A Facile Route for Producing Single-Crystalline Epitaxial Perovskite Oxide Thin Films

We report how a low vacuum pressure process followed by a few-minute annealing enables epitaxial stabilization, producing high-quality, phase-pure, single-crystalline epitaxial, and misfit dislocation-free BiFeO₃(001) thin films on SrTiO₃(001) at ∼450 °C less than current routes. These results unambiguously challenge the widely held notion that atomic layer deposition (ALD) is not appropriate for attaining high-quality chemically complex oxide films on perovskite substrates in single-crystalline epitaxial form, demonstrating applicability as an inexpensive, facile, and highly scalable route

Selective Epitaxial Growth on Germanium Nanowires via Hybrid Oxide-Stabilized/Vapor–Liquid–Solid Growth

The introduction low levels of oxygen during the vapor–liquid–solid growth (VLS) of germanium nanowires causes an oxide sheath to form at the catalyst/nanowire/vapor interface during growth. This results in extremely high aspect ratio nanowires due to the removal of homoepitaxial deposition and the finite energy required for heterogeneous nucleation of germanium on its oxide. With the removal of oxygen, the catalyzed oxide sheath terminates and conventional growth with finite sidewall deposition dominates subsequent growth. The successful transition between oxide-stabilized and conventional VLS regimes can be deliberately manipulated to grow finite conical nanowire segments with discontinuous changes in diameter

Dynamics of Photogenerated Surface Charge on BiFeO₃ Films

We report on the spatial and temporal evolution of photoinduced charge generation and carrier separation in heteroepitaxial BiFeO₃ thin films deposited on Nb:SrTiO₃ as measured in ambient at room temperature with Kelvin probe and piezoresponse force microscopy. Contributions from the self-poled and ferroelectric polarization charge are identified from the time evolution of the correlated surface potential and ferroelectric polarization in films as grown and following poling, and at different stages and intensities of optical illumination. Variations in the surface potential with bias voltage, switching history, and illumination intensity indicate how both bulk ferroelectric photovoltaic and the domain wall offset potential mechanisms contribute to the photogenerated charge

Shape-Controlled Vapor-Transport Growth of Tellurium Nanowires

A vapor transport method is employed to synthesize single crystalline tellurium nanowires with tailorable size using tellurium powder in an inert atmosphere. The average nanowire diameter is tunable between 50 and 3000 nm with an associated length of 1 to 22 μm. Growth temperature and time are used to specifically control the behavior of supersaturation, leading to nucleation and morphological control in this vapor–solid growth regime. Analysis of the resulting nanowire product provides insight into the dominant reaction kinetics involved in these growths and suggests routes by which to control growth product. This methodology provides a practical approach to synthesizing high-quality tellurium nanowires of various sizes and their incorporation into two-dimensional mesh-like structures through controlled nucleation and growth dynamics

Quantitative Phase-Change Thermodynamics and Metastability of Perovskite-Phase Cesium Lead Iodide

The perovskite phase of cesium lead iodide (α-CsPbI₃ or “black” phase) possesses favorable optoelectronic properties for photovoltaic applications. However, the stable phase at room temperature is a nonfunctional “yellow” phase (δ-CsPbI₃). Black-phase polycrystalline thin films are synthesized above 330 °C and rapidly quenched to room temperature, retaining their phase in a metastable state. Using differential scanning calorimetry, it is shown herein that the metastable state is maintained in the absence of moisture, up to a temperature of 100 °C, and a reversible phase-change enthalpy of 14.2 (±0.5) kJ/mol is observed. The presence of atmospheric moisture hastens the black-to-yellow conversion kinetics without significantly changing the enthalpy of the transition, indicating a catalytic effect, rather than a change in equilibrium due to water adduct formation. These results delineate the conditions for trapping the desired phase and highlight the significant magnitude of the entropic stabilization of this phase

Subsurface Imaging of Coupled Carrier Transport in GaAs/AlGaAs Core–Shell Nanowires

We demonstrate spatial probing of carrier transport within GaAs/AlGaAs core–shell nanowires with nanometer lateral resolution and subsurface sensitivity by energy-variable electron beam induced current imaging. Carrier drift that evolves with applied electric field is distinguished from a coupled drift-diffusion length. Along with simulation of injected electron trajectories, combining beam energy tuning with precise positioning for selective probing of core and shell reveals axial position- and bias-dependent differences in carrier type and transport along parallel conduction channels. These results indicate how analysis of transport within heterostructured nanomaterials is no longer limited to nonlocal or surface measurements

Direct Measurement of Band Edge Discontinuity in Individual Core–Shell Nanowires by Photocurrent Spectroscopy

Group III–V coaxial core–shell semiconducting nanowire heterostructures possess unique advantages over their planar counterparts in logic, photovoltaic, and light-emitting devices. Dimensional confinement of electronic carriers and interface complexity in nanowires are known to produce local electronic potential landscapes along the radial direction that deviate from those along the normal to planar heterojunction interfaces. However, understanding of selected electronic and optoelectronic carrier transport properties and device characteristics remains lacking without a direct measurement of band alignment in individual nanowires. Here, we report on, in the GaAs/Al_<i>x</i>Ga_1–<i>x</i>As and GaAs/AlAs core–shell nanowire systems, how photocurrent and photoluminescence spectroscopies can be used together to construct a band diagram of an individual heterostructure nanowire with high spectral resolution, enabling quantification of conduction band offsets

Surface Chemically Switchable Ultraviolet Luminescence from Interfacial Two-Dimensional Electron Gas

We report intense, narrow line-width, surface chemisorption-activated and reversible ultraviolet (UV) photoluminescence from radiative recombination of the two-dimensional electron gas (2DEG) with photoexcited holes at LaAlO₃/SrTiO₃. The switchable luminescence arises from an electron transfer-driven modification of the electronic structure via H-chemisorption onto the AlO₂-terminated surface of LaAlO₃, at least 2 nm away from the interface. The control of the onset of emission and its intensity are functionalities that go beyond the luminescence of compound semiconductor quantum wells. Connections between reversible chemisorption, fast electron transfer, and quantum-well luminescence suggest a new model for surface chemically reconfigurable solid-state UV optoelectronics and molecular sensing

BaTiO₃ Thin Films from Atomic Layer Deposition: A Superlattice Approach

A superlattice approach for the atomic layer deposition of polycrystalline BaTiO₃ thin films is presented as an example for an effective route to produce high-quality complex oxide films with excellent thickness and compositional control. This method effectively mitigates any undesirable reactions between the different precursors and allows an individual optimization of the reaction conditions for the Ba–O and the Ti–O subcycles. By growth of nanometer thick alternating Ba­(OH)₂ and TiO₂ layers, the advantages of binary oxide atomic layer deposition are transferred into the synthesis of ternary compounds, permitting extremely high control of the cation ratio and superior uniformity. Whereas the Ba­(OH)₂ layers are partially crystalline after the deposition, the TiO₂ layers remain mostly amorphous. The layers react to polycrystalline, polymorph BaTiO₃ above 500 °C, releasing H₂O. This solid-state reaction is accompanied by an abrupt decrease in film thickness. Transmission electron microscopy and Raman spectroscopy reveal the presence of hexagonal BaTiO₃ in addition to the perovskite phase in the annealed films. The microstructure with relatively small grains of ∼70 Å and different phases is a direct consequence of the abrupt formation reaction. The electrical properties transition from the initially highly insulating dielectric semiamorphous superlattice into a polycrystalline BaTiO₃ thin film with a dielectric constant of 117 and a dielectric loss of 0.001 at 1 MHz after annealing at 600 °C in air, which, together with the suppression of ferroelectricity at room temperature, are very appealing properties for voltage tunable devices