Coupling of the Structure and Magnetism to Spin Splitting in Hybrid Organic-Inorganic Perovskites.

Hybrid organic-inorganic perovskites are famous for the diversity of their chemical compositions, phases, phase transitions, and associated physical properties. We use a combination of experimental and computational techniques to reveal a strong coupling between structure, magnetism, and spin splitting in a representative of the largest family of hybrid organic-inorganic perovskites: the formates. With the help of first-principles simulations, we find spin splitting in both conduction and valence bands of [NH2NH3]Co(HCOO)3 induced by spin-orbit interactions, which can reach up to 14 meV. Our magnetic measurements reveal that this material exhibits canted antiferromagnetism below 15.5 K. The direction of the associated antiferromagnetic order parameter is strongly coupled with spin splitting in the centrosymmetric phase, allowing for the creation and annihilation of spin splitting through the application of a magnetic field. Furthermore, the structural phase transition to the experimentally observed polar Pna21 phase completely changes the aforementioned spin splitting and its coupling to magnetic degrees of freedom. This reveals that in [NH2NH3]Co(HCOO)3, the structure and magnetism are strongly coupled to spin splitting and can be manipulated through electric and magnetic fields. We believe that our findings offer an important step toward a fundamental understanding and practical applications of materials with coupled properties.

Enhanced Piezoelectric Response at Nanoscale Vortex Structures in Ferroelectrics.

The piezoelectric response is a measure of the sensitivity of a material's polarization to stress or its strain to an applied field. Using in operando X-ray Bragg coherent diffraction imaging, we observe that topological vortices are the source of a 5-fold enhancement of the piezoelectric response near the vortex core. The vortices form where several low-symmetry ferroelectric phases and phase boundaries coalesce. Unlike bulk ferroelectric solid solutions in which a large piezoelectric response is associated with coexisting phases in the proximity of the triple point, the largest responses for pure BaTiO3 at the nanoscale are in spatial regions of extremely small spontaneous polarization at vortex cores. The response decays inversely with polarization away from the vortex, analogous to the behavior in bulk ceramics as the cation compositions are varied away from the triple point. We use first-principles-based molecular dynamics to augment our observations, and our results suggest that nanoscale piezoelectric materials with a large piezoelectric response can be designed within a parameter space governed by vortex cores. Our findings have implications for the development of next-generation nanoscale piezoelectric materials.

2D Materials by Design: Intercalation of Cr or Mn between two VSe2 van der Waals Layers.

Insertion of metal layers between layered transition-metal dichalcogenides (TMDs) enables the design of new pseudo-2D nanomaterials. The general premise is that various metal atoms may adopt energetically favorable intercalation sites between two TMD sheets. These covalently bound metals arrange in metastable configurations and thus enable the controlled synthesis of nanomaterials in a bottom-up approach. Here, this method is demonstrated by the insertion of Cr or Mn between VSe2 layers. Vacuum-deposited transition metals diffuse between VSe2 layers with increasing concentration, arranging in ordered phases. The Cr3+ or Mn2+ ions are in octahedral coordination and thus in a high-spin state. Measured and computed magnetic moments are high for dilute Cr atoms, but with increasing Cr concentration the average magnetic moment decreases, suggesting antiferromagnetic ordering between Cr ions. The many possible combinations of transition metals with TMDs form a library for exploring quantum phenomena in these nanomaterials.

Ferrielectricity in the Archetypal Antiferroelectric, PbZrO3.

Antiferroelectric materials, where the transition between antipolar and polar phase is controlled by external electric fields, offer exceptional energy storage capacity with high efficiencies, giant electrocaloric effect, and superb electromechanical response. PbZrO3 is the first discovered and the archetypal antiferroelectric material. Nonetheless, substantial challenges in processing phase pure PbZrO3 have limited studies of the undoped composition, hindering understanding of the phase transitions in this material or unraveling the controversial origins of a low-field ferroelectric phase observed in lead zirconate thin films. Leveraging highly oriented PbZrO3 thin films, a room-temperature ferrielectric phase is observed in the absence of external electric fields, with modulations of amplitude and direction of the spontaneous polarization and large anisotropy for critical electric fields required for phase transition. The ferrielectric state observations are qualitatively consistent with theoretical predictions, and correlate with very high dielectric tunability, and ultrahigh strains (up to 1.1%). This work suggests a need for re-evaluation of the fundamental science of antiferroelectricity in this archetypal material.

Negative Longitudinal Piezoelectricity Coexisting with both Negative and Positive Transverse Piezoelectricity in a Hybrid Formate Perovskite.

Negative longitudinal piezoelectric response is a rare property, which has been found mostly in inorganic materials. We use first-principles density functional theory simulations to predict such an unusual response in [NH2NH3]Co(HCOO)3 ─a representative of a large family of hybrid organic-inorganic formate perovskites. A feature that sets aside [NH2NH3]Co(HCOO)3 from inorganic compounds with a negative longitudinal piezoelectric response is that this rare property coexists with both negative and positive transverse piezoelectric responses, which is highly desirable for tunable applications. Atomistic analysis reveals that this unusual electromechanical coupling originates from the high anisotropy of materials response to uniaxial stress. Such a deformation produces oxygen octahedral tilts in the framework, whose magnitude depends strongly on the direction of the applied strain. For hard directions, the tilts make the dominant contribution to the deformation-induced change in polarization, while for the softer direction, it is the tilts of the NH2NH3+ cation that dominate the polarization response. The latter occur as the complex hydrogen bond network responds to the octahedral tilts. As high anisotropy of mechanical properties is a common feature across the formate perovskites, we expect our findings to stimulate more discoveries of unusual electromechanical couplings in this family.

Phase Switching as the Origin of Large Piezoelectric Response in Organic-Inorganic Perovskites: A First-Principles Study.

Piezoelectrics are critical functional components of many practical applications such as sensors, ultrasonic transducers, actuators, medical imaging, and telecommunications. So far, the best performing piezoelectrics are ferroelectric ceramics, many of which are toxic, heavy, hard, and cost-ineffective. Recently, a groundbreaking discovery of extraordinarily large piezoelectric coefficients in the family of organic-inorganic perovskites gave a hope for a cheaper, environmentally friendly, inexpensive, lightweight, and flexible alternative. However, the origin of such a response in organic-inorganic ferroelectrics whose spontaneous polarization is an order of magnitude smaller than for inorganic counterparts remains unclear. In our study, we employ first-principles simulations to predict that the mechanism associated with large piezoelectric constants is of extrinsic origin and associated with switching between the stable phase and a previously overlooked energetically competitive metastable phase that can be stabilized by the external stress. The phase switching changes the polarization direction and therefore produces a large piezoelectric response similar to PbZr_{1-x}Ti_{x}O_{3} near the morphotropic phase boundary. The existence of such metastable phases is likely to manifest as the dynamical molecular disorder above the Curie temperature and therefore could be intrinsic to the entire family of organic-inorganic ferroelectrics with such disorder.

Advanced Photoemission Spectroscopy Investigations Correlated with DFT Calculations on the Self-Assembly of 2D Metal Organic Frameworks Nano Thin Films.

Metal-organic frameworks (MOFs) deposited from solution have the potential to form 2-dimensional supramolecular thin films suitable for molecular electronic applications. However, the main challenges lie in achieving selective attachment to the substrate surface, and the integration of organic conductive ligands into the MOF structure to achieve conductivity. The presented results demonstrate that photoemission spectroscopy combined with preparation in a system-attached glovebox can be used to characterize the electronic structure of such systems. The presented results demonstrate that porphyrin-based 2D MOF structures can be produced and that they exhibit similar electronic structure to that of corresponding conventional porphyrin thin films. Porphyrin MOF multilayer thin films were grown on Au substrates prefunctionalized with 4-mercaptopyridine (MP) via incubation in a glovebox, which was connected to an ultrahigh vacuum system outfitted with photoelectron spectroscopy. The thin film growth process was carried out in several sequential steps. In between individual steps the surface was characterized by photoemission spectroscopy to determine the valence bands and evaluate the growth mode of the film. A comprehensive evaluation of X-ray photoemission spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and inverse photoemission spectroscopy (IPES) data was performed and correlated with density functional theory (DFT) calculations of the density of states (DOS) of the films involved to yield the molecular-level insights into the growth and the electronic properties of MOF-based 2D thin films.

Elastocaloric Effect in Carbon Nanotubes and Graphene.

Carbon nanotubes are famous for their many extraordinary properties. We use a thermodynamical approach, experimental data from the literature, and atomistic simulations to reveal one more remarkable property of the carbon nanotubes that has so far been overlooked. Namely, we predict the existence of very large elastocaloric effect that can reach up to 30 K under moderate loads. Potentially even larger values could be achieved under extreme loads, putting carbon nanotubes in the forefront of caloric materials. Other remarkable features of the elastocaloric effect in carbon nanotubes include linearity of elastocaloric temperature change in applied force (compressive or stretching), very weak dependence on the temperature, and an absence of hysteresis. Such features are extremely desirable for practical applications in cooling devices. Moreover, a similarly large elastocaloric effect is predicted for the graphene. The prediction of a large elastocaloric effect in carbon nanotubes and graphene sets forward an unconventional strategy of targeting materials with moderate caloric responses but the ability to withstand very large loads.

Oscillatory band gap behavior in small diameter Si-clathrate nanowires.

Electronic structure analysis of small cagelike silicon nanowires is carried out and reveals many surprising features. The band gap values for all the nanowires are found to be smaller than their bulk counterparts. The most intriguing aspect appears to be the alternating sequence of direct and indirect band gaps as the diameter changes. This is attributed to the type of surface geometry. We illustrate this with two well-known clathrate forms as well as a new hexagonal clathrate structure with a direct band gap in the optical region.

Thermal conductivity in thin silicon nanowires: phonon confinement effect.

Thermal conductivity of thin silicon nanowires (1.4-8.3 nm) including the realistic crystalline structures and surface reconstruction effects is investigated using direct molecular dynamics simulations with Stillinger-Weber potential for Si-Si interactions. Thermal conductivity as a function of decreasing nanowire diameter shows an expected decrease due to increased surface scattering effects. However, at very small diameter (<1.5 nm), an increase in the thermal conductivity is observed, which is explained by the phonon confinement effect.

Structure, stability, and quantum conductivity of small diameter silicon nanowires.

Structures and energetics of various types of silicon nanowires have been investigated using both quantum and classical molecular dynamics simulations to determine the most stable forms. The tetrahedral type nanowires have been found to be the most stable and, surprisingly, the polycrystalline forms of nanowires, while having the smallest surface to bulk ratio, are found to be the least stable. We also show that the cagelike nanowires have greater thermal stability than the tetrahedral nanowires. Furthermore, their electrical conducting properties are found to be better than those of tetrahedral nanowires, suggesting useful molecular electronic applications.

Carbon nanotube "T Junctions": formation pathways and conductivity.

Using tight-binding molecular dynamics we simulate the formation of single wall carbon nanotube T junctions via the fusing of two nanotubes. We propose energetically efficient pathways for this process in which all atoms maintain their sp(2) arrangements throughout. Recent experimental advances have greatly increased the plausibility of synthesizing T junctions as proposed in the simulations. We further report I-V characteristics of the formed junctions.