Growth of Size-Tunable Ag2O Polyhedra and Revelation of Their Bulk and Surface Lattices.

By primarily adjusting the reagent amounts, particularly the volume of AgNO3 solution introduced, Ag2O cubes with decreasing sizes from 440 to 79 nm, octahedra from 714 to 106 nm, and rhombic dodecahedra from 644 to 168 nm are synthesized. 733 nm cuboctahedra are also prepared for structural analysis. With in-house X-ray diffraction (XRD) peak calibration, shape-related peak shifts are recognizable. Synchrotron XRD measurements at 100 K reveal the presence of bulk and surface layer lattices. Bulk cell constants also deviate slightly. They show a negative thermal expansion behavior with shrinking cell constants at higher temperatures. The Ag2O crystals exhibit size- and facet-dependent optical properties. Bandgaps red-shift continuously with increasing particle sizes. Optical facet effect is also observable. Moreover, synchrotron XRD peaks of a mixture of Cu2O rhombicuboctahedra and edge- and corner-truncated cubes exposing all three crystal faces can be deconvoluted into three components with the bulk and the [111] microstrain phase as the major component. Interestingly, while the unheated Cu2O sample shows clear diffraction peak asymmetry, annealing the sample to 450 K yields nearly symmetric peaks even when returning the sample to room temperature, meaning even moderately high temperatures can permanently change the crystal lattice.

Shape-Tunable BaTiO3 Crystals Presenting Facet-Dependent Optical and Piezoelectric Properties.

BaTiO3 octahedra, edge-, and corner-truncated cubes, and cubes with four tunable sizes from 132 to 438 nm are synthesized by a solvothermal growth approach. Acetic acid treatment can cleanly remove BaCO3 impurity. Rietveld refinement of X-ray diffraction patterns and Raman spectra help to confirm the particles have a tetragonal crystal structure. The crystals also exhibit size- and facet-dependent bandgap shifts. BaTiO3 octahedra show larger piezoelectric, ferroelectric, and pyroelectric effects than truncated cubes and cubes. The measured dielectric constant differences should be associated with their various facet-dependent behaviors. Piezoelectric nanogenerators fabricated from BaTiO3 octahedra consistently show the best performance than those containing truncated cubes and cubes. In particular, a nanogenerator with 30 wt.%-incorporated octahedra displays an open-circuit voltage of 23 V and short-circuit current of 324 nA. The device performance is also highly stable. The maximum output power reaches 3.9 µW at 60 MΩ. The fabricated nanogenerator can provide sufficient electricity to power light-emitting diodes. This work further demonstrates that various physical properties of semiconductor crystals show surface dependence.

Experimental Revelation of Surface and Bulk Lattices in Faceted Cu2 O Crystals.

Semiconductor crystals have generally shown facet-dependent electrical, photocatalytic, and optical properties. These phenomena have been proposed to result from the presence of a surface layer with bond-level deviations. To provide experimental evidence of this structural feature, synchrotron X-ray sources are used to obtain X-ray diffraction (XRD) patterns of polyhedral cuprous oxide crystals. Cu2 O rhombic dodecahedra display two distinct cell constants from peak splitting. Peak disappearance during slow Cu2 O reduction to Cu with ammonia borane differentiates bulk and surface layer lattices. Cubes and octahedra also show two peak components, while diffraction peaks of cuboctahedra are comprised of three components. Temperature-varying lattice changes in the bulk and surface regions also show shape dependence. From transmission electron microscopy (TEM) images, slight plane spacing deviations in surface and inner crystal regions are measured. Image processing provides visualization of the surface layer with depths of about 1.5-4 nm giving dashed lattice points instead of dots from atomic position deviations. Close TEM examination reveals considerable variation in lattice spot size and shape for different particle morphologies, explaining why facet-dependent properties are emerged. Raman spectrum reflects the large bulk and surface lattice difference in rhombic dodecahedra. Surface lattice difference can change the particle bandgap.

Synthesis of Zinc Blende-Phased CdSe Nanocrystals with Size-Tunable Optical Properties and Adjustable Valence Band Positions.

CdSe nanocrystals with average sizes of 15, 24, and 32 nm have been synthesized from an aqueous solution of Na2SeSO3, HCl, and cadmium nitrate at 15, 45, and 70 °C, respectively, for about 1 h. Mixing aqueous CdCl2, HNO3, and Na2SeSO3 at 22 °C for 4 h yields 44 nm CdSe nanocrystals. X-ray and electron diffraction analyses indicate the possession of a zinc blende crystal structure for all the samples. Despite the large particle dimensions, their absorption band red-shifts significantly from 520 to 570 nm with increasing particle sizes, and band gap values decrease from 2.03 eV for 15 nm particles to 1.68 eV for 44 nm crystals. Although these nanocrystals are not emissive, introduction of the cetyltrimethylammonium chloride surfactant during crystal growth can restore their photoluminescence attributed to the improved crystal quality, and the similarly sized CdSe nanocrystals have an emission band red-shifting from 544 nm for 15 nm particles to 583 nm for 47 nm crystals. A band diagram was constructed for these CdSe nanocrystals using information from Mott-Schottky plots. While they have close conduction band positions, the notable size-related band gap variation means that their valence band energies differ considerably with implications of electrochemical and photocatalytic properties. The 44 nm CdSe particles also show the smallest electrochemical charge-transfer resistance.

Surface-dependent band structure variations and bond deviations of GaN.

Density functional theory (DFT) calculations on a tunable number of GaN (0001) planes give an invariant band structure, density of states (DOS) diagram, and band gap of the GaN unit cell. Dissimilar band structures and DOS diagrams are obtained for 1, 3, 5, 7, and 9 layers of GaN (101̄0) planes, but the same band structure as that of the (0001) plane returns for 2, 4, 6, and 8 (101̄0) planes. Furthermore, 1 to 4 layers of GaN (101̄1) planes exhibit dissimilar band structures, but the GaN unit cell band structure is obtained for 5 (101̄1) planes. While there are no changes to the Ga-N bond length and bond geometry for the (0001) planes, the (101̄0) planes present bond length variation and bond distortion with odd numbers of layers. Bond length and bond direction deviations are also obtained for 1 to 4 (101̄1) planes. These results suggest that slight structural deviations may be present near the GaN surface to produce facet-dependent properties, and such atomic position deviations in the surface layer can be observed in various semiconductors.

Expression of concern: Surface-dependent band structure variations and bond deviations of GaN.

Expression of concern for 'Surface-dependent band structure variations and bond deviations of GaN' by Chih-Shan Tan et al., Phys. Chem. Chem. Phys., 2022, 24, 9135-9140, https://doi.org/10.1039/D2CP00100D.

Distinct Carrier Transport Properties Across Horizontally vs Vertically Oriented Heterostructures of 2D/3D Perovskites.

Two-dimensional-on-three-dimensional (2D/3D) halide perovskite heterostructures have been extensively utilized in optoelectronic devices. However, the labile nature of halide perovskites makes it difficult to form such heterostructures with well-defined compositions, orientations, and interfaces, which inhibits understanding of the carrier transfer properties across these heterostructures. Here, we report solution growth of both horizontally and vertically aligned 2D perovskite (PEA)2PbBr4 (PEA = phenylethylammonium) microplates onto 3D CsPbBr3 single crystal thin films, with well-defined heterojunctions. Time-resolved photoluminescence (TRPL) transients of the heterostructures exhibit the monomolecular and bimolecular dynamics expected from exciton annihilation, dissociation, and recombination, as well as evidence for carrier transfer in these heterostructures. Two kinetic models based on Type-I and Type-II band alignments at the interface of horizontal 2D/3D heterostructures are applied to reveal a shift in balance between carrier transfer and recombination: Type-I band alignment better describes the behaviors of heterostructures with thin 2D perovskite microplates but Type-II band alignment better describes those with thick 2D microplates (>150 nm). TRPL of vertically aligned 2D microplates is dominated by directly excited PL and is independent of the height above the 3D film. Electrical measurements reveal current rectification behaviors in both heterostructures with vertical heterostructures showing better electrical transport. As the first systematic study on comparing models of 2D/3D perovskite heterostructures with controlled orientations and compositions, this work provides insights on the charge transfer mechanisms in these perovskite heterostructures and guidelines for designing better optoelectronic devices.

Facet-Dependent Surface Trap States and Carrier Lifetimes of Silicon.

The facet-dependent electrical conductivity properties of silicon wafers result from significant band structure differences and variations in bond length, bond geometry, and frontier orbital electron distribution between the metal-like and semiconducting planes of silicon. To further understand the emergence of conductivity facet effects, electrochemical impedance measurements were conducted on intrinsic Si {100}, {110}, and {111} wafers. The attempt-to-escape frequency, obtained from temperature-dependent capacitance versus applied frequency curves, and other parameters derived from typical semiconductor property measurements were used to construct a diagram of the trap energy level (Et) and the amount of trap states Nt(Et). The trap states are located 0.61-0.72 eV above the silicon conduction band. Compared to {100} and {110} wafers, Si {111} wafer shows far less densities of trap states over the range of -0.2 to 2 V. Since these trap states inhibit direct electron excitation to the conduction band, the {111} wafer having much fewer trap states presents the best electrical conductivity property. Impedance data also provide facet-specific carrier lifetimes. The {111} surface gives consistently the lowest carrier lifetime, which reflects its high electrical conductivity. Lastly, ultraviolet photoelectron spectra and diffuse reflectance spectra were taken to obtain Schottky barriers between Ag and contacting Si wafers. The most conductive {111} surface presenting the largest Schottky barrier means the degrees of surface band bending used to explain facet-dependent electrical behaviors cannot be reliably attained this way.

Facet-Dependent Optical Properties of Semiconductor Nanocrystals.

Recent observations of facet-dependent electrical conductivity and photocatalytic activity of various semiconductor crystals are presented. Then, the discovery of facet-dependent surface plasmon resonance absorption of metal-Cu2 O core-shell nanocrystals with tunable sizes and shapes is discussed. The Cu2 O shells also exhibit a facet-specific optical absorption feature. The facet-dependent electrical conductivity, photocatalytic activity, and optical properties are related phenomena, resulting from the presence of an ultrathin surface layer with different band structures and thus varying degrees of band bending for the {100}, {110}, and {111} faces of Cu2 O to absorb light of somewhat different wavelengths. Recently, it is shown that the light absorption and photoluminescence properties of pure Cu2 O cubes, octahedra, and rhombic dodecahedra also display size and facet effects because of their tunable band gaps. A modified band diagram of Cu2 O can be constructed to incorporate these optical effects. Literature also provides examples of facet-dependent optical behaviors of semiconductor nanostructures, indicating that optical properties of nanoscale semiconductor materials are intrinsically facet-dependent. Some applications of semiconductor optical size and facet effects are considered.

Polyhedral Cu2 O Crystals for Diverse Aryl Alkyne Hydroboration Reactions.

Cu2 O cubes, octahedra, and rhombic dodecahedra have been used to examine facet-dependent catalytic activity in aryl alkyne hydroboration reactions. Although the reaction can proceed by using ethanol or other alcohols as solvent, the use of 1,4-dioxane gave the best product yield. All particle shapes gave exclusively the E-product, but the rhombic dodecahedra exposing {110} surfaces were consistently far more reactive than the other particle morphologies. A product yield of 99 % was achieved by using Cu2 O rhombic dodecahedra to catalyze the hydroboration of phenylacetylene at 60 °C for 5 h. The rhombic dodecahedra have been shown to catalyze a variety of substituted aryl alkynes, which demonstrates their potential as a versatile catalyst.

Aqueous-Phase Synthesis of Size-Tunable PbSe Nanocubes at Room Temperature for Optical Property Characterization.

A simple aqueous-phase synthesis of PbSe nanocubes with tunable sizes has been developed by first preparing a Na2 SeSO3 stock solution through dissolution of selenium powder in a solution of Na2 SO3 at 90-100 °C for 30 min, and adding part of this solution to a mixture of lead acetate and acetic acid at room temperature with stirring for only 5-8 min to complete the nanocrystal growth. Adjusting the volume of acetic acid and Na2 SeSO3 solution added enabled the size of the nanocrystals to be tuned, with average edge lengths of 13 to 121 nm attained. Changes in solution color revealed very different crystal growth rates for the 13 and 121 nm nanocubes. The PbSe cubes exhibit size-dependent absorption bands in the ultraviolet and visible-light region; the band positions show progressive redshifts with increasing particle size. Slight photocatalytic activity upon 532 nm laser irradiation of the nanocubes suggests the presence of higher energy levels in the band structure of PbSe. The synthetic conditions can be easily scaled up to obtain a large quantity of PbSe nanocubes for applications.

Germanium Wafers Possessing Facet-Dependent Electrical Conductivity Properties.

Electrical conductivity properties of Ge {100}, {110}, {111}, and {211} facets have been measured by breaking Ge (100) and (111) wafers to expose {110} and {211} surfaces and contacting the different facets with tungsten probes. Ge {111} and {211} faces are far more conductive than the already conductive Ge {100} and {110} faces, matching with recent density functional theory (DFT) predictions. Asymmetric I-V curves resembling those of p-n junctions have been collected for the {110}/{111} and {110}/{211} facet combinations. The current-rectifying effects stem from different degrees of surface band bending for the highly and less conductive faces and the direction of current flow. This work demonstrates that germanium wafers also possess facet-dependent electrical conductivity responses that can be utilized in the fabrication of novel fin field-effect transistors (finFET).

Metal-like Band Structures of Ultrathin Si {111} and {112} Surface Layers Revealed through Density Functional Theory Calculations.

Density functional theory calculations have been performed on Si (100), (110), (111), and (112) planes with tunable number of planes for evaluation of their band structures and density of states profiles. The purpose is to see whether silicon can exhibit facet-dependent properties derived from the presence of a thin surface layer having different band structures. No changes have been observed for single to multiple layers of Si (100) and (110) planes with a consistent band gap between the valence band and the conduction band. However, for 1, 2, 4, and 5 Si (111) and (112) planes, metal-like band structures were obtained with continuous density of states going from the valence band to the conduction band. For 3, 6, and more Si (111) planes, as well as 3 and 6 Si (112) planes, the same band structure as that seen for Si (100) and (110) planes has been obtained. Thus, beyond a layer thickness of five Si (111) planes at ≈1.6 nm, normal semiconductor behavior can be expected. The emergence of metal-like band structures for the Si (111) and (112) planes are related to variation in Si-Si bond length and bond distortion plus 3s and 3p orbital electron contributions in the band structure. This work predicts possession of facet-dependent electrical properties of silicon with consequences in FinFET transistor design.

Tracing the Surfactant-Mediated Nucleation, Growth, and Superpacking of Gold Supercrystals Using Time and Spatially Resolved X-ray Scattering.

The nucleation and growth process of gold supercrystals in a surfactant diffusion approach is followed by simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS), supplemented with scanning electron microscopy. The results indicate that supercrystal nucleation can be activated efficiently upon placing a concentrated surfactant solution of a nematic phase on top of a gold nanocrystal solution droplet trapped in the middle of a vertically oriented capillary tube. Supercrystal nuclei comprised of tens of gold nanocubes are observed nearly instantaneously in the broadened liquid-liquid interface zone of a steep gradient of surfactant concentration, revealing a diffusion-kinetics-controlled nucleation process. Once formed, the nuclei can sediment into the naoncrystal zone below, and grow efficiently into cubic or tetragonal supercrystals of ∼1 µm size within ∼100 min. Supercrystals matured during sedimentation in the capillary can accumulate and face-to-face align at the bottom liquid-air interface of the nanocrystal droplet. This is followed by superpacking of the supercrystals into highly oriented hierarchical sheets, with a huge number of gold nanocubes aligned for largely coherent crystallographic orientations.

Silicon Wafers with Facet-Dependent Electrical Conductivity Properties.

By breaking intrinsic Si (100) and (111) wafers to expose sharp {111} and {112} facets, electrical conductivity measurements on single and different silicon crystal faces were performed through contacts with two tungsten probes. While Si {100} and {110} faces are barely conductive at low applied voltages, as expected, the Si {112} surface is highly conductive and Si {111} surface also shows good conductivity. Asymmetrical I-V curves have been recorded for the {111}/{112}, {111}/{110}, and {112}/{110} facet combinations because of different degrees of conduction band bending at these crystal surfaces presenting different barrier heights to current flow. In particular, the {111}/{110} and {112}/{110} facet combinations give I-V curves resembling those of p-n junctions, suggesting a novel field effect transistor design is possible capitalizing on the pronounced facet-dependent electrical conductivity properties of silicon.

Synthesis of Ultrasmall Cu2 O Nanocubes and Octahedra with Tunable Sizes for Facet-Dependent Optical Property Examination.

Size-tunable small to ultrasmall Cu2 O nanocubes and octahedra are synthesized in aqueous solution without the introduction of any surfactant. These nanocrystals provide strong evidence of the existence of facet-dependent optical absorption properties of Cu2 O nanoparticles, showing nanocubes always have a more redshifted absorption band than that of octahedra having a similar volume by about 15 nm.

Novel silica stabilization method for the analysis of fine nanocrystals using coherent X-ray diffraction imaging.

High-energy X-ray Bragg coherent diffraction imaging (BCDI) is a well established synchrotron-based technique used to quantitatively reconstruct the three-dimensional morphology and strain distribution in nanocrystals. The BCDI technique has become a powerful analytical tool for quantitative investigations of nanocrystals, nanotubes, nanorods and more recently biological systems. BCDI has however typically failed for fine nanocrystals in sub-100 nm size regimes - a size routinely achievable by chemical synthesis - despite the spatial resolution of the BCDI technique being 20-30 nm. The limitations of this technique arise from the movement of nanocrystals under illumination by the highly coherent beam, which prevents full diffraction data sets from being acquired. A solution is provided here to overcome this problem and extend the size limit of the BCDI technique, through the design of a novel stabilization method by embedding the fine nanocrystals into a silica matrix. Chemically synthesized FePt nanocrystals of maximum dimension 20 nm and AuPd nanocrystals in the size range 60-65 nm were investigated with BCDI measurement at beamline 34-ID-C of the APS, Argonne National Laboratory. Novel experimental methodologies to elucidate the presence of strain in fine nanocrystals are a necessary pre-requisite in order to better understand strain profiles in engineered nanocrystals for novel device development.

Seed-Mediated Growth of Silver Nanocubes in Aqueous Solution with Tunable Size and Their Conversion to Au Nanocages with Efficient Photothermal Property.

Two seed-mediated approaches for the growth of silver nanocubes in aqueous solution have been developed. Addition of a silver-seed solution to a mixture of cetyltrimethylammonium chloride (CTAC), silver trifluoroacetate, and ascorbic acid and heating the solution at 60 °C for 1.5 h produces uniform Ag nanocubes with tunable sizes from 23 to 60 nm by simply adjusting the volume of silver-seed solution introduced. Alternatively, the silver-seed solution can be injected into a mixture of cetyltrimethylammonium bromide (CTAB), silver nitrate, copper sulfate, and ascorbic acid and heated to 80 °C for 2 h to generate 46 nm silver nanocubes. Plate-like Ag nanocrystals exposing {111} surfaces can be synthesized by reducing Ag(NH3 )2 (+) with ascorbic acid in a CTAC solution. Relatively large Ag nanocubes were converted to cuboctahedral Au/Ag and Au nanocages and nanoframes with empty {111} faces through a galvanic replacement reaction. The nanocages showed a progressive plasmonic band red-shift with increasing Au content. The nanocages exhibited high and stable photothermal efficiency with solution temperatures quickly reaching beyond 100 °C when irradiated with an 808 nm laser for large heat and water vapor generation.

Highly Facet-Dependent Photocatalytic Properties of Cu2 O Crystals Established through the Formation of Au-Decorated Cu2 O Heterostructures.

This work confirms the presence of a large facet-dependent photocatalytic activity of Cu2 O crystals through sparse deposition of gold particles on Cu2 O cubes, octahedra, and rhombic dodecahedra. Au-decorated Cu2 O rhombic dodecahedra and octahedra showed greatly enhanced photodegradation rates of methyl orange resulting from a better separation of the photogenerated electrons and holes, with the rhombic dodecahedra giving the best efficiency. Au-Cu2 O core-shell rhombic dodecahedra also displayed a better photocatalytic activity than pristine rhombic dodecahedra. However, Au-deposited Cu2 O cubes, pristine cubes, and Au-deposited small nanocubes bound by entirely {100} facets are all photocatalytically inactive. X-ray photoelectron spectra (XPS) showed identical copper peak positions for these Au-decorated crystals. Remarkably, electron paramagnetic resonance (EPR) measurements indicated a higher production of hydroxyl radicals for the photoirradiated Cu2 O rhombic dodecahedra than for the octahedra, but no radicals were produced from photoirradiated Cu2 O cubes. The Cu2 O {100} face may present a high energy barrier through its large band edge bending and/or electrostatic repulsion, preventing charge carriers from reaching to this surface. The conventional photocatalysis model fails in this case. The facet-dependent photocatalytic differences should be observable in other semiconductor systems whenever a photoinduced charge-transfer process occurs across an interface.

Facet-dependent electrical conductivity properties of Cu2O crystals.

It is interesting to examine facet-dependent electrical properties of single Cu2O crystals, because such study greatly advances our understanding of various facet effects exhibited by semiconductors. We show a Cu2O octahedron is highly conductive, a cube is moderately conductive, and a rhombic dodecahedron is nonconductive. The conductivity differences are ascribed to the presence of a thin surface layer having different degrees of band bending. When electrical connection was made on two different facets of a rhombicuboctahedron, a diode-like response was obtained, demonstrating the potential of using single polyhedral nanocrystals as functional electronic components. Density of state (DOS) plots for three layers of Cu2O (111), (100), and (110) planes show respective metallic, semimetal, and semiconducting band structures. By examining DOS plots for varying number of planes, the surface layer thicknesses responsible for the facet-dependent electrical properties of Cu2O crystals have been determined to be below 1.5 nm for these facets.