Measuring the practical particle-in-a-box: orthorhombic perovskite nanocrystals

A connection between condensed matter physics and basic quantum mechanics is demonstrated as we use the fundamental 3D particle-in-a-box model to explain the optical properties of semiconductor nanocrystals, which are substantially modified due to quantum confinement. We also discuss recent advances in the imaging and measurement capabilities of transmission electron microscopy, which have made it possible to directly image single nanocrystals while simultaneously measuring their characteristic absorption energies. We introduce the basic theory of nanocrystals and derive a simplified expression to approximate the optical bandgap energy of an orthorhombic nanocrystal. CsPbBr3 perovskite nanocrystals are used to demonstrate this model due to their cubic crystal structure, large absorption cross-section, and favourable dielectric properties, which make them ideal for exploring the applications of this simple classroom problem. Various orthorhombic shapes are explored, and the predicted values of the optical bandgap energies using the proposed model are shown to be in good agreement with the experimentally determined values

Carbon nanotubes as electrically active nanoreactors for multi-step inorganic synthesis: sequential transformations of molecules to nanoclusters, and nanoclusters to nanoribbons

In organic synthesis, the composition and structure of products are predetermined by the reaction conditions; however, the synthesis of well-defined inorganic nanostructures often presents a significant challenge yielding non-stoichiometric or polymorphic products. In this study, confinement in the nanoscale cavities of single-walled carbon nanotubes (SWNT) provides a new approach for multi-step inorganic synthesis where sequential chemical transformations take place within the same nanotube. In the first step, SWNT donate electrons to the reactant iodine molecules (I2) transforming them to iodide anions (I-). These then react with metal hexacarbonyls (M(CO)6, M = Mo or W) in the next step yielding anionic nanoclusters [M6I14]2-, the size and composition of which are strictly dictated by the nanotube cavity, as demonstrated by aberration corrected high resolution transmission electron microscopy (AC-HRTEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) spectroscopy. Atoms in the nanoclusters [M6I14]2- are arranged in a perfect octahedral geometry and can engage in further chemical reactions within the nanotube, either reacting with each other leading to a new polymeric phase of molybdenum iodide [Mo6I12]n, or with hydrogen sulphide gas giving rise to nanoribbons of molybdenum/tungsten disulphide [MS2]n in the third step of the synthesis. Electron microscopy measurements demonstrate that the products of the multi-step inorganic transformations are precisely controlled by the SWNT nanoreactor, with complementary Raman spectroscopy revealing the remarkable property of SWNT to act as a reservoir of electrons during the chemical transformation. The electron transfer from the host-nanotube to the reacting guest-molecules is essential for stabilising the anionic metal iodide 2 nanoclusters and for their further transformation to metal disulphide nanoribbons synthesised in the nanotubes in high yield

Optical orientation and alignment of excitons in ensembles of inorganic perovskite nanocrystals

We demonstrate the optical orientation and alignment of excitons in a two-dimensional layer of CsPbI$_3$ perovskite nanocrystals prepared by colloidal synthesis and measure the anisotropic exchange splitting of exciton levels in the nanocrystals. From the experimental data at low temperature (2K), we obtain the average value of anisotropic splitting of bright exciton states of the order of 120{\mu}eV. Our calculations demonstrate that there is a significant contribution to the splitting due to the nanocrystal shape anisotropy for all inorganic perovskite nanocrystrals.Comment: 10 page

Cross-Sectional Observations of Polymorphic FeGe Interphases

Cross-sectional microscopy is performed on ${\rm Fe}_x{\rm Ge}_{1-x}$ multilayers with steep concentration gradients. A profile in high angle annular dark field imaging mode demonstrates the presence of an intermetallic phase appearing brighter in Z-contrast, which is proved to be \alpha\rm \mbox{-}FeGe alloy by means of associated quantitative contrast calculations. Fourier analysis of lattice fringes observed by transmission electron microscopy reveals the previously found non-magnetic interphase as \rm \eta\mbox{-}Fe_6Ge_5. These results are discussed here with reference to recent work using Mössbauer spectroscopy, kinetic ellipsometry, and Fe L-edge EELS analysis. An attempt is then made to correlate them with phase separation models involving nanometric inhomogeneities in FeGe alloys.


Electron Energy Loss Spectroscopy with Subnanometer Spatial Resolution on Compositionally Modulated TiN x Multilayers

Compositionally modulated TiNx multilayers (0.3 < x < 1, with a period of 8 nm) are investigatedby spatially resolved electron energy loss spectroscopy with a 0.5 nrn incident probe,in order to investigate systematically the evolution of the electronic structure as the nitrogenconcentration varies. The difference in binding energy between Ti Land N K edges decreasessubstantially as the nitrogen concentration changes from the stoichiometric value (x ~ 1) towardsa non-stoichiometric one (x ~ 0.3). This can be explained by a re-distribution of the charge fronlthe removed nitrogen to the surrounding titanium atoms. Furthermore the titanium L3 peak,so-called white line, broadens in accordance with the nitrogen concentration, due to a changein the d band occupation. A shift of the volume plasma frequency towards lower energies whenthe nitrogen vacancy concentration increases is observed and explained in terms of a simplefree-electron gas modeL These results demonstrate the great power of spatially resolved electronenergy loss spectroscopy as a probe of electronic structure with subnanometer-scale spatialresolution

Structural Distortions and Charge Density Waves in Iodine Chains Encapsulated inside Carbon Nanotubes

Atomic chains are perfect systems for getting fundamental insights into the electron dynamics and coupling between the electronic and ionic degrees of freedom in one-dimensional metals. Depending on the band filling, they can exhibit Peierls instabilities (or charge density waves), where equally spaced chain of atoms with partially filled band is inherently unstable, exhibiting spontaneous distortion of the lattice that further leads to metal-insulator transition in the system. Here, using high-resolution scanning transmission electron microscopy, we directly image the atomic structures of a chain of iodine atoms confined inside carbon nanotubes. In addition to long equidistant chains, the ones consisting of iodine dimers and trimers were also observed, as well as transitions between them. First-principles calculations reproduce the experimentally observed bond lengths and lattice constants, showing that the ionic movement is largely unconstrained in the longitudinal direction, while naturally confined by thenanotube in the lateral directions. Moreover, the trimerized chain bears the hallmarks of a charge density wave. The transition is driven by changes in the charge transfer between the chain and the nanotube and is enabled by the charge compensation and additional screening provided by the nanotube.Peer reviewe

Gentle transfer method for water- and acid/alkali-sensitive 2D materials for (S)TEM study

We report a method in making transmission electron microscopy sample for both CVD-grown and exfoliated 2D materials without etching process, thus gentle to those 2D materials that are sensitive to water and reactive etchants. Large-scale WS2 monolayer grown on glass, NbS2 atomic layers grown on exfoliated h-BN flakes, and water-sensitive exfoliated TiS2 flakes are given as representative examples. We show that the as-transferred samples not only retain excellent structural integrity down to atomic scale but also have little oxidations, presumably due to the minimum contact with water/etchants. This method paves the way for atomic scale structural and chemical investigations in sensitive 2D materials