Unraveling electronic band structure of narrow-bandgap p-n nanojunctions in heterostructured nanowires.

The electronic band structure of complex nanostructured semiconductors has a considerable effect on the final electronic and optical properties of the material and, ultimately, on the functionality of the devices incorporating them. Valence electron energy-loss spectroscopy (VEELS) in the transmission electron microscope (TEM) provides the possibility of measuring this property of semiconductors with high spatial resolution. However, it still represents a challenge for narrow-bandgap semiconductors, since an electron beam with low energy spread is required. Here we demonstrate that by means of monochromated VEELS we can study the electronic band structure of narrow-gap materials GaSb and InAs in the form of heterostructured nanowires, with bandgap values down to 0.5 eV, especially important for newly developed structures with unknown bandgaps. Using complex heterostructured InAs-GaSb nanowires, we determine a bandgap value of 0.54 eV for wurtzite InAs. Moreover, we directly compare the bandgaps of wurtzite and zinc blende polytypes of GaSb in a single nanostructure, measured here as 0.84 and 0.75 eV, respectively. This allows us to solve an existing controversy in the band alignment between these structures arising from theoretical predictions. The findings demonstrate the potential of monochromated VEELS to provide a better understanding of the band alignment at the heterointerfaces of narrow-bandgap complex nanostructured materials with high spatial resolution. This is especially important for semiconductor device applications where even the slightest variations of the electronic band structure at the nanoscale can play a crucial role in their functionality.

The Role of Polarity in Nonplanar Semiconductor Nanostructures.

The lack of mirror symmetry in binary semiconductor compounds turns them into polar materials, where two opposite orientations of the same crystallographic direction are possible. Interestingly, their physical properties (e.g., electronic or photonic) and morphological features (e.g., shape, growth direction, and so forth) also strongly depend on the polarity. It has been observed that nanoscale materials tend to grow with a specific polarity, which can eventually be reversed for very specific growth conditions. In addition, polar-directed growth affects the defect density and topology and might induce eventually the formation of undesirable polarity inversion domains in the nanostructure, which in turn will affect the photonic and electronic final device performance. Here, we present a review on the polarity-driven growth mechanism at the nanoscale, combining our latest investigation with an overview of the available literature highlighting suitable future possibilities of polarity engineering of semiconductor nanostructures. The present study has been extended over a wide range of semiconductor compounds, covering the most commonly synthesized III-V (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb) and II-VI (ZnO, ZnTe, CdS, CdSe, CdTe) nanowires and other free-standing nanostructures (tripods, tetrapods, belts, and membranes). This systematic study allowed us to explore the parameters that may induce polarity-dependent and polarity-driven growth mechanisms, as well as the polarity-related consequences on the physical properties of the nanostructures.

Understanding semiconductor nanostructures via advanced electron microscopy and spectroscopy.

Transmission electron microscopy (TEM) offers an ample range of complementary techniques which are able to provide essential information about the physical, chemical and structural properties of materials at the atomic scale, and hence makes a vast impact on our understanding of materials science, especially in the field of semiconductor one-dimensional (1D) nanostructures. Recent advancements in TEM instrumentation, in particular aberration correction and monochromation, have enabled pioneering experiments in complex nanostructure material systems. This review aims to address these understandings through the applications of the methodology for semiconductor nanostructures. It points out various electron microscopy techniques, in particular scanning TEM (STEM) imaging and spectroscopy techniques, with their already-employed or potential applications on 1D nanostructured semiconductors. We keep the main focus of the paper on the electronic and optoelectronic properties of such semiconductors, and avoid expanding it further. In the first part of the review, we give a brief introduction to each of the STEM-based techniques, without detailed elaboration, and mention the recent technological and conceptual developments which lead to novel characterization methodologies. For further reading, we refer the audience to a handful of papers in the literature. In the second part, we highlight the recent examples of application of the STEM methodology on the 1D nanostructure semiconductor materials, especially III-V, II-V, and group IV bare and heterostructure systems. The aim is to address the research questions on various physical properties and introduce solutions by choosing the appropriate technique that can answer the questions. Potential applications will also be discussed, the ones that have already been used for bulk and 2D materials, and have shown great potential and promise for 1D nanostructure semiconductors.

Atomic-Resolution Spectrum Imaging of Semiconductor Nanowires.

Over the past decade, III-V heterostructure nanowires have attracted a surge of attention for their application in novel semiconductor devices such as tunneling field-effect transistors (TFETs). The functionality of such devices critically depends on the specific atomic arrangement at the semiconductor heterointerfaces. However, most of the currently available characterization techniques lack sufficient spatial resolution to provide local information on the atomic structure and composition of these interfaces. Atomic-resolution spectrum imaging by means of electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) is a powerful technique with the potential to resolve structure and chemical composition with sub-angstrom spatial resolution and to provide localized information about the physical properties of the material at the atomic scale. Here, we demonstrate the use of atomic-resolution EELS to understand the interface atomic arrangement in three-dimensional heterostructures in semiconductor nanowires. We observed that the radial interfaces of GaSb-InAs heterostructure nanowires are atomically abrupt, while the axial interface in contrast consists of an interfacial region where intermixing of the two compounds occurs over an extended spatial region. The local atomic configuration affects the band alignment at the interface and, hence, the charge transport properties of devices such as GaSb-InAs nanowire TFETs. STEM-EELS thus represents a very promising technique for understanding nanowire physical properties, such as differing electrical behavior across the radial and axial heterointerfaces of GaSb-InAs nanowires for TFET applications.

Kinetic Engineering of Wurtzite and Zinc-Blende AlSb Shells on InAs Nanowires.

Using AlSb as the model system, we demonstrate that kinetic limitations can lead to the preferential growth of wurtzite (WZ) AlSb shells rather than the thermodynamically stable zinc-blende (ZB) AlSb and that the WZ and ZB relative thickness can be tuned by a careful control of the deposition parameters. We report selective heteroepitaxial radial growth of AlSb deposited by metal-organic vapor phase epitaxy (MOVPE) on InAs nanowire core templates with engineered lengths of axial WZ and ZB segments. AlSb shell thickness, crystal phase, nanostructure, and composition are investigated as a function of the shell growth temperature, Ts, using scanning electron microscopy, transmission electron microscopy, electron tomography, and energy-dispersive X-ray spectroscopy. We find that ZB- and WZ-structured AlSb shells grow heteroepitaxially around the ZB and WZ segments of the InAs core, respectively. Surprisingly, at 390 < Ts < 450 °C, the WZ-AlSb shells are thicker than the ZB-AlSb shells, and their thickness increases with decreasing Ts. In comparison, the ZB-AlSb shell thicknesses increase slightly with increasing Ts. We find that the increased thickness of the WZ-AlSb shells is due to the formation and enhanced deposition on {112̅0} facets rather than on the more commonly grown {101̅0} sidewall facets. Overall, these results, which are in direct contrast with previous reports suggesting that heteroepitaxial radial growth of III-antimonides is always favored on the ZB-structure facets, indicate that the growth of WZ-AlSb is preferred over the thermodynamically stable ZB-AlSb at lower growth temperatures. We attribute this behavior to kinetic limitations of MOVPE of AlSb on ZB and WZ phases of InAs.

Sb Incorporation in Wurtzite and Zinc Blende InAs1-x Sbx Branches on InAs Template Nanowires.

The physical properties of material largely depend on their crystal structure. Nanowire growth is an important method for attaining metastable crystal structures in III-V semiconductors, giving access to advantageous electronic and surface properties. Antimonides are an exception, as growing metastable wurtzite structure has proven to be challenging. As a result, the properties of these materials remain unknown. One promising means of accessing wurtzite antimonides is to use a wurtzite template to facilitate their growth. Here, a template technique using branched nanowire growth for realizing wurtzite antimonide material is demonstrated. On wurtzite InAs trunks, InAs1-x Sbx branch nanowires at different Sb vapor phase compositions are grown. For comparison, branches on zinc blende nanowire trunks are also grown under identical conditions. Studying the crystal structure and the material composition of the grown branches at different xv shows that the Sb incorporation is higher in zinc blende than in wurtzite. Branches grown on wurtzite trunks are usually correlated with stacking defects in the trunk, leading to the emergence of a zinc blende segment of higher Sb content growing parallel to the wurtzite structure within a branch. However, the average amount of Sb incorporated within the branch is determined by the vapor phase composition.

Characterization of individual stacking faults in a wurtzite GaAs nanowire by nanobeam X-ray diffraction.

Coherent X-ray diffraction was used to measure the type, quantity and the relative distances between stacking faults along the growth direction of two individual wurtzite GaAs nanowires grown by metalorganic vapour epitaxy. The presented approach is based on the general property of the Patterson function, which is the autocorrelation of the electron density as well as the Fourier transformation of the diffracted intensity distribution of an object. Partial Patterson functions were extracted from the diffracted intensity measured along the [000\bar{1}] direction in the vicinity of the wurtzite 00\bar{1}\bar{5} Bragg peak. The maxima of the Patterson function encode both the distances between the fault planes and the type of the fault planes with the sensitivity of a single atomic bilayer. The positions of the fault planes are deduced from the positions and shapes of the maxima of the Patterson function and they are in excellent agreement with the positions found with transmission electron microscopy of the same nanowire.

Direct nucleation, morphology and compositional tuning of InAs1-x Sb x nanowires on InAs (111) B substrates.

III-V ternary nanowires are interesting due to the possibility of modulating their physical and material properties by tuning their material composition. Amongst them InAs1-x Sb x nanowires are good candidates for applications such as Infrared detectors. However, this material has not been grown directly from substrates, in a large range of material compositions. Since the properties of ternaries are alterable by tuning their composition, it is beneficial to gain access to a wide range of composition tunability. Here we demonstrate direct nucleation and growth of InAs1-x Sb x nanowires from Au seed particles over a broad range of compositions (x = 0.08-0.75) for different diameters and surface densities by means of metalorganic vapor phase epitaxy. We investigate how the nucleation, morphology, solid phase Sb content, and growth rate of these nanowires depend on the particle dimensions, and on growth conditions such as the vapor phase composition, V/III ratio, and temperature. We show that the solid phase Sb content of the nanowires remains invariant towards changes of the In precursor flow. We also discuss that at relatively high In flows the growth mechanism alters from Au-seeded to what is referred to as semi In-seeded growth. This change enables growth of nanowires with a high solid phase Sb content of 0.75 that are not feasible via Au-seeded growth. Independent of the growth conditions and morphology, we report that the nanowire Sb content changes over their length, from lower Sb contents at the base, increasing to higher amounts towards the tip. We correlate the axial Sb content variations to the axial growth rate measured in situ. We also report spontaneous core-shell formation for Au-seeded nanowires, where the core is Sb-rich in comparison to the Sb-poor shell.

Demonstration of Sn-seeded GaSb homo- and GaAs-GaSb heterostructural nanowires.

The particle-assisted epitaxial growth of antimonide-based nanowires has mainly been realized using gold as the seed material. However, the Au-seeded epitaxial growth of antimonide-based nanowires such as GaSb nanowires presents several challenges such as for example direct nucleation issues and crystal structure tuning. Therefore, it is of great importance to understand the role of seed material choice and properties in the growth behavior of antimonide-based nanowires to obtain a deeper understanding and a better control on their formation processes. In this report, we have investigated the epitaxial growth of GaSb and GaAs-GaSb nanowires using in situ-formed tin seeds by means of metalorganic vapor phase epitaxy technique. This comprehensive report covers the growth of in situ-formed tin seeds and Sn-seeded GaSb nanowires on both GaAs and GaSb (111)B substrates, as well as GaAs-GaSb nanowires on GaAs (111)B substrates. The growth behavior and structural properties of the obtained GaSb nanowires are further investigated and compared with the Au-seeded counterparts. The results provided by this study demonstrate that Sn is a promising seed material for the growth of GaSb nanowires.

Cubic versus hexagonal - phase, size and morphology effects on the photoluminescence quantum yield of NaGdF4:Er3+/Yb3+ upconverting nanoparticles.

Upconverting nanoparticles (UCNPs) are well-known for their capacity to convert near-infrared light into UV/visible light, benefitting various applications where light triggering is required. At the nanoscale, loss of luminescence intensity is observed and thus, a decrease in photoluminescence quantum yield (PLQY), usually ascribed to surface quenching. We evaluate this by measuring the PLQY of NaGdF4:Er3+,Yb3+ UCNPs as a function of size (ca. 15 to 100 nm) and shape (spheres, cubes, hexagons). Our results show that the PLQY of α-phase NaGdF4 Er3+,Yb3+ surpasses that of ß-NaGdF4 for sizes below 20 nm, an observation related to distortion of the crystal lattice when the UCNPs become smaller. The present study also underlines that particle shape must not be neglected as a relevant parameter for PLQY. In fact, based on a mathematical nucleus/hull volumetric model, shape was found to be particularly relevant in the 20 to 60 nm size range of the investigated UCNPs.

Towards defect-free thin films of the earth-abundant absorber zinc phosphide by nanopatterning.

Large-scale deployment of thin-film photovoltaics will be facilitated through earth-abundant components. Herein, selective area epitaxy and lateral overgrowth epitaxy are explored for the growth of zinc phosphide (Zn3P2), a promising earth-abundant absorber. The ideal growth conditions are elucidated, and the nucleation of single-crystal nanopyramids that subsequently evolve towards coalesced thin-films is demonstrated. The zinc phosphide pyramids exhibit room temperature bandgap luminescence at 1.53 eV, indicating a high-quality material. The electrical properties of zinc phosphide and the junction with the substrate are assessed by conductive atomic force microscopy on n-type, p-type and intrinsic substrates. The measurements are consistent with the p-type characteristic of zinc phosphide. Overall, this constitutes a new, and transferrable, approach for the controlled and tunable growth of high-quality zinc phosphide, a step forward in the quest for earth-abundant photovoltaics.

Silica-copper catalyst interfaces enable carbon-carbon coupling towards ethylene electrosynthesis.

Membrane electrode assembly (MEA) electrolyzers offer a means to scale up CO2-to-ethylene electroconversion using renewable electricity and close the anthropogenic carbon cycle. To date, excessive CO2 coverage at the catalyst surface with limited active sites in MEA systems interferes with the carbon-carbon coupling reaction, diminishing ethylene production. With the aid of density functional theory calculations and spectroscopic analysis, here we report an oxide modulation strategy in which we introduce silica on Cu to create active Cu-SiOx interface sites, decreasing the formation energies of OCOH* and OCCOH*-key intermediates along the pathway to ethylene formation. We then synthesize the Cu-SiOx catalysts using one-pot coprecipitation and integrate the catalyst in a MEA electrolyzer. By tuning the CO2 concentration, the Cu-SiOx catalyst based MEA electrolyzer shows high ethylene Faradaic efficiencies of up to 65% at high ethylene current densities of up to 215 mA cm-2; and features sustained operation over 50 h.

Heterotwin Zn3P2 superlattice nanowires: the role of indium insertion in the superlattice formation mechanism and their optical properties.

Zinc phosphide (Zn3P2) nanowires constitute prospective building blocks for next generation solar cells due to the combination of suitable optoelectronic properties and an abundance of the constituting elements in the Earth's crust. The generation of periodic superstructures along the nanowire axis could provide an additional mechanism to tune their functional properties. Here we present the vapour-liquid-solid growth of zinc phosphide superlattices driven by periodic heterotwins. This uncommon planar defect involves the exchange of Zn by In at the twinning boundary. We find that the zigzag superlattice formation is driven by reduction of the total surface energy of the liquid droplet. The chemical variation across the heterotwin does not affect the homogeneity of the optical properties, as measured by cathodoluminescence. The basic understanding provided here brings new propsects on the use of II-V semiconductors in nanowire technology.

Polarity and growth directions in Sn-seeded GaSb nanowires.

We here investigate the growth mechanism of Sn-seeded GaSb nanowires and demonstrate how the seed particle and its dynamics at the growth interface of the nanowire determine the polarity, as well as the formation of structural defects. We use aberration-corrected scanning transmission electron microscopy imaging methodologies to study the interrelationship between the structural properties, i.e. polarity, growth mechanism, and formation of inclined twin boundaries in pairs. Moreover, the optical properties of the Sn-seeded GaSb nanowires are examined. Their photoluminescence response is compared with one of their Au-seeded counterparts, suggesting the incorporation of Sn atoms from the seed particles into the nanowires.

p-GaN/n-ZnO heterojunction nanowires: optoelectronic properties and the role of interface polarity.

In this work, simulations of the electronic band structure of a p-GaN/n-ZnO heterointerface are presented. In contrast to homojunctions, an additional energy barrier due to the type-II band alignment hinders the flow of majority charge carriers in this heterojunction. Spontaneous polarization and piezoelectricity are shown to additionally affect the band structure and the location of the recombination region. Proposed as potential UV-LEDs and laser diodes, p-GaN/n-ZnO heterojunction nanowires were fabricated by plasma-assisted molecular beam epitaxy (PAMBE). Atomic resolution annular bright field scanning transmission electron microscopy (STEM) studies reveal an abrupt and defect-free heterointerface with a polarity inversion from N-polar GaN to Zn-polar ZnO. Photoluminescence measurements show strong excitonic UV emission originating from the ZnO-side of the interface as well as stimulated emission in the case of optical pumping above a threshold of 55 kW/cm(2).

Polarity-driven polytypic branching in cu-based quaternary chalcogenide nanostructures.

An appropriate way of realizing property nanoengineering in complex quaternary chalcogenide nanocrystals is presented for Cu2CdxSnSey(CCTSe) polypods. The pivotal role of the polarity in determining morphology, growth, and the polytypic branching mechanism is demonstrated. Polarity is considered to be responsible for the formation of an initial seed that takes the form of a tetrahedron with four cation-polar facets. Size and shape confinement of the intermediate pentatetrahedral seed is also attributed to polarity, as their external facets are anion-polar. The final polypod extensions also branch out as a result of a cation-polarity-driven mechanism. Aberration-corrected scanning transmission electron microscopy is used to identify stannite cation ordering, while ab initio studies are used to show the influence of cation ordering/distortion, stoichiometry, and polytypic structural change on the electronic band structure.

Enhanced photovoltaic performance of nanowire dye-sensitized solar cells based on coaxial TiO2@TiO heterostructures with a cobalt(II/III) redox electrolyte.

The growth of a TiO shell at the surface of TiO2 nanowires (NWs) allowed us to improve the power conversion efficiency of NW-based dye-sensitized solar cells (DSCs) by a factor 2.5. TiO2@TiO core-shell NWs were obtained by a two-step process: First, rutile-phase TiO2 NWs were hydrothermally grown. Second, a hongquiite-phase TiO shell was electrochemically deposited at the surface of the TiO2 NWs. Bare TiO2 and heterojunction TiO2@TiO NW-based DSCs were obtained using a cobalt(II/III) redox electrolyte and LEG4 as the dye. With this electrolyte/dye combination, DSCs with outstanding Voc values above 900 mV were systematically obtained. While TiO2@TiO NW-based DSCs had slightly lower Voc values than bare TiO2 NW-based DSCs, they provided 3-fold higher photocurrents, overall reaching 2.5-fold higher power conversion efficiencies. The higher photocurrents were associated with the larger surface roughness and an enhanced charge-carrier separation/transfer at the NW/dye interface.