Core-shell Nanowire Heterostructures Research Articles

Enhancing and broadening the photoresponse of CdS nanowire by constructing core–shell heterostructure

Limited by the severe surface recombination and large bandgap, it is still a challenge to achieve a high-performance broad-spectrum photodetector based on CdS nanowires (NWs). In this work, the CdS/GeS core–shell heterostructure is constructed to enhance and broaden the photoresponse of CdS NWs. The CdS/GeS core–shell heterostructure NWs are prepared by the chemical vapor deposition method, exhibiting smooth surfaces, controlled shell thicknesses, and compositions. From the UV-Vis-near-infrared (NIR) diffuse reflectance spectrum, PL, and time-resolved photoluminescence studies, it is found that the surface recombination is weakened and the light absorption range is widened after the constructing core–shell heterostructure. When configured into photodetectors, the responsivity of the CdS/GeS core–shell heterostructure NWs is up to 76.8 A W−1, which is 10 folds higher than that of pristine CdS NWs. Furthermore, the photoresponse wavelength of the CdS/GeS core–shell heterostructure NWs is extended from 405 to 850 nm. The improved photodetection performance is attributed to the effective separation of photogenerated carriers at the heterostructure interface, weakened surface recombination, and excellent light absorption of the GeS shell. All results imply that constructing core–shell heterostructures is an effective strategy for constructing high-performance photodetectors.

Enhancing and broadening the photoresponse of CdS nanowire by constructing core–shell heterostructure

Limited by the severe surface recombination and large bandgap, it is still a challenge to achieve a high-performance broad-spectrum photodetector based on CdS nanowires (NWs). In this work, the CdS/GeS core–shell heterostructure is constructed to enhance and broaden the photoresponse of CdS NWs. The CdS/GeS core–shell heterostructure NWs are prepared by the chemical vapor deposition method, exhibiting smooth surfaces, controlled shell thicknesses, and compositions. From the UV-Vis-near-infrared (NIR) diffuse reflectance spectrum, PL, and time-resolved photoluminescence studies, it is found that the surface recombination is weakened and the light absorption range is widened after the constructing core–shell heterostructure. When configured into photodetectors, the responsivity of the CdS/GeS core–shell heterostructure NWs is up to 76.8 A W−1, which is 10 folds higher than that of pristine CdS NWs. Furthermore, the photoresponse wavelength of the CdS/GeS core–shell heterostructure NWs is extended from 405 to 850 nm. The improved photodetection performance is attributed to the effective separation of photogenerated carriers at the heterostructure interface, weakened surface recombination, and excellent light absorption of the GeS shell. All results imply that constructing core–shell heterostructures is an effective strategy for constructing high-performance photodetectors.

Lattice-mismatch-free construction of III-V/chalcogenide core-shell heterostructure nanowires

Growing high-quality core-shell heterostructure nanowires is still challenging due to the lattice mismatch issue at the radial interface. Herein, a versatile strategy is exploited for the lattice-mismatch-free construction of III-V/chalcogenide core-shell heterostructure nanowires by simply utilizing the surfactant and amorphous natures of chalcogenide semiconductors. Specifically, a variety of III-V/chalcogenide core-shell heterostructure nanowires are successfully constructed with controlled shell thicknesses, compositions, and smooth surfaces. Due to the conformal properties of obtained heterostructure nanowires, the wavelength-dependent bi-directional photoresponse and visible light-assisted infrared photodetection are realized in the type-I GaSb/GeS core-shell heterostructure nanowires. Also, the enhanced infrared photodetection is found in the type-II InGaAs/GeS core-shell heterostructure nanowires compared with the pristine InGaAs nanowires, in which both responsivity and detectivity are improved by more than 2 orders of magnitude. Evidently, this work paves the way for the lattice-mismatch-free construction of core-shell heterostructure nanowires by chemical vapor deposition for next-generation high-performance nanowire optoelectronics.

Shape and Composition Evolution in an Alloy Core-Shell Nanowire Heterostructure Induced by Adatom Diffusion.

A core-shell nanowire heterostructure is an important building block for nanowire-based optoelectronic devices. In this paper, the shape and composition evolution induced by adatom diffusion is investigated by constructing a growth model for alloy core-shell nanowire heterostructures, taking diffusion, adsorption, desorption and incorporation of adatoms into consideration. With moving boundaries accounting for sidewall growth, the transient diffusion equations are numerically solved by the finite element method. The adatom diffusions introduce the position-dependent and time-dependent adatom concentrations of components A and B. The newly grown alloy nanowire shell depends on the incorporation rates, resulting in both shape and composition evolution during growth. The results show that the morphology of nanowire shell strongly depends on the flux impingement angle. With the increase in this impingement angle, the position of the largest shell thickness on sidewall moves down to the bottom of nanowire and meanwhile, the contact angle between shell and substrate increases to an obtuse angle. Coupled with the shell shapes, the composition profiles are shown as non-uniform along both the nanowire and the shell growth directions, which can be attributed to the adatom diffusion of components A and B. The impacts of parameters on the shape and composition evolution are systematically investigated, including diffusion length, adatom lifetime and corresponding ratios between components. This kinetic model is expected to interpret the contribution of adatom diffusion in growing alloy group-IV and group III-V core-shell nanowire heterostructures.

Hydrothermally grown SnS2/Si nanowire core-shell heterostructure photodetector with excellent optoelectronic performances

Core-shell nanowire heterostructure is a new architecture for photodetector application with enlarged active surface area enhancing light absorption and photodetector performance. As an emanating coating material, SnS2 has a growing interest in next-generation optoelectronic materials. Here, we reported the enhanced optoelectronic performance of the hydrothermally grown SnS2 and Si nanowire (SiNWs) core–shell heterostructure. Hydrothermally grown SnS2 on Si nanowire creates a uniform coating over the entire surface of nanowires which enhances the heterostructure’s effective junction area and improves optoelectronic performance over the broad spectral range (300 – 1100 nm). Specially, under 340 nm illumination, the core–shell photodetector exhibits large responsivity (∼383 A/W) and extremely high external quantum efficiency (∼2 × 105 %) at very low optical power (∼20 nW/mm2). This SnS2/SiNWs core–shell heterostructure with significantly improved optoelectronic performance will be favourable for the development of photodetector with an ability to work with extremely high efficiency.

Electron-hole coherence in core-shell nanowires with partial proximity-induced superconductivity

By solving the Bogoliubov--de Gennes Hamiltonian, the electron-hole coherence within a partially proximitized $n$-doped semiconductor shell of a core-shell nanowire heterostructure is investigated numerically and compared with the Andreev reflection interpretation of proximity induced superconductivity. Partial proximitization is considered to quantify the effects of a reduced coherence length. Three cases of partial proximitization of the shell are explored: radial, angular, and longitudinal. For the radial case, it is found that the boundary conditions impose localization probability maxima in the center of the shell in spite of off-center radial proximitization. The induced superconductivity gap is calculated as a function of the ratio between the proximitized shell thickness and the total shell thickness. In the angular case, the lowest-energy state of a hexagonal wire with a single proximitized side is found to display the essence of Andreev reflection, only by lengthwise summation of the localization probability. In the longitudinal case, a clear correspondence with Andreev reflection is seen in the localization probability as a function of length along a half-proximitized wire. The effect of an external magnetic field oriented along the wire is explored.

Extended-SWIR Photodetection in All-Group IV Core/Shell Nanowires

Group IV Ge1-xSnx semiconductors hold the premise of enabling broadband silicon-integrated infrared optoelectronics due to their tunable bandgap energy and directness. Herein, we exploit these attributes along with the enhanced lattice strain relaxation in Ge/Ge0.92Sn0.08 core-shell nanowire heterostructures to implement highly responsive, room-temperature short-wave infrared nanoscale photodetectors. Atomic-level studies confirm the uniform shell composition and its higher crystallinity with respect to thin films counterparts. The demonstrated Ge/Ge0.92Sn0.08 p-type field-effect nanowire transistors exhibit superior optoelectronic properties achieving simultaneously a relatively high mobility, a high ON/OFF ratio, and a high responsivity, in addition to a broadband absorption in the short-wave infrared range. Indeed, the reduced bandgap of the Ge0.92Sn0.08 shell yields an extended cutoff wavelength of 2.1 um, with a room-temperature responsivity reaching 2.7 A/W at 1550 nm. These results highlight the potential of Ge/Ge1-xSnx core/shell nanowires as silicon-compatible building blocks for nanoscale integrated infrared photonics.

Water-Selective Nanostructured Dehumidifiers for Molecular Sensing Spaces.

Humidity and moisture effects, frequently called water poisoning, in surroundings are inevitable for various molecular sensing devices, strongly affecting their sensing characteristics. Here, we demonstrate a water-selective nanostructured dehumidifier composed of ZnO/TiO2/CaCl2 core-shell heterostructured nanowires for molecular sensing spaces. The fabricated nanostructured dehumidifier is highly water-selective without detrimental adsorptions of various volatile organic compound molecules and can be repeatedly operated. The thermally controllable and reversible dehydration process of CaCl2·nH2O thin nanolayers on hydrophilic ZnO/TiO2 nanowire surfaces plays a vital role in such water-selective and repeatable dehumidifying operations. Furthermore, the limitation of detection for sensing acetone and nonanal molecules in the presence of moisture (relative humidity ∼ 90%) was improved more than 20 times using nanocomposite sensors by operating the developed nanostructured dehumidifier. Thus, the proposed water-selective nanostructured dehumidifier offers a rational strategy and platform to overcome water poisoning issues for various molecular and gas sensors.

Optimization of the Sb2S3 Shell Thickness in ZnO Nanowire-Based Extremely Thin Absorber Solar Cells.

Extremely thin absorber (ETA) solar cells made of ZnO/TiO2/Sb2S3 core–shell nanowire heterostructures, using P3HT as the hole-transporting material (HTM), are of high interest to surpass solar cell efficiencies of their planar counterpart at lower material cost. However, no dimensional optimization has been addressed in detail, as it raises material and technological critical issues. In this study, the thickness of the Sb2S3 shell grown by chemical spray pyrolysis is tuned from a couple of nanometers to several tens of nanometers, while switching from a partially to a fully crystallized shell. The Sb2S3 shell is highly pure, and the unwanted Sb2O3 phase was not formed. The low end of the thickness is limited by challenges in the crystallization of the Sb2S3 shell, as it is amorphous at nanoscale dimensions, resulting in the low optical absorption of visible photons. In contrast, the high end of the thickness is limited by the increased density of defects in the bulk of the Sb2S3 shell, degrading charge carrier dynamics, and by the incomplete immersion of the P3HT in the structure, resulting in the poor hole collection. The best ETA solar cell with a short-circuit current density of 12.1 mA/cm2, an open-circuit voltage of 502 mV, and a photovoltaic conversion efficiency of 2.83% is obtained for an intermediate thickness of the Sb2S3 shell. These findings highlight that the incorporation of both the absorber shell and HTM in the core–shell heterostructures relies on the spacing between individual nanowires. They further elaborate the intricate nature of the dimensional optimization of an ETA cell, as it requires a fine-balanced holistic approach to correlate all the dimensions of all the components in the heterostructures.

Drastic Effect of Sequential Deposition Resulting from Flux Directionality on the Luminescence Efficiency of Nanowire Shells.

Core-shell nanowire heterostructures form the basis for many innovative devices. When compound nanowire shells are grown by directional deposition techniques, the azimuthal position of the sources for the different constituents in the growth reactor, substrate rotation, and nanowire self-shadowing inevitably lead to sequential deposition. Here, we uncover for In0.15Ga0.85As/GaAs shell quantum wells grown by molecular beam epitaxy a drastic impact of this sequentiality on the luminescence efficiency. The photoluminescence intensity of shell quantum wells grown with a flux sequence corresponding to migration enhanced epitaxy, that is, when As and the group-III metals essentially do not impinge at the same time, is more than 2 orders of magnitude higher than for shell quantum wells prepared with substantially overlapping fluxes. Transmission electron microscopy does not reveal any extended defects explaining this difference. Our analysis of photoluminescence transients shows that co-deposition has two detrimental microscopic effects. First, a higher density of electrically active point defects leads to internal electric fields reducing the electron-hole wave function overlap. Second, more point defects form that act as nonradiative recombination centers. Our study demonstrates that the source arrangement of the growth reactor, which is of mere technical relevance for planar structures, can have drastic consequences for the material properties of nanowire shells. We expect that this finding holds good also for other alloy nanowire shells.

Transition from elastic to plastic strain release in core−shell nanowires revealed by in-plane x-ray diffraction

We investigate the strain evolution and relaxation process as function of increasing lattice mismatch between the GaAs core and surrounding In x Ga1−x As shell in core–shell nanowire heterostructures grown on Si(111) substrates. The dimensions of the core and shell are kept constant whereas the indium concentration inside the shell is varied. Measuring the and in-plane Bragg reflections normal to the nanowire side edges and side facets, we observe a transition from elastic to plastic strain release for a shell indium content x > 0.5. Above the onset of plastic strain relaxation, indium rich mounds and an indium poor coherent shell grow simultaneously around the GaAs core. Mound formation was observed for indium contents x = 0.5 and 0.6 by scanning electron microscopy. Considering both the measured radial reflections and the axial 111 Bragg reflection, the 3D strain variation was extracted separately for the core and the In x Ga1−x As shell.

Self-organization of various “phase-separated” nanostructures in a single chemical vapor deposition

Chemical vapor deposition (CVD) is one of the most versatile techniques for the controlled synthesis of functional nanomaterials. When multiple precursors are induced, the CVD process often gives rise to the growth of doped or alloy compounds. In this work, we demonstrate the self-assembly of a variety of ‘phase-separated’ functional nanostructures from a single CVD in the presence of various precursors. In specific, with silicon substrate and powder of Mn and SnTe as precursors, we achieved self-organized nanostructures including Si/SiOx core-shell nanowire heterostructures both with and without embedded manganese silicide particles, Mn11Si19 nanowires, and SnTe nanoplates. The Si/SiOx core-shell nanowires embedded with manganese silicide particles were grown along the direction of the crystalline Si via an Au-catalyzed vapor-liquid-solid process, in which the Si and Mn vapors were supplied from the heated silicon substrates and Mn powder, respectively. In contrast, direct vapor-solid deposition led to particle-free -oriented Si/SiOx core-shell nanowires and -oriented Mn11Si19 nanowires, a promising thermoelectric material. No Sn or Te impurities were detected in these nanostructures down to the experimental limit. Topological crystalline insulator SnTe nanoplates with dominant {100} and {111} facets were found to be free of Mn (and Si) impurities, although nanoparticles and nanowires containing Mn were found in the vicinity of the nanoplates. While multiple-channel transport was observed in the SnTe nanoplates, it may not be related to the topological surface states due to surface oxidation. Finally, we carried out thermodynamic analysis and density functional theory calculations to understand the ‘phase-separation’ phenomenon and further discuss general approaches to grow phase-pure samples when the precursors contain residual impurities.

Core–Shell Tunnel Junction Nanowire White-Light-Emitting Diode

We have demonstrated a new class of phosphor-free white LEDs with the use of tunnel junction structure in nonpolar core-shell InGaN nanowires. It is confirmed that the tunnel junction based nanowire LEDs can eliminate the use of the resistive p-GaN:Mg contact layer, leading to significantly enhanced hole injection and dramatically reduced voltage loss. The nonpolar core-shell nanowire heterostructure showed the enhanced carrier injection efficiency through the widened shell n-GaN contact area. The TEM analysis verified that the core-shell Al tunnel junction layers were uniformly grown on nonpolar surfaces of the GaN wurtzite crystal nanowire structure. We have also showed the monolithic integration of multiple-color emission on a single chip by using the multiple-stacked tunnel junction core-shell nanowire heterostructure. Compared to the conventional film based quantum well LEDs, the demonstrated nonpolar core-shell tunnel junction nanowire LEDs will be a very promising candidate for future solid-state lighting applications as well as phosphor-free white LEDs.

Fabrication of WO3/TiO2 core-shell nanowire arrays: Structure design and high electrochromic performance

Unique WO3/TiO2 core-shell nanowire heterostructures are rationally constructed based on a solvothermal method combining magnetron sputtering. High resolution transmission electron microscopy (HRTEM) analysis evidences the existence of a semi-coherent interface between a hexagonal phase WO3 core and anatase TiO2 shell. X-ray photoelectron spectroscopy and optical absorption spectra suggest an interfacial interaction between the WO3 core and TiO2 shell. The WO3/TiO2 core-shell nanowires exhibit superior electrochromic performance in both visible and near infrared regions with large optical modulation (87.0% at 1500 nm and 85.3% at 633 nm), short bleaching–coloration times (1.1 s and 3.0 s), decent coloring efficiency (102.1 cm2 C−1) and excellent cycling stability (95.6% after 3000 cycles). The obtained WO3/TiO2 core-shell nanowire heterostructures may find great potential in producing electrochromic nanodevices and other energy-efficient applications.

Full-Color InGaN/AlGaN Nanowire Micro Light-Emitting Diodes Grown by Molecular Beam Epitaxy: A Promising Candidate for Next Generation Micro Displays.

We have demonstrated full-color and white-color micro light-emitting diodes (μLEDs) using InGaN/AlGaN core-shell nanowire heterostructures, grown on silicon substrate by molecular beam epitaxy. InGaN/AlGaN core-shell nanowire μLED arrays were fabricated with their wavelengths tunable from blue to red by controlling the indium composition in the device active regions. Moreover, our fabricated phosphor-free white-color μLEDs demonstrate strong and highly stable white-light emission with high color rendering index of ~ 94. The μLEDs are in circular shapes with the diameter varying from 30 to 100 μm. Such high-performance μLEDs are perfectly suitable for the next generation of high-resolution micro-display applications.

Correlated Nanoscale Analysis of the Emission from Wurtzite versus Zincblende (In,Ga)As/GaAs Nanowire Core–Shell Quantum Wells

While the properties of wurtzite GaAs have been extensively studied during the past decade, little is known about the influence of the crystal polytype on ternary (In,Ga)As quantum well structures. We address this question with a unique combination of correlated, spatially resolved measurement techniques on core-shell nanowires that contain extended segments of both the zincblende and wurtzite polytypes. Cathodoluminescence hyperspectral imaging reveals a blue-shift of the quantum well emission energy by 75 ± 15 meV in the wurtzite polytype segment. Nanoprobe X-ray diffraction and atom probe tomography enable k·p calculations for the specific sample geometry to reveal two comparable contributions to this shift. First, there is a 30% drop in In mole fraction going from the zincblende to the wurtzite segment. Second, the quantum well is under compressive strain, which has a much stronger impact on the hole ground state in the wurtzite than in the zincblende segment. Our results highlight the role of the crystal structure in tuning the emission of (In,Ga)As quantum wells and pave the way to exploit the possibilities of three-dimensional band gap engineering in core-shell nanowire heterostructures. At the same time, we have demonstrated an advanced characterization toolkit for the investigation of semiconductor nanostructures.

Strain deformation in GaAs/GaAsBi core-shell nanowire heterostructures

We present the growth and strain deformation of sharp-facetted GaAs/GaAsBi core–shell heterostructure nanowires on a Si (111) substrate. The nanowires have a 90 nm wide GaAs core surrounded by an 80 nm thick GaAsBi shell. The sample was analyzed using microbeam synchrotron x-ray diffraction to resolve the local strain status at the GaAs/GaAsBi heterointerface. GaAsBi showed identical lattice constants for its vertical and lateral lattice planes. From the lattice constants, the Bi concentration in GaAsBi was estimated to be approximately 1.3%. In contrast, the GaAs core showed biaxial strain deformation, exhibiting an enlarged vertical lattice constant identical to that of the GaAsBi shell. These layers are coherently grown at the sidewalls of the nanowires, preserving identical vertical lattice constants between the constituent layers in the heterostructure.

Type-II ZnO/ZnS core-shell nanowires: Earth-abundant photoanode for solar-driven photoelectrochemical water splitting.

A core-shell structure, formed in a nanostructured photoanode, is an effective strategy to achieve high solar-to-hydrogen conversion efficiency. In this study, we present a facile and simple synthesis of a unique vertically aligned ZnO/ZnS core-shell heterostructure nanowires (NWs) on a Si substrate. Well-aligned ZnO NWs were grown on Si (100) substrates on a low-temperature ZnO buffer layer by metal-organic chemical vapor deposition. The ZnO NWs were then coated with various thicknesses of ZnS shell layers using atomic layer deposition. The structural characterizations exhibit the well-developed ZnO/ZnS core-shell NWs heterostructure. The as-prepared ZnO/ZnS core-shell NWs was applied as photoanode for photoelectrochemical (PEC) water splitting. This unique ZnO/ZnS core-shell NWs photoanode shows photocurrent density of 1.21 mA cm-2, which is 8.5 times higher than bare ZnO NWs. The PEC performance and the applied-bias-photon-to-current conversion efficiency of ZnO/ZnS core-shell NWs photoanode are further improved with the optimized ZnS shell. The type-II band alignment of the heterostructure photoanode is the key factor for their excellent PEC performance. Importantly, this type of core-shell NWs heterostructure provides useful insights into novel electrode design and fabrication based on earth abundant materials for low-cost solar fuel generation.

ZnO/CuCrO2 Core–Shell Nanowire Heterostructures for Self‐Powered UV Photodetectors with Fast Response

AbstractAn original self‐powered UV photodetector integrating ZnO/CuCrO2 core–shell nanowire heterostructures is fabricated using low‐cost and scalable chemical deposition techniques operating at moderate temperatures. A 35 nm thick delafossite phase CuCrO2 shell is formed with high uniformity by aerosol‐assisted chemical vapor deposition over an array of vertically aligned ZnO nanowires grown by chemical bath deposition. The CuCrO2 shell consists of columnar grains at the top of ZnO nanowires as well as nanograins with some preferential orientations on their vertical sidewalls. The ZnO/CuCrO2 core–shell nanowire heterostructures exhibit significant diode behavior, with a rectification ratio approaching 1.2 × 104 at 1 V and ‐1 V, as well as a high optical absorptance above 85% in the UV part of the electromagnetic spectrum. A high UV responsivity at zero bias under low‐power illumination of up to 3.43 mA W−1 under a 365 nm UV lamp, and up to 5.87 mA W−1 at 395 nm from spectrally resolved measurements, alongside a high selectivity with a UV‐to‐visible (395–550 nm) rejection ratio of 106 is measured. The short rise and decay times of 32 and 35 µs, respectively, both measured at zero bias, further establish these devices as promising candidates for cost‐efficient, all‐oxide self‐powered UV photodetectors.

Efficient ultraviolet photodetector device based on modulated wide band gap Type-II CuO/CdSe core-shell nanowires

A high-performance ultraviolet photodetector has been successfully fabricated based on new CuO/CdSe core-shell nanowires (NWs). The obtained modulated optical band gap due to quantum confinement effect has the capability to detect photons with energies (3.96 eV) significantly larger than the band gap of CuO (1.2 eV) and CdSe (1.7 eV), which is mainly attributed to the abrupt interface between the CuO core and CdSe shell. The fabricated CuO/CdSe core-shell heterostructure NWs as photodetector device showed responsivity 0.63A/W with external quantum efficiency approximately 3.14 × 102% under illumination of wavelength 254 nm. The strong response of CuO/CdSe NWs photodetector is explained by the Type-II band alignment which cause the separation of photo generated electron–hole pairs and their charge transfer at the junction.