescence (PL) emission characteristics of the samples have been reported and they are found to consist of a strong near band-edge UV emission, which is systematically red shifted due to annealing from 371 to 383nm for the capped and from 372 to 385nm for the uncapped samples. Such biodegradable capped nanoparticles having UV PL emission might find potential applications as biophotonic materials.

The nondestructive observation of correlation between optical characteristic and morphologic feature within capped ZnCdSe/ZnSe quantum dots system

The correlation between optical property and morphologic appearance within capped ZnCdSe/ZnSe quantum dots (QDs) system is investigated by photoluminescence (PL) spectrum measured at temperatures from 11 to 300K. The coexistent dense QDs and atypical two-dimensional non-uniform structures (neither wetting layer nor small platelet) are identified by their unique temperature evolutions of full width at half maximum (FWHM), distinct quenching rates of spectral integrated intensity and extra redshifts of emission peak energy. The divergent lateral exciton transfer modes between these two quantum structures are demonstrated by the differently morphologic-induced spectral features. The incompletely three-dimensional excitonic confinement within QDs assemblies is observed by comparing the carrier repopulation-induced redistribution of emission energy with that of isolated QDs system. In comparison with general surface detection methods of the uncapped sample, utilizing PL as spectral probe is a nondestructive technique to explore inner morphologic characteristics of capped multiple quantum structure within functional device.

Raman spectroscopy study of silicon nanoribbons on Ag(110)

The grating of self-assembled Si nanoribbons at the Ag(110) surface has been studied by Raman spectroscopy. The study, conducted in situ with uncapped samples, resulted in phonon frequencies in disagreement with the results of theoretical calculations reported in literature for freestanding silicene sheets and nanoribbons. These results suggest that the structure of these silicon nanoribbons is very different from the planar and lightly buckled structural models and that the influence of the substrate might be underestimated in the discussed structural models.

Intrinsic antiferromagnetic/insulating phase at manganite surfaces and interfaces

In this work we investigate interfacial effects in bilayer systems integrated by La2/3Sr1/3MnO3 (LSMO) thin films and different capping layers by means of surface-sensitive synchrotron radiation techniques and transport measurements. Our data reveal a complex scenario with a capping-dependent variation of the Mn oxidation state by the interface. However, irrespective of the capping material, an antiferromagnetic/insulating phase is also detected at the interface, which is likely to originate from a preferential occupancy of Mn 3d 3z2−r2 eg orbitals. This phase, which extends approximately to two unit cells, is also observed in uncapped LSMO reference samples, thus pointing to an intrinsic interfacial phase separation phenomenon, probably promoted by the structural disruption and inversion symmetry breaking at the LSMO free surface/interface. These experimental observations strongly suggest that the structural disruption, with its intrinsic inversion symmetry breaking at the LSMO interfaces, plays a major role in the observed depressed magnetotransport properties in manganite-based magnetic tunneling junctions and explains the origin of the so-called dead layer.

Surface-treated biocompatible ZnS quantum dots: Synthesis, photo-physical and microstructural properties

In the present study, the ZnS semiconductor quantum dots were successfully synthesized via an aqueous method utilizing glutathione (GSH), thioglycolic acid (TGA) and polyvinyl pyrrolidone (PVP) as capping agents. The structural, morphological and photo-physical properties and biocompatibility were investigated using comprehensive characterization techniques such as x-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), dynamic light scattering (DLS), Fourier transform infrared spectrometry (FT-IR), UV-Vis optical absorption, photoluminescence (PL) spectrometer and MTT assay. The XRD patterns showed a cubic zinc blende crystal structure and a crystallite size of about 2–3 nm using Scherrer’s equation confirmed by the electron micrographs and Effective Mass Approximation (EMA). The DLS and zeta-potential results revealed that GSH capped ZnS nanoparticles have the narrowest size distribution with an average size of 27 nm and relatively good colloidal stability. Also, the FT-IR spectrum confirmed the interaction of the capping agent groups with ZnS nanoparticles. According to the UV-Vis absorption results, optical bandgap of the spherical capped nanoparticles is higher compared to the uncapped sample and could be wider than 3.67 eV (corresponding to the bulk ZnS), which is due to the quantum confinement effect. From photoluminescence spectra, it was found that the emission becomes more intensive and shifts towards the shorter wavelengths in the presence of the capping agent. Moreover, the emission mechanism of uncapped and capped ZnS was discussed in detail. Finally, the MTT results revealed the satisfactory (>94%) biocompatibility of GSH capped ZnS quantum dots which would be a promising candidate applicable in fluorescent biological labels.

Effect of polyethylene glycol on the particle size and photoluminescence emissions characteristics of chemically synthesized ZnO nanoparticles

ZnO nanoparticles have been prepared by a simple wet chemical approach at a low temperature of 75°C by using polyethylene glycol (PEG) as capping agent. It is demonstrated that by varying the concentration of the PEG the particle size can be varied within 7.7 to 21nm. The nanostructures of the samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Optical properties are measured by UV–Visible absorption spectrophotometer and photoluminescence (PL) spectroflourimeter. It is found that the uncapped sample emits strong UV light, whereas the capped samples exhibit strong and broad visible emission. An enhancement of green PL emission intensity by the factor of ~30 has been obtained, in a capped sample with the minimum particle size and thus with more defect centers, in comparison to that of uncapped one. Also some interesting results of enhanced green PL emissions as well as co-emission of light in blue and green wavelength region have been reported by excitation wavelength dependent PL emission measurements. These results might attract the attention of the researchers to use the synthesized materials as biomarkers, if the nontoxic nature of ZnO is coupled with the photoluminescence properties reported in this work.

Influence of a stabilized cap layer on the photodegradation of coextruded high density polyethylene/wood‐flour composites

The effect of light stabilizer's addition method into wood‐plastic composites (WPCs), i.e., surface versus bulk, on their photostability was evaluated. Blends of ultraviolet absorbers (benzotriazole or hydroxyphenyltriazine) with a hindered amine light stabilizer were used as the stabilizing additives. Both unstabilized and photostabilized uncapped (control) samples, as well as coextruded WPCs counterparts, were exposed to up to 3000 h of accelerated artificial weathering. The light transmittance, surface morphology, and color of the samples before and after weathering were analyzed by UV‐vis spectroscopy, SEM, and Chroma Meter. The experimental results indicated that the method of adding the light stabilizer had a significant effect on the WPC photostability. While bulk addition reduced the degree of fading in uncapped composite, it did not suppress it completely. On the other hand, coextruded WPCs with photostabilized cap layers showed no visible signs of fading, thus clearly indicating that the stabilized cap layers blocked most of the UV radiation, and thereby prevent of UV light to reach the surface of the inner layer of coextruded composites. Cost‐analysis considerations indicated that 50 times more light stabilizer was needed when it was incorporated into the bulk of the composites rather than in the cap layer of coextruded samples. Clearly, these results suggest that adding light stabilizers at the surface of WPCs not only protects them against UV degradation, but also is a most efficient and cost‐effective method of photostabilization than bulk addition. J. VINYL ADDIT. TECHNOL., 19:239–249, 2013. © 2013 Society of Plastics Engineers

Influence of a low-temperature capping on the crystalline structure and morphology of InGaN quantum dot structures

Abstract The structure and morphology of uncapped and capped InGaN quantum dots formed by spinodal decomposition was studied by AFM, SEM, XRD, and EXAFS. As result of the spinodal decomposition, the uncapped samples show a meander structure with low Indium content which is strained to the GaN template, and large, relaxed Indium-rich islands. The thin meander structure is responsible for the quantum dot emission. A subsequently deposited low-temperature GaN cap layer forms small and nearly unstrained islands on top of the meander structure which is a sharp interface between the GaN template and the cap layer. For an InGaN cap layer deposited with similar growth parameters, a similar morphology but lower crystalline quality was observed. After deposition of a second GaN cap at a slightly higher temperature, the surface of the quantum dot structure is smooth. The large In-rich islands observed for the uncapped samples are relaxed, have a relatively low crystalline quality and a broad size distribution. They are still visible after capping with a low-temperature InGaN or GaN cap at 700 °C but dissolve after deposition of the second cap layer. The low crystalline quality of the large islands does not influence the quantum dot emission but is expected to increase the number of defects in the cap layer. This might reduce the performance of complex devices based on the stacking of several functional units.

Fabrication of Thin Graphene Layers on a Stacked 6H-SiC Surface in a Graphite Enclosure

Thin and homogeneous epitaxial graphene (EG) layers on a 6H-SiC (0001̄) substrate are fabricated and they cover the whole substrate (10 × 10 mm2). The sample surface is capped by another 6H-SiC (0001̄) wafer in a graphite enclosure to form a relatively high Si partial pressure between them, which significantly reduces the extremely high growth rate of EG. The structure and morphology of the EG layers are investigated by Raman spectroscopy, atomic force microscopy and field-emission scanning electronic microscopy. The results are compared with an uncapped sample surface, and reveal the obvious existence of ridges on the surface of the EG, and show that capping is indeed beneficial to obtain homogeneous graphene.

Site-controlled growth of InP/GaInP islands on periodic hole patterns in GaAs substrates produced by microsphere photolithography

We present microsphere photolithography in combination with wet chemical etching as a fast and low-cost method to produce regular hole arrays in a (100) GaAs surface, which are suitable for controlled nucleation of self-organized InP islands in a metal–organic vapor-phase epitaxy (MOVPE) system. A hexagonal close-packed monolayer of microspheres is used as an array of microlenses to focus UV-light on UV-sensitive photoresist. In this way, regular arrays of holes with 2μm spacing can be realized in the photoresist with controllable feature size in the sub-μm range, which are transferred to a GaAs buffer, using an isotropic etchant. These templates are used to study the site-controlled nucleation of InP islands on a Ga0.51In0.49P barrier. For this purpose the subsequent overgrowth of the templates with a GaAs buffer layer and the GaInP barrier is investigated previous to the additional deposition of the InP. The template quality is monitored during structuring and overgrowth experiments using atomic force microscopy (AFM). Preferred nucleation of InP islands inside the almost filled holes can be observed for uncapped samples. The correlation between the initial patterning and the optical signal of the InP islands is investigated by micro-photoluminescence (micro-PL) measurements. We observe site-controlled nucleation of large, but optically active, InP islands on these templates.

In-plane mapping of buried InGaAs quantum rings and hybridization effects on the electronic structure

This work reports the investigation on the structural differences between InAs quantum rings and their precursor quantum dots species as well as on the presence of piezoelectric fields and asymmetries in these nanostructures. The experimental results show significant reduction in the ring dimensions when the sizes of capped and uncapped ring and dot samples are compared. The iso-lattice parameter mapped by grazing-incidence x-ray diffraction has revealed the lateral extent of strained regions in the buried rings. A comparison between strain and composition of dot and ring structures allows inferring on how the ring formation and its final configuration may affect optical response parameters. Based on the experimental observations, a discussion has been introduced on the effective potential profile to emulate theoretically the ring-shape confinement. The effects of confinement and strain field modulation on electron and hole band structures are simulated by a multiband k.p calculation.

Dependences of photoluminescence from P-implanted epitaxial Ge

A systematic investigation has been carried out to study the influence of various annealings and implantations on the photoluminescence (PL) properties of phosphorus (P)-implanted Ge epitaxial films on Si substrate. For un-capped Ge samples, rapid thermal annealing (RTA) at 700 °C for 300 seconds yields the strongest PL emission peaked at 1550 nm. The influence of employing various capping layers (i.e., SiO(2), Si(3)N(4), and α-Si ) on the PL properties has been investigated. The capping layers are found to effectively decrease the dopant loss, leading to a significant PL enhancement. Si(3)N(4) is found to be the most efficient capping layer to prevent dopant out-diffusion and thus lead to strongest PL. Furthermore, it has been found that capping layers not only enhance the PL intensities but also make PL emission peak red- and blue- shift, depending on the stress type of the capping films. The effect of implantation dose on the PL has been also investigated.

The effect of a SiC cap on the growth of epitaxial graphene on SiC in ultra high vacuum

Thin and homogeneous graphenes with excellent thickness uniformity were produced on the carbon-rich surface of a SiC crystal using an ultra high vacuum technique. The sample surface was capped by another SiC substrate with a silicon-rich face to form a shallow cavity between them. During the graphene growth by high temperature annealing, silicon atoms sublimated from the capped sample were trapped inside the cavity between the two substrates. The confined vapor phase silicon maintains a relatively high partial pressure at the sample surface which significantly reduces the extremely high growth rate of epitaxial graphene to an easily controllable range. The structure and morphology of the graphene samples grown with this capping method are characterized by low energy electron diffraction and Raman spectroscopy and the results are compared with those of layers grown on an uncapped sample surface. The results show that capping yields much thinner graphene with excellent uniformity.

Optical and structural studies of InN/GaN dots with varying GaN cap thickness

This paper reported a structural and optical study of single InN/GaN dots with varying GaN cap thickness. The surface morphology of the 10-nm thick GaN capping layer shows a truncated pyramidal shape and changed to a dome shape when the thickness of the capping layer increases to 20-nm. The increase in compressive strain of InN dots with the increase in the cap thickness was analyzed by high resolution x-ray diffraction (HRXRD). A redshift was observed in the photoluminescence (PL) peak energy of the InN dots with increasing GaN capping thickness. The redshift of the PL peak energy of the 20-nm thick GaN capping layer sample was believed to be due to the GaN capping avoids InN decomposition and decreases the surface defect density. In addition, the maximum PL intensity of the 20-nm thick GaN capping layer sample was ∼2.5 times higher than that of the uncapped sample.

Elevated-Temperature Annealing Effects on AlGaN/GaN Heterostructures

The effect of high-temperature annealing of undoped AlGaN/GaN heterostructures on different substrates was systematically studied between 1100°C and 1230°C. An AlN spacer layer was found to add stability to structures on sapphire substrates. AlGaN/GaN heterostructures on SiC substrates demonstrated excellent robustness for the temperature range studied, maintaining their mobility, sheet resistance, and sheet concentration values, even after annealing. A silicon nitride, SiNx, capping layer was found to assist in minimizing surface roughness during annealing and maintaining the electrical characteristics of the heterostructures. AlGaN/GaN heterostructures on SiC substrates showed a 20% decrease in mobility for uncapped samples compared with SiNx-capped samples.