Strain Mapping of Single Nanowires using Nano X-ray Diffraction

Nanowires are explored as basic components for a large range of electronic devices. The nanowire format offersseveral benefits, including reduced material consumption and increased potential for combining materials to formnew novel heterostructures. Several factors, such as mechanical stress from contacting or a lattice mismatch in aheterostructure, can strain and change the lattice tilt. The strain is often intertwined with small gradients ofcomposition. The strain relaxation can differ significantly from bulk due to the small diameters, but the mechanismsare not fully comprehended. X-rays have a penetrating power that makes it possible to investigate embeddedsamples without preparation or slicing. The high flux of coherent X-ray beams from synchrotron radiation facilities,combined with the nano-focus capabilities developed in recent years, have made it possible to probe nano-crystals.The 4th generation of synchrotrons, including MAX IV in Lund, Sweden, has even higher brilliance than previoussources. Diffraction imaging techniques using synchrotron radiation can reveal small strains down to 10-4-10-5. Thefield of coherent imaging pushes the limits of resolutions below the size of the focus. With Bragg ptychography, thedisplacement field in a crystal can be probed with resolution beyond the probe focus by numerically reconstructingthe phase.This thesis includes the development of X-ray nano-diffraction methods for the characterizing of nanowires, includingGaInP/InP barcode nanowires, p-i-n InP nanowire devices and metal halide perovskite CsPbBr3 nanowires. Itincludes a theoretical background of the scattering mechanisms in Thomson scattering in nano-crystals, goesthrough the formalism for coherent diffraction imaging, crystal structure and deformation in nanoobjects and thetechnical aspects of the experimental setup and measurement. Moreover, theoretical modelling of elastic strainrelaxation in these nanowires was performed with finite element modelling.Single III-V nanowire heterostructures and III-V nanowire devices were probed with scanning XRD and Braggprojection ptychography (BPP). How the techniques compare to each other and how the results are affected by thedifferent approximations that are made in the respective technique was explored. Finite element simulationscombined with nano-diffraction revealed that the lattice mismatch of 1.5% could be relaxed elastically for thediameter of 180 nm. From the strain mapping of the nanowire device, we found how the contacting of the nanowirebends the nanowire resulting in a tilt normal to the substrate.Single perovskite metal-halide perovskite CsPb(Br(1-x)Clx)3 nanowire heterostructures were characterized withscanning nano-XRD and XRF, which showed that the lattice spacing was affected by composition and strain.Composition gradients revealed that Cl diffusion had taken place within the heterostructure. Furthermore, extractingthe lattice tilts from shifts of the Bragg peak revealed a ferroelastic domain structure with simultaneously existinglattice tilts. These findings are beneficial for the further development of MHP nanowires devices

Thermopower simulation of a two level spinless quantum dot

Quantum dots are interesting candidates for a broad variety of electronic components, with single electron transistors and LEDs being two examples already well on their way. In nanostructures, such as quantum dots, quantum effects greatly influence the transport. In a spin polarized quantum dot system with two energy levels, interference effects have been found to cause a strong suppression of conductance [Phys. Rev. Lett. 104, 186804 (2010)]. In the present work, this system is further investigated with thermopower acting as probing tool. Thermopower is a measure of the voltage induced by a temperature difference, attributed to the Seebeck effect, at vanishing current. While conductance probes transport at and around the Fermi level, thermopower does so for a wider range of energies. For the system addressed in this work, thermopower is evaluated as a probing tool complementary to conduction. To simulate transport, a generalized master equation approach is used; the second order von Neumann approach. This method takes into account second order tunneling as well as interference effects; coherence and correlations. The simulations show that the conductance suppression manifest itself also in the thermopower and furthermore, with a more prominent signal

Coherent Bragg imaging of 60 nm Au nanoparticles under electrochemical control at the NanoMAX beamline

Nanoparticles are essential electrocatalysts in chemical production, water treatment and energy conversion, but engineering efficient and specific catalysts requires understanding complex structure–reactivity relations. Recent experiments have shown that Bragg coherent diffraction imaging might be a powerful tool in this regard. The technique provides three-dimensional lattice strain fields from which surface reactivity maps can be inferred. However, all experiments published so far have investigated particles an order of magnitude larger than those used in practical applications. Studying smaller particles quickly becomes demanding as the diffracted intensity falls. Here, in situ nanodiffraction data from 60 nm Au nanoparticles under electrochemical control collected at the hard X-ray nanoprobe beamline of MAX IV, NanoMAX, are presented. Two-dimensional image reconstructions of these particles are produced, and it is estimated that NanoMAX, which is now open for general users, has the requisites for three-dimensional imaging of particles of a size relevant for catalytic applications. This represents the first demonstration of coherent X-ray diffraction experiments performed at a diffraction-limited storage ring, and illustrates the importance of these new sources for experiments where coherence properties become crucial.This work was supported by the AForsk Foundation through grant 17-408. JS-G acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante (UATALENTO16-02). The MAX IV Laboratory receives funding through the Swedish Research Council under grant no 2013-02235

The Flow and Pressure Relationships in Different Tubes Commonly Used for Semi-occluded Vocal Tract Exercises

This experimental study investigated the back pressure (pback) versus flow (U) relationship for 10 different tubes commonly used for semi-occluded vocal tract exercises (SOVTE), i.e., 8 straws of different lengths and diameters, a resonance tube and a silicone tube similar to a Lax Vox tube. All tubes were assessed with the free end in air. The resonance tube and silicone tube were further assessed with the free end under water at the depths from 1 to 7 cm in steps of 1 cm. The results showed that relative changes in the diameter of straws affect pback considerably more compared to the same amount of relative change in length. Additionally, once tubes are submerged into water, pback needs to overcome the pressure generated by the water depth before flow can start. Under this condition, only a small increase in pback was observed as the flow was increased. Therefore, the wider tubes submerged into water produced an almost constant pback determined by the water depth, while the thinner straws in air produced relatively large changes to pback as flow was changed. These differences may be taken advantage of when customizing exercises for different users and diagnoses and optimizing the therapy outcome

Combining Nanofocused X-Rays with Electrical Measurements at the NanoMAX Beamline

The advent of nanofocused X-ray beams has allowed the study of single nanocrystals and complete nanoscale devices in a nondestructive manner, using techniques such as scanning transmission X-ray microscopy (STXM), X-ray fluorescence (XRF) and X-ray diffraction (XRD). Further insight into semiconductor devices can be achieved by combining these techniques with simultaneous electrical measurements. Here, we present a system for electrical biasing and current measurement of single nanostructure devices, which has been developed for the NanoMAX beamline at the fourth-generation synchrotron, MAX IV, Sweden. The system was tested on single InP nanowire devices. The mechanical stability was sufficient to collect scanning XRD and XRF maps with a 50 nm diameter focus. The dark noise of the current measurement system was about 3 fA, which allowed fly scan measurements of X-ray beam induced current (XBIC) in single nanowire devices

Resonance tube phonation in water – the effect of tube diameter and water depth on back pressure and bubble characteristics at different airflows

Objectives: Resonance tube phonation with tube end in water is a voice therapy method in which the patient phonates through a glass tube, keeping the free end of the tube submerged in water, creating bubbles. The purpose of this experimental study was to determine flow-pressure relationship, flow thresholds between bubble types, and bubble frequency as a function of flow and back volume. Methods: A flow-driven vocal tract simulator was used for recording the back pressure produced by resonance tubes with inner diameters of 8 and 9 mm submerged at water depths of 0–7 cm. Visual inspection of bubble types through video recording was also performed. Results: The static back pressure was largely determined by the water depth. The narrower tube provided a slightly higher back pressure for a given flow and depth. The amplitude of the pressure oscillations increased with flow and depth. Depending on flow, the bubbles were emitted from the tube in three distinct types with increasing flow: one by one, pairwise, and in a chaotic manner. The bubble frequency was slightly higher for the narrower tube. An increase in back volume led to a decrease in bubble frequency. Conclusions: This study provides data on the physical properties of resonance tube phonation with the tube end in water. This information will be useful in future research when looking into the possible effects of this type of voice training

Nanoscale mapping of carrier collection in single nanowire solar cells using X-ray beam induced current

Here it is demonstrated how nanofocused X-ray beam induced current (XBIC) can be used to quantitatively map the spatially dependent carrier collection probability within nanostructured solar cells. The photocurrent generated by a 50 nm-diameter X-ray beam was measured as a function of position, bias and flux in single p–i–n doped solar-cell nanowires. The signal gathered mostly from the middle segment decays exponentially toward the p- and n-segments, with a characteristic decay length that varies between 50 nm and 750 nm depending on the flux and the applied bias. The amplitude of the XBIC shows saturation at reverse bias, which indicates that most carriers are collected. At forward bias, the relevant condition for solar cells, the carrier collection is only efficient in a small region. Comparison with finite element modeling suggests that this is due to unintentional p-doping in the middle segment. It is expected that nanofocused XBIC could be used to investigate carrier collection in a wide range of nanostructured solar cells

Nanobeam X-ray Fluorescence Dopant Mapping Reveals Dynamics of in Situ Zn-Doping in Nanowires

The properties of semiconductors can be controlled using doping, making it essential for electronic and optoelectronic devices. However, with shrinking device sizes it becomes increasingly difficult to quantify doping with sufficient sensitivity and spatial resolution. Here, we demonstrate how X-ray fluorescence mapping with a nanofocused beam, nano-XRF, can quantify Zn doping within in situ doped III-V nanowires, by using large area detectors and high-efficiency focusing optics. The spatial resolution is defined by the focus size to 50 nm. The detection limit of 7 ppm (2.8 × 1017 cm-3), corresponding to about 150 Zn atoms in the probed volume, is bound by a background signal. In solar cell InP nanowires with a p-i-n doping profile, we use nano-XRF to observe an unintentional Zn doping of 5 × 1017 cm-3 in the middle segment. We investigated the dynamics of in situ Zn doping in a dedicated multisegment nanowire, revealing significantly sharper gradients after turning the Zn source off than after turning the source on. Nano-XRF could be used for quantitative mapping of a wide range of dopants in many types of nanostructures

High resolution strain mapping of a single axially heterostructured nanowire using scanning X-ray diffraction

Axially heterostructured nanowires are a promising platform for next generation electronic and optoelectronic devices. Reports based on theoretical modeling have predicted more complex strain distributions and increased critical layer thicknesses than in thin films, due to lateral strain relaxation at the surface, but the understanding of the growth and strain distributions in these complex structures is hampered by the lack of high-resolution characterization techniques. Here, we demonstrate strain mapping of an axially segmented GaInP-InP 190 nm diameter nanowire heterostructure using scanning X-ray diffraction. We systematically investigate the strain distribution and lattice tilt in three different segment lengths from 45 to 170 nm, obtaining strain maps with about 10−4 relative strain sensitivity. The experiments were performed using the 90 nm diameter nanofocus at the NanoMAX beamline, taking advantage of the high coherent flux from the first diffraction limited storage ring MAX IV. The experimental results are in good agreement with a full simulation of the experiment based on a three-dimensional (3D) finite element model. The largest segments show a complex profile, where the lateral strain relaxation at the surface leads to a dome-shaped strain distribution from the mismatched interfaces, and a change from tensile to compressive strain within a single segment. The lattice tilt maps show a cross-shaped profile with excellent qualitative and quantitative agreement with the simulations. In contrast, the shortest measured InP segment is almost fully adapted to the surrounding GaInP segments. [Figure not available: see fulltext.]